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EXPLANATORY NOTES

HYDROGEOLOGICAL AND HYDROCHEMICAL MAPSOF DODOLA NB 37-7

Astatike Kiflu (Chief Compiler)

Jiri Sima (Editor)

The Main Project Partners

The Czech Development Agency (CzDA)cooperates with the Ministry of Foreign Affairs on the establishment of an institutional framework of Czech development cooperation and actively participates in the creation and financing of development cooperation programs between the Czech Republic and partner countries.

www.czda.cz

The Geological Survey of Ethiopia (GSE)which is accountable to the Ministry of Mines and Energy, collects and assesses geology, geological engineering and hydrogeology data for publication. The project beneficiary.www.geology.gov.et (www.mome.gov.et)

AQUATEST a.s. a Czech consulting and engineering company in water management and environmental protection. The main contractor.www.aquatest.cz

The Czech Geological Service collects data and information on geology and processes it for political, economical and environmental management. The main subcontractor.www.geology.cz

Copyright © 2011 AQUATEST a.s., Geologicka 4, 152 00 Prague 5, Czech RepublicFirst editionISBN 978-80-260-0332-8

aquatest

Acknowledgment

Field work and primary compilation of the map and explanatory notes was done by a team from the Geological Survey of Ethiopia (GSE) consisting of staff from the Groundwater Resources Assessment Department; the Czech experts from AQUATEST a.s. and the Czech Geological Survey in the framework of the Czech Official Development Assistance Program. The team is greatly indebted to the Guji, West Arsi and Bale zone administration of Oromia regional state for their limitless cooperation. We are indebted to the Bale, West Arsi and Sidama zone water resources office for providing relevant data which were crucial for our hydrogeological mapping. The team is grateful to the management of the Geological Survey of Ethiopia, particularly to Director General (GSE) Mr. Masresha G/Selassie and Mr. Yohannes Belete, Head of Groundwater Resources Assessment Department (GSE) and Mr. Muhudin Abdela, Senior Hydrogeologist and Project Coordinator. Special thanks go to the NGOs and private water drilling and consultant companies for providing data from private databases. Finally, the team acknowledges the untiring support of the people in the Dodola area who assisted the team by all means possible and facilitated the data collection and those who helped us in different ways.

Acknowledgment

ContentsAcknowledgment .........................................................................................................................................................................3Extended Summary................................................................................................................................................................... 13Introduction ................................................................................................................................................................................ 171. Basic Characteristics of the Area .................................................................................................................................191.1 Location and Accessibility .......................................................................................................................................................191.2 Population, Settlements and Health Status ........................................................................................................................201.3 Land Use ......................................................................................................................................................................................272. Selected Physical and Geographical Settings ........................................................................................................... 292.1 Geomorphology .........................................................................................................................................................................302.2 Soil and Vegetation Cover .......................................................................................................................................................302.3 Climatic Characteristics ...........................................................................................................................................................342.3.1 Climatic Zones and Measurements...................................................................................................................................342.3.2 Precipitation ............................................................................................................................................................................402.4 Hydrography and Hydrology of the Area ............................................................................................................................482.4.1 Surface Water Network Development ..............................................................................................................................482.4.2 Surface Water Regime .........................................................................................................................................................502.4.3 Baseflow ..................................................................................................................................................................................632.5 Water balance ............................................................................................................................................................................722.6 Drought and Climate Changes...............................................................................................................................................743. Geological Settings ...........................................................................................................................................................773.1 Previous Work ............................................................................................................................................................................773.2 Stratigraphy ................................................................................................................................................................................783.3 Lithology ......................................................................................................................................................................................793.3.1 Crystalline Basement Rocks and Associated Intrusive Rocks ....................................................................................793.3.2 Mesozonic Sedimentary Formations ................................................................................................................................803.3.3 Tertiary Volcanic Rocks ........................................................................................................................................................813.3.4 Quaternary Sediments and Basalts ..................................................................................................................................813.4 Structure ......................................................................................................................................................................................823.5 Geological History .....................................................................................................................................................................824. Hydrogeology .................................................................................................................................................................... 854.1 Water Point Inventory ...............................................................................................................................................................854.2 Hydrogeological Classification/Characterization ..............................................................................................................874.3 Elements of the Hydrogeological System of the Area (Aquifers) ..................................................................................884.3.1 Local and Moderately Productive Porous Aquifers .......................................................................................................894.3.2 Extensive and Moderately Productive Fissured and Karstic Aquifers ......................................................................904.3.3 Extensive and Low Productive Fissured Aquifers ..........................................................................................................954.3.4 Extensive Formation Consisting of a Minor Fissured Aquifer with Local and Limited Groundwater Resources – Aquitard ......................................................................................................................................................................964.4 Hydrogeological Conceptual Model .....................................................................................................................................984.5 Annual Recharge in the Area .............................................................................................................................................. 1005. Hydrogeochemistry ....................................................................................................................................................... 1035.1 Sampling and Analysis .......................................................................................................................................................... 1035.2 Classification of Natural Waters ......................................................................................................................................... 1015.2.1 Precipitation ......................................................................................................................................................................... 107

5.2.2 Surface Water...................................................................................................................................................................... 1075.2.3 Groundwater in Tertiary Volcanic Rocks ....................................................................................................................... 1075.2.4 Groundwater in Mesozoic and Quaternary Sediments ............................................................................................. 1085.2.5 Groundwater in Basement Rock ..................................................................................................................................... 1085.3 Water Quality .......................................................................................................................................................................... 1095.3.1 Domestic Use ....................................................................................................................................................................... 1095.3.2 Irrigation Use ........................................................................................................................................................................ 1115.3.3 Industrial Use ........................................................................................................................................................................ 1115.4 Mineral and Thermal Water ..................................................................................................................................................1136. Natural Resources of the Area .....................................................................................................................................1156.1 Economic Geology ..................................................................................................................................................................1156.2 Water Resources .....................................................................................................................................................................1166.2.1 Surface Water Resources Development ........................................................................................................................1176.2.2 Groundwater Resources Development ..........................................................................................................................1186.3 Human and Land Use Resources and Development..................................................................................................... 1266.4 Wind and Solar Energy Development ............................................................................................................................... 1266.5 Environmental Problems and their Control / Management ........................................................................................ 1266.6 Touristic Potential of the Area ............................................................................................................................................. 128Conclusions ......................................................................................................................................................................................131References ....................................................................................................................................................................................... 133Annex No. 1 .................................................................................................................................................................................... 135Annex No. 2 .................................................................................................................................................................................... 149Annex No. 3 .................................................................................................................................................................................... 153

List of Figures

Fig. 1.1 Location map ............................................................................................................................................................ 19Fig. 1.2 The main roads and settlements..........................................................................................................................20Fig. 1.3 Administrative zones ..............................................................................................................................................22Fig. 1.4 Malaria risk in Ethiopia ...........................................................................................................................................23Fig. 1.5 Land use ....................................................................................................................................................................27Fig. 2.1 Generalized physiographic units ..........................................................................................................................29Fig. 2.2 Distribution of soil types ........................................................................................................................................ 31Fig. 2.3 Afro–Alpine vegetation at Sanetti Plateau (Bale National Park) ...................................................................33Fig. 2.4 The Herele forest near Delo Mena town ............................................................................................................33Fig. 2.5 Climatic zones ..........................................................................................................................................................36Fig. 2.6 Temperature [°C] at Dodola meteo-station ........................................................................................................38Fig. 2.7 Mean monthly sunshine [hour] and radiation [MJ/km2/d] at Dodola meteo-station .............................38Fig. 2.8 Mean monthly relative humidity [%] and wind speed [km/d] at Dodola meteo-station .........................39Fig. 2.9 ETo data calculated for Dodola meteo-station ..................................................................................................39Fig. 2.10 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996) ........................... 41Fig. 2.11 The Goro meteo-station precipitation pattern...................................................................................................43Fig. 2.12 Long-term fluctuation of precipitation in Goro meteo-station .......................................................................43Fig. 2.13 The Rira meteo-station precipitation pattern ....................................................................................................44Fig. 2.14 Long-term fluctuation of precipitation in Rira meteo-station ........................................................................44Fig. 2.15 The Delo Mena meteo-station precipitation pattern .......................................................................................45Fig. 2.16 Long-term fluctuation of precipitation in Delo Mena meteo-station ...........................................................45Fig. 2.17 The Melka Amana meteo-station precipitation pattern ..................................................................................46Fig. 2.18 Long-term fluctuation of precipitation in Melka Amana meteo-station ......................................................46Fig. 2.19 The Dodola meteo-station precipitation pattern .............................................................................................. 47Fig. 2.20 Long-term fluctuation of precipitation in Dodola meteo-station .................................................................. 47Fig. 2.21 The principal river basins in the area ..................................................................................................................48Fig. 2.22 Welmel River – waterfall ........................................................................................................................................49Fig. 2.23 Flow diagram of the Shawe River from the Mes Project river gauge ..........................................................53Fig. 2.24 Annual variability of the mean annual flow of Shawe River at the Mes project river gauge .................53Fig. 2.25 Flow diagram of the Yadot River from the Delo Mena river gauge ..............................................................54Fig. 2.26 Annual variability of the mean annual flow of Yadot River at the Delo Mena river gauge .....................54Fig. 2.27 Flow diagram of the Halgol River at Delo Mena river gauge.........................................................................55Fig. 2.28 The annual variability of the mean annual flow of Halgol River at Delo Mena river gauge ...................55Fig. 2.29 Flow diagram of the Welmel River at Melka Amana river gauge .................................................................56Fig. 2.30 The annual variability of the mean annual flow of Welmen River at Melka Amana river gauge ..........56Fig. 2.31 Flow diagram of the Deyiu River at Deyiu Harewa river gauge ....................................................................57Fig. 2.32 The annual variability of the mean annual flow of Deyiu River at Deyiu Harewa river gauge ...............57Fig. 2.33 Flow diagram of the Tegona River at Goba river gauge .................................................................................58Fig. 2.34 The annual variability of the mean annual flow of Tegona River at Goba river gauge ............................58Fig. 2.35 Flow diagram of the Ukuma River at Dodola river gauge ..............................................................................59Fig. 2.36 The annual variability of the mean annual flow of Ukuma River at Dodola river gauge .........................59

Fig. 2.37 Flow diagram of the Herero River at Herero river gauge ...............................................................................60Fig. 2.38 The annual variability of the mean annual flow of Herero River at Herero river gauge ..........................60Fig. 2.39 Flow diagram of the Maribo River at Adaba river gauge ............................................................................... 61Fig. 2.40 The annual variability of the mean annual flow of Maribo River at Adaba river gauge .......................... 61Fig. 2.41 Flow diagram of the Leliso River at Adaba river gauge ..................................................................................62Fig. 2.42 The annual variability of the mean annual flow of Leliso River at Adaba river gauge .............................63Fig. 2.43 Method of Kille baseflow assessment ................................................................................................................64Fig. 2.44 Kille baseflow separation – gauging stations within Genale-Dawa basin ..................................................65Fig. 2.45 Kille baseflow separation – gauging stations within Wabe Shebelle-Dawa basin ...................................66Fig. 2.46 Method of baseflow separation ...........................................................................................................................67Fig. 2.47 Hydrograph baseflow separation – gauging stations within Genale-Dawa basin ....................................67Fig. 2.48 Hydrograph baseflow separation – gauging stations within Wabe Shebelle-Dawa basin .....................69Fig. 2.49 The most drought prone areas of Ethiopia (source: RRC, 1985) ..................................................................75Fig. 3.1 Granitic rock near Hora Kore .................................................................................................................................79Fig. 3.2 Bale basalt at the Sanetti Plateau ........................................................................................................................ 81Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after Gani et al., 2008) ........................................................................................................................83Fig. 4.1 Marsh developed on elluvial deposits in the Meleyu area .............................................................................89Fig. 4.2 Extent and location of moderately productive fissured and karst aquifers ................................................90Fig. 4.3 The Harawa spring Csp-48 .................................................................................................................................... 91Fig. 4.4 Frequency of yield of springs and wells in fissured aquifer developed in sedimentary rocks ...............92Fig. 4.5 Forest nearby Alabada spring ...............................................................................................................................93Fig. 4.6 Frequency of yield of springs and wells in fissured aquifer developed in volcanic rocks .......................94Fig. 4.7 Extent and location of the low productive fissured aquifer developed in basement rocks ....................95Fig. 4.8 Frequency of yield of springs and wells in fissured aquifers developed in basement rocks .................96Fig. 4.9 Extent and location of aquitard ............................................................................................................................ 97Fig. 4.10 Intermittent springs at the contact of Alkali trachyte flows (Tpt) and tuff (Tbt) ....................................... 97Fig. 4.11 Conceptual hydrogeological model of southeastern highlands and lowlands .........................................98Fig. 4.12 Fissures in the roof of the Sof Omar cave ..........................................................................................................99Fig. 5.1 Level of cation-anion balance ............................................................................................................................ 104Fig. 5.2 Piper diagram for classification of natural waters ........................................................................................ 106Fig. 5.3 Content of nitrate in analysis of water in the study area .............................................................................. 111Fig. 5.4 Multiple eyes of Hora Kore hot spring ..............................................................................................................114Fig. 6.1 Mount Tullu Deemtu – Bale Mountains National Park ..................................................................................129

List of TablesList of authors and professionals participating in the project ............................................................................................ 18Tab. 1.1 Population in the study area ................................................................................................................................. 21Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006) .........................................................................................................24Tab. 1.3 Rural water facilities by Zones ..............................................................................................................................24Tab. 1.4 Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region ......................25Tab. 2.1 Ethiopian climate classification ............................................................................................................................35Tab. 2.2 Climatic stations in the Dodola area ...................................................................................................................37Tab. 2.3 Mean monthly temperature at Dodola meteo-station.....................................................................................37Tab. 2.4 Characterization of the precipitation pattern in Ethiopia ...............................................................................40Tab. 2.5 Monthly long-term average precipitation at selected meteo-stations of the Dodola sheet [mm] ........42Tab. 2.6 Long-term monthly precipitation at Goro meteo-sation [mm] (fully recorded years only) .....................42Tab. 2.7 Data on the river gauging stations ......................................................................................................................50Tab. 2.8 Runoff ......................................................................................................................................................................... 51Tab. 2.9 Baseflow data for the Dodola area ...................................................................................................................... 71Tab. 2.10 Water balance input data ......................................................................................................................................72Tab. 2.11 Water balance of Shaya basin ..............................................................................................................................73Tab. 2.12 Comparison of water losses in water balance with estimated deep base flow ........................................73Tab. 3.1 Litho stratigraphy of the mapped area ...............................................................................................................78Tab. 4.1 Aquifer classification based on well yield for Genale-Dawa basin ...............................................................86Tab. 4.2 Aquifer classification by Lahmeyer (2005) .......................................................................................................86Tab. 4.3 Summary of field inventory ...................................................................................................................................87Tab. 4.4 Basic hydraulic characteristics of wells in the Dodola sheet ........................................................................89Tab. 4.5 Estimated minimum recharge to groundwater from stations of the Genale-Dawa basin ....................101Tab. 4.6 Rainfall infiltration factor for Wabe Shebelle basin by WWDST (2003) ..................................................101Tab. 5.1 Level of balance .................................................................................................................................................... 104Tab. 5.2 Summary of hydrochemical types .................................................................................................................... 105Tab. 5.3 Groundwater descriptive statistics of TDS, EC and Cl values .....................................................................107Tab. 5.4 Chemical composition of rain water .................................................................................................................107Tab. 5.5 Groundwater chemistry compared to drinking water standards and guidelines ...................................110Tab. 5.6 Suitability of water for irrigation ........................................................................................................................ 111Tab. 5.7 Suitability of water for use in industry .............................................................................................................. 112Tab. 5.8 Concentration limits for incrustation ................................................................................................................113Tab. 5.9 Concentration limits for corrosion .....................................................................................................................113Tab. 5.10 Chemical composition of the Hora Kore hot spring ......................................................................................114Tab. 6.1 Aquifers of the area ...............................................................................................................................................116Tab. 6.2 Assessment of water resources of the Dodola area ...................................................................................... 117Tab. 6.3 Sites proposed geophysical investigation and drilling ..................................................................................119Tab. 6.4 Results of vertical electrical sounding - Bururi - BVES-1 ...............................................................................120Tab. 6.5 Results of vertical electrical sounding - Bururi - BVES-2 ...............................................................................120Tab. 6.6 Well log of Yedi abandoned well ........................................................................................................................121Tab. 6.7 Results of vertical electrical sounding - Yedi - YVES-1 ..................................................................................121

Tab. 6.8 Results of vertical electrical sounding - Mekana Gobelle - MGVES-1 ........................................................122Tab. 6.9 Results of vertical electrical sounding - Mekana Gobelle - MGVES-2 ........................................................122Tab. 6.10 Results of vertical electrical sounding - Gerbigallo - GGVES-1 ....................................................................123Tab. 6.11 Results of vertical electrical sounding - Gerbigallo - GGVES-2 ....................................................................123Tab. 6.12 Results of vertical electrical sounding - Gerbigallo - GGVES-3 ....................................................................123Tab. 6.13 Results of vertical electrical sounding - Chefa - OVES-1 ...............................................................................124

Under Separate Cover (see attached CD)

Annexes:Annex 1 Field Inventory DataAnnex 2 Water ChemistryAnnex 3 Well Logs

Maps:Hydrogeological Map of Dodola NB 37–7 - full size and A3 sizeHydrochemical Map of Dodola NB 37–7 - full size and A3 size

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Extended SummaryThe Dodola area is located in Eastern Ethiopia on the Dodola map sheet (NB 37-7) at the scale of

1:250,000, covering an area of 18,356 km2. The area is a part of the Oromia and SNNP regional states and is inhabited by 0.8 million people and only small part of the area is cultivated.

The southern and eastern part of the Dodola area is below 1,500 m above sea level (a.s.l.) and is represented by the Sirima and Somaya plains. This area rises to the northwest to the Harenna erosional escarpment and the Warka Aramfama hills with an altitude of 1,500–2,300 m a.s.l. and to the Sanetti Plateau which is on top of the Bale Mountains with an altitude of 3,477 m a.s.l. (Mt. Tullu Deemtu). The area is a part of the Genale-Dawa river basin, except for the northwestern corner which is a part of the Wabe Shebelle basin. The rainy season is bimodal from March to May and from September to November; the annual mean rainfall for the Dodola area was adopted as being 850 mm/year. There are several permanent rivers (Dumal, Genale Shewa, Yadot, Tegona and Welmel) and intermittent rivers particularly in the southeastern part of the area. Specific surface runoff was adopted as being a value of 10.5 l/s.km2. The adopted value of specific baseflow is 2.8 l/s.km2 representing 178 mm/year and 21 % precipitation. The Dodola area faced severe Kiremt drought in 1969, 1970 and 1987. The years when drought was most serious in the Dodola area were 1969, 1973 and 1977. The area shows high Kiremt drought probability (third highest in Ethiopia).

The aquifer system has been defined based on the hydrogeological characteristics of lithological units described by the geological maps and data from the field inventory and desk study. The characterization of the area shows the following aquifer/aquitard systems:

1. Extensive and moderately productive porous aquifers with spring and well yield Q = 0.51–5 l/s developed in Quaternary unconsolidated deposits.

2. Extensive and moderately productive fissured and karst aquifers with spring and well yield Q = 0.51–5 l/s developed in Hamanlei limestone and Adigrad sandstone in the lower reaches of rivers, calcareous sandstone of the plateau area and Tertiary volcanic rocks (not forming plugs).

3. Extensive and low productive fissured aquifers with spring and well yield Q = 0.051–0.5 l/s developed in basement rocks.

4. Minor formation consisting of minor fissured aquifers with local and limited groundwater resources – Aquitard with spring and well yield Q < 0.05 l/s developed in plugs forming trachyte in the plateau area.

The water balance, hydrograph separation and Kille methods show that the infiltration coefficient (recharge) is about 21 % of the total precipitation. Part of the groundwater infiltrates

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directly from precipitation and groundwater flows laterally to local and or regional drainage base levels represented by rivers in deep valleys and an area at the foot of the Harenna escarpment where it emerges as springs or flows vertically recharging deeper aquifers. This type of front recharge is dominant in the northern highlands, but aquifers in lowlands with low precipitation depth and limited surplus of water for infiltration have limited direct recharge. Recharge from areas with higher precipitation in the north to aquifers in limestone at southeast is inferred. The outcrops of basement rocks are recharged directly by precipitation which is adequate enough to form good groundwater resources. The intermittent and ephemeral rivers and flood episodes of perennial rivers in the lowlands contribute significantly to the recharge of aquifers along river banks. Bank recharge provides a relatively large amount of good quality groundwater with low TDS for development in the alluvial aquifers of the lowlands.

Chemistry of groundwater in the Dodola area is uniform reflecting fast circulation in volcanic, sedimentary and basement rocks. The dominant hydrochemical type of groundwater is bicarbonate type. The basic Ca–HCO

3 type occurs in the centre of the sheet with margins of transitional Ca–

HCO3 type. In general, TDS increases from the northwest with higher precipitation to the more

arid southeastern part of the sheet. Groundwater TDS varies from 35 mg/l to 3,256 mg/l and only 4 % of samples is not convenient for drinking. The use of groundwater can be limited by pollution particularly of human and animal origin and some samples show increasing concentrations of nitrates in addition to high TDS.

The total amount of water resources of the area has been assessed to be 6,087 Mm3/year. The use of surface water for irrigation is the most important development factor and 80 % of available surface water resources will be used for irrigation. This portion represents 4,870 Mm3/ year. Considering that about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 487,000 ha (4,870 km2) which is about 26 % of the Dodola area.

The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 1,591 Mm3/year. Considering the total number of people living within the area is 0.8 million the need for water supply can be nearly 5.7 Mm3/year (20 l/c.d). The figure shows that recent demand represents less than 2 % of renewable groundwater resources of active aquifers, meaning that aquifers can provide adequate drinking water even in the future considering the trends in population growth and can be also used for supply of areas adjacent to the Dodola area.

Most of the people within the area live in small towns and villages. There is a good practice to develop big springs which form regional drainage of aquifers developed in volcanic rocks for drinking water supply of towns. Additional to development of big springs the water supply based on drilled wells represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells as follows:

a) In aquifers developed in Hamanlei limestone with a depth of about 150–250 m. Each of the wells can yield about 2 l/s (recent average). Each of these wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,640 inhabitants considering a daily consumption of 20 l/c.d.

b) In aquifers developed in volcanic rocks with a depth of about 50–200 m. Each of the wells can yield about 2 l/s (recent average). Each of these wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,640 inhabitants.

c) In aquifers developed in basement rocks with a depth of about 30–70 m. Each of the wells can yield about 1 l/s (recent average). Each of these wells can provide 86,400 l/d and can supply a small town or group of villages with about 4,320 inhabitants.

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Drilling should be done in sites where there is not an adequate water supply and or the quality of water at the existing water source is not safe for drinking purpose and where groundwater resources are abundant but not effectively utilized. Twelve drilling sites were selected and recommended for investigation by geophysical measurements (VES). The sites are shown on the hydrogeological map.

The minimum required distance of water supply wells and potential pollution sources should be maintained during the development of groundwater resources for towns and villages. In addition to priority in development of groundwater for safe drinking water supply it should be possible to select the most fertile soil to be developed by small scale irrigation and livestock watering based on groundwater to increase the stability of food supply in prolonged periods of drought in the Dodola area.

Soil erosion and protection is one of the limiting factors of sustainable development of agriculture within the area and should be addressed in all development projects, but data about soil erosion are scarce in the area.

The work which is summarized in the presented explanatory notes shows the excellent water, agricultural, industrial, human as well touristic potential of the Dodola area.

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BackgroundEthiopia is a country affected by environmental degradation including recurrent droughts which

lead to food insecurity, and drought stricken areas have been constantly degraded over the past several decades by improper utilization of natural resources. The eastern dissected highlands and lowlands of Ethiopia are no exception to the above mentioned fact. The rugged topography and high gradient coupled with increasing population and intense deforestation aggravates the problem. It is therefore important to compile a map of water resources to be able to propose and implement appropriate protection measures during development efforts. It is also vital in identifying and tackling existing problems and proposing their solution. In this context the project for hydrogeological investigation of the “Groundwater Resources Assessment of the Southeastern Highlands and Associated Lowlands of Ethiopia” was performed in the Dodola sheet in 2010 by the Geological Survey of Ethiopia. The publication of the project results was conducted in the framework of bilateral cooperation between the Czech and Ethiopian governments, where the participation of the Czech experts was financed by the Czech Development Agency in the framework of the Czech Republic Development Assistance Program and the project entitled “Capacity Building in the Field of Hydrogeology and Engineering Geology”. Participation of the Ethiopian professionals was financed by the Ethiopian government. This report deals with the assessment of hydrogeological and hydrochemical characteristics and other environmental parameters acquired during the desk and field work and discussion between stakeholders and the joint Czech-Ethiopian team of professionals.

Objective and ScopeWater is a finite resource and must be managed in a sustainable way. For sustainable development,

water resource investigation can play an important role in the efficient and optimal utilization of the water resources available to a country. The main objectives of the study for hydrogeological mapping were to identify water-bearing lithological units and their basic characteristics, to indentify recharge and discharge areas as well as groundwater flow direction, to categorize water quality within water bearing formations, to indicate the suitability of groundwater for different purposes, and to compile hydrogeological and hydrochemical maps with accompanying explanatory notes of the study area based on the information and analysis made. The work covers the interpretation of aerial photos and satellite images, meteorological and hydrological data analysis, quantification of inventoried water points, collection of representative water samples and data for hydrochemical studies, and evaluation of water resource management of the area. The hydrogeological investigation of the Dodola map sheet is part of the project entitled “Groundwater Resources Assessment of the Southeastern Lowlands and Associated Highlands” that was conducted between 2009 and 2011 to alleviate water shortage in the area.

IntroductionIntroduction

The desk and field work was carried out by a group of Ethiopian hydrogeologists. Final assessment and publication of the map was carried out by a joint Czech-Ethiopian team of professionals. The names of participating experts are shown in the following list.

List of professionals participating in the project

Name Institution Participation field

Jiri Sima AQUATEST a.s. Editor

Astatike Kiflu Geological Survey of Ethiopia Chief compiler

Muhedin Abdela Geological Survey of Ethiopia Project coordinator

Ondrej Nol AQUATEST a.s. Hydrogeological expert

Antonin Orgon AQUATEST a.s. GIS expert

Romana Suranova AQUATEST a.s. Printing expert

Craig Hampson AQUATEST a.s. Language revision

Mulugeta H/Mariam Geological Survey of Ethiopia Data acquisition and evaluation

Dana Capova Czech Geological SurveyAEGOS project expert – coordination, tech-nical architecture, interoperability

Vladimir Ambrozek Czech Geological SurveyAEGOS project expert data conversion and processing

Petr Coupek Czech Geological SurveyAEGOS project expert – data on-line provi-sion

Shiferaw Ayele Geological Survey of Ethiopia AEGOS project country representative

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1.1 Location and AccessibilityThe study area is located in Eastern Ethiopia, in part of the Eastern Ethiopian highlands (plateau)

and the adjacent lowlands of the Ogaden plain. Geographically the study area is bounded from north to south by latitudes 6°00‘N and 7°00‘ N, and from west to east by longitudes 39°00‘ E and 40°30‘ E. The area covers approximately 18,337 km2 of the topographic map sheet of Dodola –Goba (NB 37-7) at a scale of 1:250,000. The location of the map is illustrated in Fig. 1.1. The sheet is bounded by the Asela sheet to the north, the Dila sheet to the west, the Magalo sheet to the east and the Negele sheet to the south.

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Basic Characteristics of the Area

20 Basic Characteristics of the Area

The study area is about 430 km from Addis Ababa and can be reached via either the Addis Ababa – Asela or Addis Ababa – Shashemene roads. Both the Addis Ababa – Asela (175 km) and Addis Ababa – Shashamane (250 km) roads are asphalted, the rest of the way to Goba being under construction. The road joining Goba with Negele, which crosses the Sanetti Plateau and passes through the central parts of the map sheet, makes the area fairly accessible. The dry weather gravel road running west out of the town of Delo Mena and to the northeast of the study area enables important sites to be reached. The dry weather road that starts at Dinsho village outside the map runs 40 km southwards to the center of Bale Mountain National Park (BMNP) while the gravel road from Bale-Robe to Goro running just to the north of the Dodola map sheet also enables Rayitu village to be reached. Throughout the entire project area there is a good network of footpaths and traditional tracks joining different villages except in the central western and northern parts where impenetrable forests and rugged topography are prevalent. The main accessible roads and settlements are shown in Fig. 1.2.

1.2 Population, Settlements and Health StatusDodola sheet is mainly located within the Oromia Regional state, with the most western part

being located in the Southern Nations Nationalities and Peoples (SNNP) Regional state (in the Bale and West Arsi, Guji zonal administrative region of the Oromia and Sidama zone of SNNP). The population density varies from place to place in the highlands and lowlands. The population density as well as the number of settlements in the eastern lowlands is not high due to the lack of sustainable water resources, harsh climate conditions, and the way of living (most people of the

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21Basic Characteristics of the Area

area are pastoralists). The density of population is higher in the highlands in the western part of the study area because of the favorable climatic and living conditions; especially where there is better access to sufficient farmland and a sustainable water supply for the community, as well as the proximity of the villages to roads and markets, etc. The highest part of the Sanetti Plateau within the Bale National Park is not inhabited. Population density varies from place to place in the urban areas and rural villages of the lowlands and highlands with 50– 150 inhabitants per km2 in the northwestern highland part of the area and 7–15 inhabitants per km2 in the southeastern lowland part of the area with an average of 88 inhabitants per km2.

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Tab. 1.1 Population in the study area

Region Zone Wereda

Wereda area in mapped area Total

population

Assessed populationin mapped area[km2] [%]

Oromia Bale Sinana 81 8 119,208 9,537

Oromia Bale Goba 1,380 80 40,737 32,590

Oromia Bale Berbere 1,383 100 84,503 84,503

Oromia Bale Gura Demole 1,683 39 28,651 11,174

Oromia Bale Delo Mena 4,360 84 80,593 67,698

Oromia Bale Dinsho 61 10 36,151 3,615

Oromia Bale Harenna Buluk 1,798 100 76,590 76,590

Oromia Bale Meda Welabu 1,794 57 94,826 54,051

Oromia Bale Goro 256 19 75,147 14,278

Oromia Guji Adola 235 16 113,735 18,198

Oromia Guji Wadera 38 4 45,348 1,814

Oromia Guji Girja 856 97 49,176 47,701

Oromia West Arsi Adaba 1,094 48 126,559 60,748

Oromia West Arsi Kofele 0.01 0.001 163,411 16

Oromia West Arsi Dodola 998 69 159,774 110,244

Oromia West Arsi Kokosa 0.44 0.10 141,283 141

Oromia West Arsi Nensebo 1,640 100 108,462 108,462

SNNP Sidama Hula 116 16 122,177 19,548

SNNP Sidama Bursa 560 59 98,340 58,021

Total 18,337 778,928

Source: Population by Zone, Central Statistics Authority Statistical Abstract (2007)


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The study area within the Bale, West Arsi and Sidama zones is inhabited mainly by the Arsi and Guji tribes of the Oromo. The central western and southeastern parts of the map area are inhabited by Sidama and Somali (Gura) peoples, respectively. Amahara settlers in and around the towns are also present. The Oromos cultivate mainly corn, teff and in some places barley, sorghum and wheat to subsidize their subsistence. The Somalis raise camel, goats and practice farming in some places.

To calculate the total number of people living within the mapped area the number of people living in Weredas was assessed from the total Wereda population and by the percentage of the area within the map sheets. Tab. 1.1 shows the population in the different Weredas within the mapped area.

Based on the data provided by the Central Statistics Authority, the total population is assumed to be 778,928; however, this figure could in reality be several thousand higher. The urban population comprises only 20 % and the remaining 80 % of the population live in rural areas.

Considering the trends in population growth, access to water will become worse by 2015 in urban areas and 2025 in rural areas, respectively. People in the area could face a water scarcity i.e. less than 1,000 m3/year, and/or even water stress i.e. availability of water is less than 500 m3/ year (Tesfay Tafese, 2001).

The life expectancy at birth is 49 years for males and 51 years for females (WHO, 2006). As in most developing countries, Ethiopia‘s main health problems are communicable diseases caused by poor sanitation and malnutrition. Mortality and morbidity data are based primarily on

Fig. 1.3 Administrative zones


health facility records which show that the leading causes of hospital deaths are dysentery and gastroenteritis, tuberculosis, pneumonia, malnutrition and anemia, and liver diseases including hepatitis, tetanus, and malaria. The situation is complicated by the fact that Ethiopia´s population mainly lives in rural areas (84 %) where access to healthcare is more complicated than in urban areas.

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Fig. 1.4 Malaria risk in Ethiopia


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The country faces chronic problems with malaria (Fig. 1.4) which is endemic over 70 % of the country, and was once a scourge in areas below 1,500 m a.s.l. Fortunately, practically the entire area of the Dodola sheet has a minimal malaria risk. The threat of malaria had declined considerably as a result of government efforts supported by the WHO and AID, but sporadic seasonal outbreaks are common. The UNICEF estimated that the number of malaria cases per year is about 9 million and the number of extra cases in an epidemic year is about 6 million. The occurrence of outbreaks is largely a result of heavy rain, unusually high temperatures, and the settling of peasants in new lowland locations. An example of the different diseases in Ethiopia is shown in Tab. 1.2.

Type of disease Total inhabitants [%] Children under 5

Respiratory 12 22

HIV/AIDS 12 4

Prenatal / neonatal 8 30

Diarrheal 6 17

Tuberculosis 4

Measles 4 4

Cardio-vascular 3

Ischemic heart diseases 3

Malaria / injuries 3 2

Syphilis / others 2 14

Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006)

Tab. 1.3 Rural water facilities by Zones (Part 1)

Zone / Wereda Number of facilities Number of inhabitants per facility

Bale / Sinana 23 bono, 9 tankers 3,725

Bale / Goba 2 springs 20,369

Bale / Berbere 3 bono, 2 springs, 1 tanker 14,084

Bale / Gura Demole 1 bono, 1 pond 28,651

Bale / Delo Mena 2 bono 40,297

Bale / Dinsho 3 bono 12,050

Bale / Harenna Buluk 2 bono, 1 tanker 25,530

Guji / Girja none 49,176

Bale / Meda Welabu 5 bono 18,965

Bale / Goro 27 bono, 6 tankers, 7 springs, 5 ponds 1,670


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Zone / Wereda Number of facilities Number of inhabitants per facility

Guji / Adola 16 bono 7,108

Guji / Wadera 6 bono, 1 pond 6,478

Guji / Girja none 49,176

West Arsi / Adaba 8 bono, 1 water pump, 4 tankers 9,735

West Arsi / Kofele none 163,411

West Arsi / Dodola 9 bono, 6 tankers 10,652

West Arsi / Kokosa 3 bono 47,094

West Arsi / Nensebo 1 water pump 108,462

Sidama / Hula none 145,323

Sidama / Bursa 10 bono 9,834

Tab. 1.3 Rural water facilities by Zones (Part 2)

Tab. 1.4 Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region

Source: Oromia Regional Health Bureau

Rank Diagnosis No. of all cases % of all cases

1 Acute upper respiratory tract infection 103,154 5.93

2 Other helminthes 106,620 4.81

3 Other unspecified malaria 103,154 4.65

4 Gastritis and duodenitis 102,252 4.61

5 Homicide and injury purposelyinflicted by another person (not in war)

100,161 4.52

6 All other diseases of gento–urinal system 82,493 3.72

7 Bronchopneumonia 78,189 3.53

8 Infection of skin and subcutaneous tissue 72,881 3.29

9 Muscular rheumatism and rheumatism unspecified 72,868 3.29

10 Pyrexia of unknown origin 70,226 3.17

Total of leading diseases 920,411 41.50

Total of other diseases 1,297,453 58.50

Total of all diseases 2,217,864 100.00


Access to safe drinking water is limited and some statistics suggest that only 15 % of rural inhabitants have access to safe drinking water. The WHO (2006) statistics show that 31 % of the rural population has sustainable access to improved drinking water sources (96 % of the urban population). This low number is alarming because 70 % of contagious diseases are caused by contaminated water. This is a serious problem for Ethiopia in the effort to establish a strong agricultural community that will be able to safeguard the supply of food for the whole country. One of the priorities of government policy is therefore to provide safe drinking water to rural communities.

The supply of safe water is not equal in all of the Zones of the region. The total number of facilities and the number of inhabitants for a single facility are shown in Tab. 1.3. Preliminarily results of the population and housing census of 2007 show that a relatively large number of ponds serving for water supply can provide an adequate volume of water, but do not follow the requirements for safe water supply to inhabitants.

The leading causes of hospital and health center morbidity in 2008/2009 in the Oromia are shown in Tab. 1.4.

Conclusions of a review made by the Ethiopian Health Sector Development Program (HSDP, 2008) show that despite the significant rise in access to water and improved sanitation, there is no data on rates of usage of these services. Ethiopia still suffers from a heavy disease burden that is directly related to poor hygiene practices and sanitation services. Each year, the average Ethiopian child has five to twelve diarrhea episodes and diarrheal illnesses kill between 50,000 to 112,000 children each year. Women and girls are most affected by inadequate sanitation services as they are forced to spend more time fetching water and caring for the sick than participating in income-generating activities or attending school.

During the last few years, there has been an increased level of political commitment to hygiene and environmental health services in Ethiopia leading to the Ministry of Health defining a Hygiene and Environmental Health Program (www.moh.gov.et). The program is based on key policies such as the National Sanitation Strategy and Protocol and the Millennium Sanitation Movement has established a framework that serves to motivate and align relevant actors to speed up sanitation coverage and hygiene behavioral change. In addition, three key ministries – Health, Water Resources and Education – have joined to launch the National WASH program, which provides a strategic framework for achieving a national vision for universal access to hygiene sanitation.

The Ministry of Health has defined the following objectives of the program: • Increase sanitation measures including latrine coverage and ensure facilities are properly

handled, sustained and utilized.• Promote communal solid waste disposal sites, including improvement of medical and other

waste management systems in public and private health institutions.• Increase drinking water quality monitoring; and monitor food safety and food processing

industries.

Health Extension Workers (HWEs) play a significant role in carrying out the key activities of the program throughout communities. HEWs promote personal and environmental hygiene and provide support to the community; increase community awareness and involvement in safe water supply and prevention of water contamination; promote behavioral change to improve food safety and control vector born diseases; build a “Healthy House Model” and work with the relevant institutions to ensure irrigation development projects and water conservation schemes.

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Improving safe water supply to people living in the mapped area basin contributes to an improvement in their health which is one of the fundamental problems for the creation of strong pastoral and farm communities capable of full time engagement in agricultural activity.

1.3 Land UsePoor land use practices, improper management systems and lack of appropriate soil conservation

measures have played a major role in causing land degradation problems in the country. Because of the rugged terrain, the rates of soil erosion and land degradation in Ethiopia are high. Setegn (2010) mentions the soil depth of more than 34 % of the Ethiopian territory is already less than 35 cm, indicating that Ethiopia loses a large volume of fertile soil every year and the degradation of land through soil erosion is increasing at a high rate. The highlands are now so seriously eroded that they will no longer be economically productive in the foreseeable future.

The land and water resources are in danger due to the rapid growth of the population, deforestation and overgrazing, soil erosion, sediment deposition, storage capacity reduction, drainage and water

Fig. 1.5 Land use

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logging, flooding, and pollutant transport. In recent years, there has been an increased concern over climate change caused by increasing concentrations of CO

2 and other trace gases in the

atmosphere. A major effect of climate change is alterations in the hydrologic cycles and changes in water availability. Increased evaporation combined with changes in precipitation characteristics has the potential to affect runoff, frequency and intensity of floods and droughts, soil moisture, and water supplies for irrigation and generation of hydroelectric power.

Only a small percentage of the Dodola area is classified as intensively and moderately cultivated land (Fig. 1.5). A large part of the area at the foot of the Sanetti Plateau is covered with high forest where coffee is cultivated. The southeastern part in lowland is open grass to woodland and is used mainly for pasture; crop cultivation is not common in this area.

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29

The entire study area is located on the eastern shoulder of the southeastern Ethiopian plateau with a general slope to the southeast. The area is composed of highlands of the Sanetti Plateau with Mt. Tullu Deemtu (Mt. Batu) (4,377 m a.s.l.), hill domes with summits greater than 2,000 m a.s.l. and plains with an elevation of 1,000 to 1,200 m a.s.l. (Fig. 2.1). The highland (Sanetti Plateau) covers the northern part, the hill domes the western part and the plains are located in the southeastern part of the area together with deep valleys of the Dumal and Welmel rivers. The bottoms of these river valleys with an altitude below 600 m a.s.l. are located in the southeastern corner of the map sheet.

The most distinct physiographic units are as follows:• Highlands of the Sanetti Plateau at the north.• Erosional escarpment of Harenna in the center.

2. Selected Physical and Geographical Settings2.

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Selected Physical and Geographical Settings

Fig. 2.1 Generalized physiographic units

30 Selected Physical and Geographical Settings

• Plains of Sirima and Somaya representing an unpopulated relative lowland in the east and southeast.

• Hill domes of Warka – Aramfama undulating and rugged topography in the west of the study area.

2.1 GeomorphologyThe geomorphology of the area is variable and it is generally the result of repeated tectonic

events with the associated erosion of Mesozoic sedimentary, crystalline and igneous rocks as well as deposition processes. The tectonic activity and lithological variation in the area also partly or wholly control the drainage density and drainage pattern. Most of the river channels follow the young lineaments. The maximum elevation is 4,377 m a.s.l. (Mt. Tullu Deemtu – the highest peak in Southern Ethiopia) in the northern part of the Dodola sheet and a minimum elevation of less than 600 m on the bank of the Dumal River on the south eastern border of the map sheet.

One of the most important phenomena of the area results from the general slope of the Eastern Ethiopian plateau to the southeast, which is a result of global tectonics and influences the recent direction of surface as well as groundwater flow.

The boundary between the Sanetti Plateau and in the north and plain areas of Harenna, Sirima and Somaya is marked by the 2,000 and 1,500 m contours on the west and east, respectively. The highlands are built from various Cenozoic volcanic rocks. The Sanetti Plateau, which is a part of the Bale Mountain massif, is located to the east of the shoulder of the Eastern Ethiopian plateau. It lies above 3,500 m and falls away rapidly to 2,000 m to the south over a distance of only 8 km. This topographic zone is termed the Harenna erosional escarpment, its general trend being E-W. In places, the Sanetti Plateau is characterized by very sharp peaks and broken rocky surfaces formed by relatively recent lava flows.

In the northern and central parts of the area, numerous continuous and/or discontinuous northerly, north westerly and north easterly trending upstanding volcanic ridges are common; these are broad based, flat topped and occasionally gently undulating.

The Harenna plain and associated lowlands, which lie south of the Bale Mountains, and specifically the south of the Harenna escarpment occupies the largest part of the map area. Starting from the foot of the escarpment to the south the slope is relatively gentle. The area is composed of Mesozoic sedimentary units (limestone).

Hill domes of undulating and rugged Warka – Aramfama topography in the west of the study area are composed of a Precambrian basement which is formed by N-S trending units of various crystalline rocks. The N-S trending structures forming hill ridges are also morphologically visible in the southwestern part of the sheet (see Fig. 2.1).

2.2 Soil and Vegetation CoverSoil and vegetation cover reflects the basic climatic condition of the area as well as the regional

and site specific geological, geomorphological and erosion characteristics.

Soil

Soil stores rainwater in its pores before it infiltrates to greater depths and recharges the aquifer system. Water stored in upper layers evaporates directly. Soil water that is stored in deeper layers is absorbed by vegetation roots then transpires to leaves where it is evaporated. The amount

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31Selected Physical and Geographical Settings

of evapotranspiration from soil is controlled by soil attributes such as soil texture, soil structure and soil moisture content therefore the ability of soil to store and transport water is different for every soil type. Deeper soil has a larger soil moisture reserve than thinner soil, which can supply more water to evaporate. The development of soils depends primarily on geological and climatic conditions. The soils were formed from different types of rocks and occurrence is restricted to these parent rocks and along the transporting agents. The hydrology of the soil is dependent on the texture of the rocks and the degree of weathering. Soils derived from coarse grained rocks inherit a coarse texture, whereas those derived from fine grained rocks are characterized by a fine texture. They are variable in spatial distribution even under the same climate zone. The soil classes used for the soil water balance and groundwater recharge evaluation are based on the hydrological property of the FAO classification of soil (FAO, 1978).

According to the soil map provided by the Ministry of Agriculture, the study area is mainly covered with five major types of soil. These are Cambisols, Rendzinas, Lithosols, Luvisols, Nitosols and Vertisols. Distribution of soil types is shown in Fig. 2.2.

Cambisols

Most Cambisols are medium-grained and have a good structural stability, high porosity, good water holding capacity and good internal drainage. Most Cambisols also contain at least some weatherable minerals in the silt and sand fractions. Based on these characteristics, Cambisols

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Fig. 2.2 Distribution of soil types


have a good infiltration capacity to recharge groundwater. Hydrologically, Chromic Cambisols is more permeable than Eutric Cambisols. Eutric Cambisols is mainly exposed in southwestern and central southern parts of the Dodola map sheet.

Rendzinas

It is dark, grayish-brown and humus rich. It is one of the soils most closely associated with the bedrock type and an example of the initial stages of soil development. The soil of this type contains a significant amount of gravel and stones. It is usually developed beneath grassland formed by the weathering of soft rock types: usually carbonate rocks (dolomite, limestone, marl, chalk) but occasionally sulfate rocks (gypsum). The soil occurs in the southeastern corner of the map.

Lithosols

Lithosols are mineral soils less than 10 cm thick, developed over hard rock. These soils have no agricultural value. They are often referred to as ”skeletal soils” because of their extreme shallowness and steepness and consequently their high erosion hazard.

Nitosols

Nitosols are the most inherently fertile of the tropical soils because of their high nutrient content and deep permeable structure. They can be exploited for plantation agriculture.

Luvisols

Luvisols have a distinct clay accumulation horizon. Most Luvisols are well-drained but Luvisols in depression areas with shallow groundwater may develop gleyic soil properties in and below the argic horizon. Stagnant (more impermeable) properties are found where a dense illuvial horizon obstructs downward percolation and the surface soil becomes saturated with water for extended periods of time. Chromic Luvisols are exposed in the northern, northwestern and southwestern parts of the area and mainly as a weathering product of the volcanic unit of the area.

Vertisols

Vertisols is commonly known as black cotton soil containing smectite clay characterized by a sticky nature, a high water holding capacity and a low infiltration capacity. Vertisols become very hard in the dry season and are sticky in the wet season. They are the second widely exposed soil type in the area. Eutric Vertisols covers the central southern and the gently sloping and low laying eastern areas and are composed mainly of silt, clay and sandy particles. Eutric Vertisols are commonly red in color and highly susceptible to erosion. As observed in the field it is produced from weathering of Quaternary basalt that is exposed throughout the area. This soil forming, erosional and depositional land feature at the Sirima plain is hard and prevents water from percolating. It has water logging characteristics and during the rainy seasons it is very difficult to plough due to high content of clay.

Vegetation

The type of vegetation and its distribution in the area varies mainly based on temperature, altitude, soil type and humidity or precipitation. Vegetation is widely variable in the study area and comprises of Afro-Alpine vegetation, coniferous forest and woodland and savanna regions. The entire Dodola map sheet is characterized by high vegetation cover relative to other parts of Ethiopia due to the existence of the Bale Mountain National Park and more than 50 % of the map is covered by mixed dense high forest.

Afro-Alpine Vegetation Region

The central northern parts of the area, mainly the Sanetti Plateau, are characterized by Alpine vegetation (Fig. 2.3). This vegetation is very short in height and when flowering, looks like snow

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cover from a distance. It occurs at altitudes of between 3,400–4,300 m a.s.l. in the case of the Dodola map sheet, but normally alpine vegetation flowers in areas above 3,000 m a.s.l with low mean annual temperatures of less than 10 °C and average annual precipitation from 800 to

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Fig. 2.3 Afro–Alpine vegetation at Sanetti Plateau (Bale National Park)

Fig. 2.4 The Herele forest near Delo Mena town


1,600 mm. Nevertheless, in our case the area between 2,800–3,400 m a.s.l. is marked either by eroded escarpment or cliff forming rocks.

Coniferous forest Region

The coniferous forest region of the study area includes highly impenetrable Harenna forest (Hawo, Herelle, Shisha and Kumbi forests) in the center of map sheet and moderately densely developed vegetations in the west and northwest. The Harenna forest with many different plant species is a part of the Bale Mountain National Park (BMNP). The Herele forest is shown in Fig. 2.4.

According to the Atlas of Ethiopia (1981) coniferous forest regions are referred to as temperate forests and develop at altitudes ranging from 2,300–3,100 m a.s.l with mean annual rainfall varying from 500 to 1,100 mm. In the case of the study area the forest region developed at altitude ranging from 1,200–2,800 m a.s.l with annual mean rain fall ranging from 840 to 990 mm (according to Goba, Rira, and Delo Mena meteorological stations). North of Delo Mena, the surroundings of Angetu, Hawo, Werka, west of Meti and Jala are covered by this type of region. This coniferous forest region is dominated by >30 m high Prodocarpus (Zigba), juniperous (Tid), Hagenia Abyssinia (Heto) and broad leafed trees. Recently this forest region has started to be deforested by local peoples for household consumption and land for plowing.

Woodland and savanna region

This region mainly covers the southeastern part of the study area and is composed of Acacia and other bush forming low woods. This vegetation region is normally found between an altitude of 400 and 2,000 m a.s.l with rainfall of about 250–1,300 mm. In the case of the study area it occurs between 600–1,200 m a.s.l. to the south of Jibri village, around Berak and to the east of Delo Mena town (Hayoda area). Along the Delo Mena – Melka Amana road this vegetation type swells its roots to accumulate soil water and keep it for use during dry periods of the year.

2.3 Climatic CharacteristicsThe area is very heterogeneous and is mainly characterized by Alpine, Temperate, Sub-tropical

and Tropical climate types in which the rainy season passes from March to May and from October to November. The mean annual rainfall is between 500 and 400 mm in the south-eastern lowlands and 1,500 and 1,600 mm in the northern highlands based on rainfall assessment within the Genale-Dawa Basin. The mean maximum annual temperature is 22 °C and the mean minimum annual temperature is 7.5 °C based on the temperature – elevation relationship for the Genale-Dawa basin.

2.3.1 Climatic Zones and MeasurementsThe climatic conditions of Ethiopia are mostly dominated by altitude. According to Daniel

Gamatchu (1977) there are wide varieties in climatic zones. Climatic zones defined by Javier Gozálbez and Dulce Cebrián (2006) and Tesfaye Chernet (1993) are shown in Tab. 2.1.

A climatic zoning map (Fig. 2.5) has been compiled based on the climatic region classifications given in Tab. 2.1 and the elevation of the study area. The project area was found to belong to five climatic regions: Afro-Alpine (High Wurch), Sub Afro-Alpine (Wurch), Temperate (Dega), Sub tropical (Weina Dega) and Tropical (Kolla). From elevation data of the Dodola map sheet there is also a Desert climatic region but it covers a small area in the extreme southeastern part and is restricted to Dumal River canyon.

The outstanding modern quantitative climatic classification of Koeppen (1989) defines the climatic types according to the values of temperature and precipitation regardless of the geographic

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location of the region. Criteria for classification of the principal climatic types in a modified Koppen system are based on the mean annual and mean monthly precipitation and temperature values. The actual application of the Koeppen system to climatological statistics shows that the Ethiopian climate is grouped into three main categories, each divided into three or more types making a total of 11 principal climatic types.

The highlands of the northwestern part of the Dodola sheet belong to Zone H – characterized by cool highland climate (Sanetti Plateau). The mean temperature of the warmest month is below 10 °C and the mean rainfall is about 2,000 mm/year. This type of climate prevails in areas at altitudes above 3,500 m a.s.l.

The area along Harenna escarpment belongs to Zone Cwb – characterized by a warm temperate rainy climate where the mean temperature of the coldest months is below 18 °C and for four months the mean temperature is above 10 °C. The rainfall is heavy in areas covered by forests and moderate rainfall is typical for areas covered with grass. This type of climate prevails in areas at altitudes between 1,750 and 3,200 m a.s.l.

The lowlands of the southeastern part of the sheet belong to Aws and Cws zones – characterized by a tropical and warm temperate climate. The mean temperature of the coldest month is above or below 18 °C and the mean annual rainfall is 680–1,200 mm. This type of climate prevails up to

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Tab. 2.1 Ethiopian climate classification

Name / Altitude / Mean annual temperature

Precipitationbelow 900 mm

Precipitationbetween 900 and 1,400 mm

Precipitationabove 1,400 mm

High Wurch (Kur)above 3,700 mbelow 5 °C

Afro-alpinemeadows of grazing land and steppes, no farmingHelichrysum, Lobelia

Wurch (Kur)3,700–3,200 m 5–10 °C

Sub-afroalpine barleyErica, Hypericum

Sub-afroalpine barleyErica, Hypericum

Dega3,200–2,300 m10–15 °C

Afro-mountain (temperate)forest – woodlandbarley, wheat, pulsesJuniperus, Hagenia, Podo-carpus

Afro-mountain (temperate)bamboo forestbarley, wheat, nug, pulsesJuniperus, Hagenia, Podo-carpu, bamboo

Weina Dega2,300–1,500 m15–20 °C

Savannah (sub-tropical)wheat, teff, some cornacacia savannah

Shrub-savannah(sub-tropical)corn, sorghum, teff, enset, nug, wheat, barleyAcacia, Cordia, Ficus

Wooded savannah(sub-tropical)corn, teff, nug, enset, barleyAcacia, Cordia, Ficus, bam-boo

Kolla1,500–500 m above 30 °C

Tropicalsorghum and teffacacia bushes

Tropicalsorghum, teff, nug, peanutsAcacia, Cordia, Ficus

Wet tropicalmango, sugar cane, corn, coffee, orangesCyathea, Albizia

Berehabelow 500 mabove 40 °C

Semi-desert and desertcrops only with irrigationthorny acacias, Commi-phora

Remark: after Javier Gozálbez and Dulce Cebrián (2006), Tesfaye Chernet (1993)


an elevation of 1,750 m a.s.l. This climate is characterized by tall grass usually intermingled with trees. Two wet periods are separated by dry periods.

There are 12 meteorological stations operated by the Meteorological Institute within the mapped area. The stations located in Goba, Dodola, Rira, Delo Mena, and Melka Amana provide basic climatic characteristics for mountain and lowland areas.

The air temperature shows seasonal changes with an annual average of 19.3 °C. It is difficult to calculate an average temperature for the sheet because of the vast differences in altitude. The seasonal variation of temperature at Dodola meteo-station is shown in Tab. 2.3 as an example.

The temperature is negatively correlated with elevation. Temperature is high in the southern and southeastern parts of the area, whereas low temperatures are observed in the northwestern part of the Dodola map sheet. Temperature is usually the most important factor for evapotranspiration. The evaporation will continue to increase at an increased rate as the temperature rises as long as there is water to evaporate.

The minimum and maximum temperature in Dodola meteo-station ranges from 0.01 °C to 3.6 °C and from 20.8 °C to 27.3 °C with an annual mean of 12.8 °C. Fig. 2.6 confirms the seasonal variation of temperature is more or less constant throughout the year. The mean temperatures recorded in the major stations are relatively similar. The hot months are from February to September, but the mean temperature is low in August. Temperatures in September to January are relatively low.

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Fig. 2.5 Climatic zones


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Tab. 2.2 Climatic stations in the Dodola area

Map ID Station name Class X UTM Y UTMAltitude[m a.s.l.]

Data type Sub-basin

RF8 Goba-Mission 1 610455 775638 2,545 P, T, E ,H, S Genale

RF16 Bule School 4 434537 696492 1,770 P Genale

RF17 Goba NMSA 1 607491 774479 2,545 P,T Genale

RF31 Goro 1 663424 773362 1,760 P, T, E ,H, S, W Genale

RF32 Woshi 4 500000 734991 2,500 P Genale

RF45 Delo Mena 1 594228 709533 1,295 P, T, E ,H, S, W Genale

RF55 Angetu 4 564947 707971 1,450 P Genale

RF58 Nensebo 1 511826 727985 1,835 P, T, S Genale

RF60 Rira 4 579785 748344 2,900 P Genale

RF63 Melka Amana 4 588164 690626 1,090 P Genale

RF64 Aborso 3 543842 677605 1,580 P Genale

B5 Dodola 1 520000 770050 2,500 P, TWabe Shebelle

Tab. 2.3 Mean monthly temperature at Dodola meteo-station

Month t max t min t av

1 25.0 0.01 12.5

2 27.3 1.5 14.4

3 23.8 3.6 13.7

4 25.1 3.1 14.1

5 24.1 3.4 13.8

6 24.6 1.5 13.1

7 20.8 2.6 11.7

8 25.3 2.3 13.8

9 22.2 2.0 12.1

10 22.4 1.2 11.8

11 22.3 0.05 11.2

12 23.7 0.01 11.9

mean 23.9 1.8 12.8


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Sunshine and radiation are two of the climatic agents that affect the rate of evapotranspiration. The maximum sunshine is recorded in the Dodola meteo-station in March and the minimum sunshine is observed in July (Fig. 2.7).

Relative humidity and wind are also closely related to evapotranspiration because if the relative humidity is close to its holding capacity, the ability of plants to transpire may be inhibited. The higher the relative humidity, the lower is the evaporation rate; the drier the air above the surface, the faster the evaporation. Minimum evapotranspiration rates generally occur during the coldest months of the year (December to February) and maximum from July to October.

Fig. 2.6 Temperature [°C] at Dodola meteo-station

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25

30

1 2 3 4 5 6 7 8 9 10 11 12

month

t [o C] t_max

t_min

t_mean

Fig. 2.7 Mean monthly sunshine [hour] and radiation [MJ/km2/d] at Dodola meteo-station

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1 2 3 4 5 6 7 8 9 10 11 12

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[h]

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Sun_hours

Radia on


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Wind speed influences the moisture gradient has a direct relation with evaporation. As water vaporizes into the atmosphere, the boundary layer between the earth and air becomes more and more saturated so that the water vapor has to continuously be removed and replaced with drier air. The values of relative humidity and wind speed are plotted in Fig. 2.8.

Evapotransparation is the combination of soil evaporation and vegetation transpiration and has a considerable impact on the water balance in the study area. It is difficult to make accurate field measurements of ETo. It is commonly computed from climatic data like mean daily maximum temperature, mean daily minimum temperature, mean relative humidity, mean wind speed and

Fig. 2.8 Mean monthly relative humidity [%] and wind speed [km/d] at Dodola meteo-station

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160

1 2 3 4 5 6 7 8 9 10 11 12

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win

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[%]

Wind

R_humidity

Fig. 2.9 ETo data calculated for Dodola meteo-station

0,0

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1 2 3 4 5 6 7 8 9 10 11 12

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[mm

/d]

ETo


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mean sunshine hours. The Water Resources, Development and Management Service and the Environment and the Natural Resources Service of the Food and Agriculture Organization of the UN (FAO) offer data for Dodola, Adaba, Bale Robe, and Goba stations. The mean evapotranspiration calculated using the Penman-Monteith method for Adaba, Dodola, Bale Robe and Goba meteo-stations is 3.66, 3.71, 3.63, and 3.15 mm/d, respectively. In general, ETo increases from January and reaches a peak in March and then starts to fall in July. Fig. 2.9 shows ETo calculated for Dodola meteo-station.

2.3.2 Precipitation The Ethiopian territory is divided into four zones marked as A, B, C, and D, each of them with

different precipitation patterns. The seasonal classification and precipitation regimes of Ethiopia (after NMSA, 1996) are characterized in Tab. 2.4 and shown in Fig. 2.10.

The mapped area belongs to regions C and A which are similar in rainfall pattern and are characterized by four distinct seasons and by bimodal precipitation patterns with peaks in April and August and October. In general the annual rainfall depends on the regional altitude variation of the area and precipitation decreases from west to east. The highland in the north receives mean annual precipitation above 1,500 mm/year. The mean annual rainfall is less than 500 mm/year, for the arid lowland regions and is less than 200 mm in the most arid parts of the Ogaden. The low precipitation regions have higher intensity of precipitation than those areas which have a higher amount of annual precipitation. The intensity of precipitation of more than 100 mm a day in the lowlands and less than 50 mm a day in the highlands is common.

Basic precipitation data from Goro, Rira, Delo Mena, Melka Amana and Dodola are shown in Tab. 2.5.

The average precipitation at the Goro meteo-station is 915.5 mm. The station is located in precipitation regime C (Fig. 2.11), however very near to the border the precipitation pattern is A.

Tab. 2.4 Characterization of the precipitation pattern in Ethiopia

Zone Precipitation pattern

A

This region mainly covers the central and central eastern part of the country. It is characterized by three distinct seasons, and by bimodal precipitation patterns with small peaks in April and the main rainy season during mid June to mid September with peaks in July.

B

This region covers the western part of the country. It is characterized by a single pre-cipitation peak. Two distinct seasons, one being wet and the other dry, are encoun-tered in this region. The analysis of mean monthly precipitation patterns shows that this zone can be split into southwestern (b1) with the wet season during February/March to October/November, western (b2) with the wet season during April/May to October/November, and northwestern (b3) with the wet season during June to September parts.

CThis region mainly covers the southern and southeastern parts of the country. It has two distinct precipitation peaks with a dry season between. The first wet season is from March to May and the second is from September to November.

DThe Red Sea region in the extreme northeastern part of the country receives diffused precipitation with no distinct pattern; however precipitation occurs mainly during the winter.


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Fig. 2.10 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996)


The A pattern is not different from C in the area of the Dodola sheet. The Goro station has the most complete precipitation data and years with a full set of data were extracted from the review of data from 1981 to 1999 and are illustrated in Tab. 2.6 and Fig. 2.12.

The average precipitation in the Rira meteo-station is 836.1 mm. The station is located in precipitation regime A (Fig. 2.13), however very near the border the precipitation pattern is C. The C pattern is not very different from A in the area of the Dodola sheet despite higher rainfall in July. Years with a full set of data were extracted from the review of data from the period from 1988 to 2000 and are illustrated in Fig. 2.14.

The average precipitation in the Delo Mena meteo-station is 958.4 mm. The station is located in precipitation regime C (Fig. 2.15), however very near the border the precipitation pattern is A. The A pattern is not very different from the C in the area of the Dodola sheet. The Delo Mena years with a full set of data were extracted from the review of data from the period from 1973 to 1998 and are illustrated in Fig. 2.16.

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Tab. 2.5 Monthly long-term average precipitation at selected meteo-stations of the Dodola sheet [mm]

St / M Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Goro 19.5 27.8 66.8 201.4 154.8 36.7 25.9 48.0 86.5 172.9 58.3 16.4

Rira 32.4 45.7 94.9 127.6 66.1 46.9 92.2 74.1 61.0 123.5 43.9 27.7

Delo Mena

22.5 59.9 90.4 204.8 184.8 28.5 14.2 20.7 54.2 206.2 52.7 19.7

MelkaAmana

19.1 33.5 77.0 129.5 81.6 57.1 94.0 105.1 88.4 76.3 27.2 13.8

Dodola 36.4 34.7 46.6 75.7 46.4 79.0 164.4 157.2 109.6 58.4 20.0 14.8

Tab. 2.6 Long-term monthly precipitation at Goro meteo-sation [mm] (fully recorded years only)

Year/Month

1 2 3 4 5 6 7 8 9 10 11 12 Total

1981 0.0 57.8 289.8 303.3 31.8 11.7 15.6 40.3 97.8 96.9 19.0 7.4 971.4

1982 4.2 20.5 22.6 122.4 198.6 29.5 18.9 1.7 78.2 143.2 52.6 22.2 714.6

1983 69.7 25.8 11.5 167.3 81.9 70.0 11.5 119.0 154.0 152.5 124.7 3.4 991.1

1985 9.4 40.0 0.0 186.2 257.0 106.0 59.0 11.0 18.0 153.4 133.7 4.6 978.3

1987 4.4 33.5 135.5 172.9 240.6 22.1 0.0 6.6 115.0 127.8 36.3 0.0 894.8

1988 1.7 48.8 69.4 181.3 99.0 16.0 72.4 71.4 86.1 109.1 24.0 0.0 779.2

1990 14.8 59.4 93.2 157.6 103.2 37.4 15.4 54.4 50.6 92.8 118.7 6.8 804.3

1994 0.0 0.0 31.1 115.1 279.7 30.8 28.2 55.9 74.9 202.9 66.1 17.1 901.8

1995 0.0 16.0 118.7 289.1 113.5 13.8 42.0 32.9 90.1 159.0 2.4 14.5 892.0

1997 12.4 0.0 53.2 210.3 57.3 59.9 6.8 112.0 72.8 294.2 183.7 40.6 1,103.6

1999 0.0 0.0 58.9 125.4 220.6 8.5 52.0 88.4 145.0 408.8 52.6 0.0 1,160.3


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The average precipitation in the Melka Amana meteo-station is 802.5 mm. The station is located in precipitation regime C (Fig. 2.17), however very near to the border the precipitation pattern is A. The A pattern is not very different from the C in the area of Dodola sheet. The Melka Amana years with a full set of data were extracted from the review of data from the period from 1988 to 1991 and are illustrated in Fig. 2.18.

Fig. 2.11 The Goro meteo-station precipitation pattern

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

month

aver

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[mm

/mon

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1400

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

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1991

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1996

1997

1998

1999

2000 Avg

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prec

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[mm

/yea

r]

Fig. 2.12 Long-term fluctuation of precipitation in Goro meteo-station


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The average precipitation in the Dodola meteo-station is 843.2 mm. The station is located in precipitation regime A (Fig 2.19) and is different from other precipitation regimes which usually

Fig. 2.13 The Rira meteo-station precipitation pattern

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month

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1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Avg

year

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[mm

/yea

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Fig. 2.14 Long-term fluctuation of precipitation in Rira meteo-station


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show a C pattern in the area of the Dodola sheet. Years with a full set of data were extracted from the review of data from the period from 1955 to 1978 and are illustrated in Fig. 2.20.

Fig. 2.15 The Delo Mena meteo-station precipitation pattern

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Fig. 2.16 Long-term fluctuation of precipitation in Delo Mena meteo-station

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1972

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The graphs show the high fluctuations in precipitation. Differences in precipitation can exceed 100 % in some years.

Fig. 2.17 The Melka Amana meteo-station precipitation pattern

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Fig. 2.18 Long-term fluctuation of precipitation in Melka Amana meteo-station

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1988 1989 1990 1991 Avg

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The annual precipitation of the area is 900 mm for Sanetti Plateau, Harenna escarpment and Warka-Aramfama hills and 800 mm for lowlands represented by Sirima and Somaya plains.

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Fig. 2.19 The Dodola meteo-station precipitation pattern

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Fig. 2.20 Long-term fluctuation of precipitation in Dodola meteo-station

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1954

1956

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[mm

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2.4 Hydrography and Hydrology of the AreaThe Dodola area is found within the Genale-Dawa and Wabe Shebelle basins. The Genale-Dawa

basin covers about 95 % of the Dodola area. It is known as the third biggest basin in Ethiopia and covers an area of about 172,880 km2. The basin has a relatively low runoff with a mean flow of 125 m3/s. The minimum flow of the Genale and Dawa is from December to March and the maximum flow is from August to November. This is due to the dominant precipitation pattern and arid character of the climate. The general drainage trend in the area is from the elevated northwestern mountainous area to the southeastern lowland plains of the Ogaden basin and the Indian Ocean. Wabe Shebelle is the second biggest Ethiopian river basin, with a size of about 202,679 km2, but it also has low runoff because of the dominant arid character of the basin. The general drainage trend is to the north within the Wabe Shebelle basin and the rest of the northern part of the sheet, but to the south in the majority of the sheet. The principal river basins of the area are shown in Fig. 2.21.

2.4.1 Surface Water Network DevelopmentThe drainage pattern in the Dodola sheet is mainly characterized by a sub-parallel pattern

and controlled by structures. The drainage density depends on the slope, nature and attitude of bedrocks and the existing regional and local fracture patterns. They reflect the lithology and structure of a given area and can be of great value for groundwater resources evaluation. Parallel types of drainage patterns are indicative for the presence of structures that act as conduits or storage for sub-surface water. It is a measure of how dissected a basin is, and it is expected that drainage density affects the transformation of rainfall into runoff (Reddy, 2001). Many studies have integrated lineaments and drainage maps to infer the groundwater recharge potential zone (Edet et al., 1998; Shaban et al., 2006).

Fig. 2.21 The principal river basins in the area


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Genale-Dawa basin

The basin comprises three main sub-basins including the Genale sub-basin Weyb sub-basin and Dawa sub-basin. Among these, the Genale sub-basin is the one which mostly covers the Dodola map sheet. The Geberticha, Iya, Welmel, Yadot, Deyu, Hambela, Dumal, Weyb, Tegona, and Shaya rivers are the main tributaries of the Genale River.

Wabe Shebelle basin

The basin covers only a small part of the northwestern part of the Dodola map sheet. The Herero, Furuna and Maribo rivers are perennial tributaries of the Wabe Shebelle River in its upper catchment. The Ukuma, Herero Maribo, and Lelisso rivers are tributaries of the Wabe Shebelle.

Genale River originates from the western extension of volcanic rocks covering the Bale highland and flows in a southeastern direction and crosses the Ethio-Somali frontiers, after which the river is named “Juba”.

Geberticha River flows from Mt. Kurduro directly to the south at its upper reach, deviating to southeast at its lower reach and joins the Genale River. It is a perennial river which follows the western rugged topography of the area.

Iya River also flows from the central part of the Bale highland and flows due south to join the Genale River a few km beyond the sheet area. It is a perennial stream with relatively high discharge but it has no gauging station. This river flows through the rugged western area of the sheet and it is difficult to develop irrigation along its source, but local electric power stations can be developed similar to on the Yadot River due to its reasonable discharge.

Welmel River rises from the Sanetti Plateau and flows to the southern lowland. Recent Quaternary lava forms the channel bed of the Welmel and it has a series of waterfalls. One of the highest

Fig. 2.22 Welmel River – waterfall


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waterfalls is located about 3 km SE of Melka Amana and is about 15 to 25 m high (Fig. 2.22). River flow is very fast due to the high topographic gradient, eroding the Quaternary basalt which covers the river bed A number of springs recharge this river from its Tertiary basalt which covers the upper catchment. River discharge of 5.6 m3/s was measured using the floating method on 5th of February, 2010, during the area’s last dry season, at the bridge on the way from Delo to Bidre.

Yadot River is a perennial river which rises from the foot of Tullu Deemtu (Mt. Batu). Currently, about 10 % of the water flow is used for local electric power production to supply the town of Delo Mena. River discharge of 2.86 m3/s was measured using the floating method on 22th of January, 2010 at 586816 E, 712778 N, before the channel is diverted to the power plant.

2.4.2 Surface Water RegimeThere are about 38 river gauging stations within the Genale-Dawa basin and 50 river gauging

stations within the Wabe Shebelle basin. Some of them are operational but lots of stations have no data. In the Dodola sheet there are seven registered gauging stations. Other river gauging stations are on the border of the Asela and Dodola sheets and inside the neighboring sheets of Asela and Dila. Data from stations located in the Dodola sheet were used for surface and baseflow assessment. Data from other rivers were also calculated for assessment of surface as well as baseflow values and used to compare and correct data from the Dodola area. The selected river stations are summarized in Tab. 2.7.

Tab. 2.7 Data on the river gauging stations (Part 1)

Map ID River Station X UTM Y UTMAltitude[m a.s.l.]

Area [km2]

Basin/ Sub-basin

Map

RG11 Genale Chenemasa 559250 630852 1,120 9,190.3 Genale Negele

RG12 Mana Robi Goro RD. 654745 775353 2,010 272.5 Genale Asela

RG13 Shawe Mes Project 575301 710810 1,450 193.7 Genale Dodola

RG14 Yadot Delo Mena 592610 709060 1,250 451.9 Genale Dodola

RG15 Halgol Delo Mena 592205 699802 1,200 182.1 Genale Dodola

RG16 Welmel Melka Amana 586880 689892 1,060 1,395.8 Genale Dodola

RG17 Deyiu Deyu Harewa 607140 717828 1,205 111.1 Genale Dodola

RG18 Logita Bensa 471810 718651 1,900 729.4 Genale Dila

RG22 Gelana Bona Kike 470569 720471 1,960 376.2 Genale Dila

RG23 Ererte Bona Kike 466086 722055 2,130 99.3 Genale Dila

RG24 Morodo Bona Kike 465069 721278 2,140 85.9 Genale Dila

RG25 Konkona Bensa Daye 480506 720998 1,880 52.3 Genale Dila

RG26 Gambetu Aroressa 486754 703726 1,510 270.3 Genale Dila

RG27 U.Genale Girja 494472 685417 1,360 3,177.4 Genale Dila

RG28 Dumel Dildila 629019 744377 1,160 207.5 Genale Dodola

RG29 Denka Dinsho 585771 785149 3,050 89.6 Weyb Asela

RG30 Tegona Shalo Village 622525 783151 2,400 467.8 Weyb Asela


Records from all stations reflect the fact that the river discharge is directly proportional to the intensity of rainfall within the basin. There is a high discharge fluctuation between the wet and dry season of the year. The highest flow period is from June to October and the peak flow for all rivers is usually recorded in August. Runoff data are summarized in Tab. 2.8.

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Map ID River Station X UTM Y UTMAltitude[m a.s.l.]

Area [km2]

Basin/ Sub-basin

Map

RG31 Weyb Agarfa 590171 806929 2,370 794.0 Weyb Asela

RG32 Weyb Denbel 610622 815284 2,320 1,048.0 Weyb Asela

RG33 Weyb Alem Kerem 663138 779169 1,750 3,722.6 Weyb Asela

RG34 Shaya Robe 607676 792364 2,380 450.9 Weyb Asela

RG35 Tegona Goba 606966 771922 2,770 84.4 Weyb Dodola

RG37 Weyb Sofumer 703729 763790 1,175 4,546.3 Weyb Magalo

RG39/H1 Wabe bridge 504300 775500 2,500 1,040.0Wabe Shebelle

Asela

RG41/H3 Ukuma Dodola 505500 773800 2,450 137.0Wabe Shebelle

A/Dodola

RG45/H7 Herero Herero 536100 773800 2,355 122.0Wabe Shebelle

A/Dodola

RG50/H12 Maribo Adaba 537500 773800 2,430 192.0Wabe Shebelle

A/Dodola

RG49/H11 Lelisso Adaba 5412500 773800 2,345 120.0Wabe Shebelle

A/Dodola

RG42/H4 Fruna Adaba 545500 775700 2,405 86.0Wabe Shebelle

Asela

Tab. 2.7 Data on the river gauging stations (Part 2)

Tab. 2.8 Runoff (Part 1)

Map ID River StationMean flow [m3/s]

Annual flow[mm]

Area [km2]

Specific runoff[l/s.km2]

Sub-basin

Aquifer

RG11 Genale Chenemasa 92.1 316.3 9,190.3 10.0 GenaleVolcanic/basement

RG12 Mana Robi Goro RD. 0.48 55.6 272.5 1.8 Genale Volcanic

RG13 Shawe Mes Project 3.01 489.6 193.7 15.9 Genale Volcanic

RG14 Yadot Delo Mena 6.7 467.8 451.9 14.8 Genale Volcanic

RG15 Halgol Delo Mena 0.75 130.0 182.1 4.1 Genale Volcanic

RG16 Welmel Melka Amana 17.3 391.1 1,395.8 12.4 Genale Volcanic

RG17 Deyiu Deyiu Harewa 1.0 284.3 111.1 8.7 Genale Volcanic

RG18 Logita Bensa 15.9 688.3 729.4 21.9 Genale Volcanic


Measured discharge of the Shawe River at the Mes Project river gauge between 1983 and 2005 is shown in Fig. 2.23. The figure shows that flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year The lowest daily discharge of 0.341 m3/s (9.–10.3.1992) and the highest daily discharge 70.697 m3/s (5.8.2005) were recorded by the river gauge. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where the Shawe River rises, which receive the highest precipitation within the Genale-Dawa catchment.

Map ID River StationMean flow [m3/s]

Annual flow[mm]

Area [km2]


Sub-basin

Aquifer

RG22 Gelana Bona Kike 7.1 595.9 376.2 18.8 Genale Volcanic

RG23 Ererte Bona Kike 1.3 414.4 99.3 13.3 Genale Volcanic

RG24 Morodo Bona Kike 0.9 330.3 85.9 10.1 Genale Volcanic

RG25 Konkona Bensa Daye 1.0 606.9 52.3 19.2 Genale Volcanic

RG26 Gambetu Aroressa 4.9 572.7 270.3 18.1 Genale Basement

RG27 U.Genale Girja 74.1 736.0 3,177.4 23.3 Genale Volcanic

RG28 Dumel Dildila No data 207.5 Genale Volcanic

RG29 Denka Dinsho 0.85 298.0 89.6 9.4 Weyb Volcanic

RG31 Weyb Agarfa 4.6 182.8 794.0 5.8 Weyb Volcanic

RG32 Weyb Denbel 4.1 123.5 1,048.0 3.9 Weyb Volcanic

RG33 WeybAlem Kerem

12.2 103.4 3,722.6 3.3 Weyb Volcanic

RG34 Shaya Robe 4.3 300.9 450.9 9.6 Weyb Volcanic

RG35 Tegona Goba 1.4 523.5 84.4 16.6 Weyb Volcanic

RG37 Weyb Sofumer 23.46 162.8 4,546.3 5.2 Weyb Limestone

RG39/H1 Wabe bridge 1.7 69.8 1,040.0 1.7 Wabe Shebelle Volcanic

RG41/H3 Ukuma Dodola 1.5 345.5 137.0 11.0 Wabe Shebelle Volcanic

RG45/H7 Herero Herero 1.6 413.9 122.0 12.8 WabeShebelle Volcanic

RG50/H12 Maribo Adaba 3.0 493.1 192.0 15.6 WabeShebelle Volcanic

RG49/H11 Lelisso Adaba 1.4 368.2 120.0 11.7 Wabe Shebelle Volcanic

RG42/H4 Fruna Adaba 3.2 1,174.2 86.0 36.9 WabeShebelle Volcanic

Tab. 2.8 Runoff (Part 2)

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The annual variability of the mean annual flow of the Shawe River in the Mes Project river gauge is shown in Fig. 2.24.

Measured discharge of the Yadot River at the Delo Mena river gauge between 1984 and 2006 is shown in Fig. 2.25. The figure shows that the flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.088 m3/s (12.3.1994) and the highest daily discharge 64.419 m3/s (27.7.1995) were recorded at the river gauge. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where Yadot River rises, which receive the highest precipitation within the Genale-Dawa catchment.

Fig. 2.23 Flow diagram of the Shawe River from the Mes Project river gauge

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Fig. 2.24 Annual variability of the mean annual flow of Shawe River at the Mes project river gauge

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The annual variability of the mean annual flow of Yadot River at Delo Mena river gauge is shown in Fig. 2.26.

Measured discharge of the Halgol River at the Delo Mena river gauge between 1986 and 2006 is shown in Fig. 2.27. The figure shows that flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.012 m3/s (23.7.1987) and the highest daily discharge 25.017 m3/s (3.3.2005) were recorded at the river gauge. Flow was not recorded several times in the period from September to

Fig. 2.25 Flow diagram of the Yadot River from the Delo Mena river gauge

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Fig. 2.26 Annual variability of the mean annual flow of Yadot River at the Delo Mena river gauge

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December 1995. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where Halgol River rises, which receive the highest precipitation within the Genale-Dawa catchment.

The annual variability of the mean annual flow of Halgol River at Delo Mena river gauge is shown in Fig. 2.28.

Measured discharge of the Welmel River at Melka Amana river gauge between 1985 and 2006 is shown in Fig. 2.29. The figure shows that flow is relatively regular, however its total annual flow

Fig. 2.27 Flow diagram of the Halgol River at Delo Mena river gauge

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Fig. 2.28 The annual variability of the mean annual flow of Halgol River at Delo Mena river gauge

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and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 1.786 m3/s (26.–29.3.2000) and the highest daily discharge 199.389 m3/s (27.5.1993) were recorded at the river gauge. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where Welmel River rises, which receive the highest precipitation within the Genale-Dawa catchment.

The annual variability of the mean annual flow of Welmel River at Melka Amana river gauge is shown in Fig. 2.30.

Fig. 2.29 Flow diagram of the Welmel River at Melka Amana river gauge

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Fig. 2.30 The annual variability of the mean annual flow of Welmen River at Melka Amana river gauge

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Measured discharge of the Deyiu River at Deyiu Harewa river gauge between 1991 and 2007 is shown in Fig. 2.31. The figure shows that flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.116 m3/s (18. 1. 2005) and the highest daily discharge 12.428 m3/s (18. 5. 2005) were recorded at the river gauge. Flow was not recorded several times in the period from 7th to 13th of April, 2003. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where Deyiu River rises, which receive the highest precipitation within the Genale-Dawa catchment.

Fig. 2.31 Flow diagram of the Deyiu River at Deyiu Harewa river gauge

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Fig. 2.32 The annual variability of the mean annual flow of Deyiu River at Deyiu Harewa river gauge

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The annual variability of the mean annual flow of Deyiu River at Deyiu Harewa river gauge is shown in Fig. 2.32

Measured discharge of the Tegona River at Goba river gauge between 1984 to 2006 and shown in Fig. 2.33. The figure shows that flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.003 m3/s (16. 10. 1996) and the highest daily discharge 41.01 m3/s (30. 3. 2005) were recorded at the river gauge. The calculated mean annual flow of 3.01 m3/s represents flow generated mainly in the eastern highlands, where Tegona River rises, which receive the highest precipitation within the Genale-Dawa catchment.

Fig. 2.33 Flow diagram of the Tegona River at Goba river gauge

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Fig. 2.34 The annual variability of the mean annual flow of Tegona River at Goba river gauge

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The annual variability of the mean annual flow of Tegona River at Goba river gauge is shown in Fig. 2.34.

Measured discharge of the Ukuma River at Dodola river gauge between 1976 and 2006 is shown in Fig. 2.35. The figure shows that flow is relatively regular, however its total annual flow and in particular maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.002 m3/s (27.2.2001) and the highest daily discharge 52.278 m3/s (30.10.1977) were recorded at the river gauge. Flow was not recorded several times in the period from the 5th to 10th of February, 2002 and from 21st to 26th of May, 2004. The

Fig. 2.35 Flow diagram of the Ukuma River at Dodola river gauge

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Fig. 2.36 The annual variability of the mean annual flow of Ukuma River at Dodola river gauge

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calculated mean annual flow of 1.7 m3/s represents flow generated mainly in the eastern highlands, where Ukuma River rises, which receive the highest precipitation within the Wabe Shebelle catchment.

The annual variability of the mean annual flow of Ukuma River at Goba river gauge is shown in Fig. 2.36.

Measured discharge of the Herero River at Herero river gauge between 1976 and 2006 is shown in Fig. 2.37. The figure shows that flow is relatively regular, however its total value of annual flow

Fig. 2.37 Flow diagram of the Herero River at Herero river gauge

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Fig. 2.38 The annual variability of the mean annual flow of Herero River at Herero river gauge

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and particularly maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.001 m3/s (17.–18. 3. 1985) and the highest daily discharge 18.143 m3/s (27. 10. 1998) were recorded at the river gauge. Flow was not recorded several times in 1976, 1987, 1989, 1990 and 1991. The calculated mean annual flow of 1.6 m3/s represents flow generated mainly in the eastern highlands, where Herero River rises, which receive the highest precipitation within the Wabe Shebelle catchment.

The annual variability of the mean annual flow of Herero River at Herero river gauge is shown in Fig. 2.38.

Fig. 2.39 Flow diagram of the Maribo River at Adaba river gauge

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Fig. 2.40 The annual variability of the mean annual flow of Maribo River at Adaba river gauge

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Measured discharge of the Maribo River at Adaba river gauge between 1976 and 2006 is shown in Fig. 2.39. The figure shows that flow is relatively regular, however its total value of annual flow and particularly maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.044 m3/s (30.1.1995) and the highest daily discharge 27.584 m3/s (31.7.1999) were recorded at the river gauge. The calculated mean annual flow of 3.0 m3/s represents flow generated mainly in the eastern highlands, where Maribo River rises and which receive the highest precipitation within the Wabe Shebelle catchment.

The annual variability of the mean annual flow of Maribo River at Adaba river gauge is shown in Fig. 2.40.

Measured discharge of the Leliso River at Adaba river gauge between 1975 and 2006 is shown in Fig. 2.41. The figure shows that flow is relatively regular, however its total value of annual flow and particularly maximal monthly flow can vary substantially throughout the year. The lowest daily discharge of 0.001 m3/s (24.–25. 1. 1983) and the highest daily discharge 26.917 m3/s (17. 7. 1978) were recorded at the river gauge. The calculated mean annual flow of 1.4 m3/s represents flow generated mainly in the eastern highlands, where Herero River rises, which receive the highest precipitation within the Wabe Shebelle catchment.

The annual variability of the mean annual flow of Leliso River at Adaba river gauge is shown in Fig. 2.42.

The assessment of specific runoff is based on data from flow measurements and calculated specific runoff in gauging stations shown in Tab. 2.8 and the appropriate area of the pertinent river basin within the Dodola sheet considering the altitude and rock composition of the area. The specific runoff for Genale-Dawa is 0.72 l/km2. The specific runoff is assessed for the basaltic rock basement and sedimentary rocks separately. The specific runoff for volcanic rocks was assessed to be 12.1 l/s.km2 based on data from all 24 river gauges shown in Tab. 2.8. The specific runoff for sedimentary rocks was not assessed. The specific runoff for basement rocks was assessed to be 14 l/s.km2 based on data from the Gambetu at Aroresa and the Upper Genale at Girja in the highlands where the catchments receive high volumes of precipitation (depth). This is a generic assessment and specific runoff will be highly variable based on the location of the basement rocks.

Fig. 2.41 Flow diagram of the Leliso River at Adaba river gauge

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Evaporation and usage of surface water for water supply and irrigation has not been considered in the assessment.

2.4.3 BaseflowThe same gauging stations were used for calculation of baseflow, because these stations have

provided flow data for several years.

Baseflow represents one of the most important types of information on groundwater resources in the basin. The methods were analyzed by Bogena et al. (2005) and it was found by means of a correlation analysis that the appropriate baseflow values can be determined on the basis of daily river discharge data. The baseflow can be identified from a series of observed monthly low-water runoff values (MoLR) as the simplest assessment method. It has been shown that a long-term average of MoLR of a 20-year period is a good approximation for groundwater recharge in unconsolidated rock areas. However, in consolidated rock areas the MoLR values are often affected by interflow leading to a significant overestimation of groundwater recharge. Hence, a more sophisticated hydrograph separation method based on the Kille method is recommended in these areas.

The Kille method (see Fig. 2.43) for calculation of baseflow was used in the study together with separation of hydrographs where baseflow data is deduced from the discharge record of a stream by separating the baseflow component from the total discharge.

The application of the method can be summarized as follows:1. For each month in a year the minimum daily discharge rate (Q in m3/s) was selected. In

total, the number of Q values is n = 12 × length of the record set in years.

Fig. 2.42 The annual variability of the mean annual flow of Leliso River at Adaba river gauge

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2. Sort the n rates into ascending order and plot them against the corresponding orders (i). In general, a subset of points of low discharge in the scatter plot fits on a straight line.

3. The linear zone of the distribution curve represents the baseflow. The MoLR is calculated by means of the gradient m, the number of values n and the axis intercept y0: MoLR = m × n / 2 + y0. If the hydrographic basin is closed (i.e. there is no water flowing in/out from/to an adjacent basin) and the aquifer is in steady state with respect to storage on an annual basis, then the average groundwater recharge rate R = MoLR.

4. Convert R into a value in mm/y, i.e. multiply the value in m3/s by 60 × 60 × 24 × 365 × 1,000 and subsequently divide the result by the drainage area of the basin in m2.

Data on baseflow assessed by the Kille method is shown in Fig. 2.44–2.45 and in Tab. 2.9 together with baseflow data assessed by the hydrograph separation method.

Separation of the hydrograph (see Fig. 2.46) is another method that was used for assessment of baseflow. Baseflow separation techniques use the time-series record of stream flow to derive the baseflow signature. The common separation methods are either graphical which tend to focus on defining the points where baseflow intersects the rising and falling limbs of the quickflow response, or involve filtering where data processing of the entire stream hydrograph derives a baseflow hydrograph.

The graphical method was used for assessment of baseflow for the rivers of the area. The daily flow data were used to plot the baseflow component of a flood hydrograph event, including the point where the baseflow intersects the falling limb. Stream flow subsequent to this point was assumed to be entirely baseflow, until the start of the hydrographic response to the next significant rainfall event. These graphical approaches (Fig. 2.46) to partitioning baseflow vary in complexity and include (Linsley, 1958):

a) the constant discharge method (green line on the chart) assuming that baseflow is constant during the storm hydrograph; the minimum streamflow immediately prior to the rising limb is used as the constant value;

b) the constant slope method (blue line on the chart) connecting the start of the rising limb with the inflection point on the receding limb; this assumes an instant response in baseflow to the rainfall event;

Fig. 2.43 Method of Kille baseflow assessment

MoLR = 0.00060 n/2 + 0.02296

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Fig. 2.44 Kille baseflow separation – gauging stations within Genale-Dawa basin

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Shawe1.12 m3/s

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c) the concave method (violet line on the chart) attempting to represent the assumed initial decrease in baseflow during the climbing limb by projecting the declining hydrographic trend evident prior to the rainfall event to directly under the crest of the flood hydrograph; this minimum is then connected to the inflection point on the receding limb of storm hydrograph to model the delayed increase in baseflow.

Separation of hydrograph and results of separation are shown in Fig 2.47 and Fig. 2.48.

Comparison of the assessment of baseflow using the Kille method and hydrograph separation is shown in Tab. 2.9. Results show very small differences between assessment of baseflow using the Kille method and hydrograph separation.

The assessment of specific baseflow is based on data from flow measurements and using the Kille method. The specific baseflow is assessed for volcanic and basement rocks separately.

Fig. 2.45 Kille baseflow separation – gauging stations within Wabe Shebelle-Dawa basin

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-0,5

-0,75

-1

-1,25

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Ukuma0.11 m3/s

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Maribo0.56 m3/s

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Herero0.52 m3/s

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Lelisso0.3 m3/s

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The specific baseflow is not assessed for sedimentary rocks. The specific baseflow for aquifers developed in volcanic rocks was assessed to be 4.1 l/s.km2 based on data from all stations located in the Dodola sheet and its near surroundings. The specific baseflow for aquifers developed in basement rocks was assessed to be 5.3 l/s.km2 based on data from the Gambetu River at Aroresa with consideration of data from the Upper Genale River at Girja and Chenemasa in the highlands where catchments receive high volumes of precipitation (depth). This assessment is a general one and specific runoff will be highly variable based on the location of basement rocks.

Fig. 2.46 Method of baseflow separation

b

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Shawe (2003)1.99 m3/s

Fig. 2.47 Hydrograph baseflow separation – gauging stations within Genale-Dawa basin (Part 1)


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Yadot (2005)2.12 m3/s

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Welmel (1988)8.63 m3/s

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Deyiu (1999)0.51 m3/s

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Tegona (2000)0.42 m3/s

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Fig. 2.48 Hydrograph baseflow separation – gauging stations within Wabe Shebelle-Dawa basin (Part 1)

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Fig. 2.48 Hydrograph baseflow separation – gauging stations within Wabe Shebelle-Dawa basin (Part 2)

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Maribo (2003)1.25 m3/s

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Leliso (2001)0.66 m3/s

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Tab. 2.9 Baseflow data for the Dodola area

Map ID RiverArea [km2]


Kille method [m3/s]

Hydrograph separation [m3/s]

Specific baseflow[l/s.km2]

Aquifer

RG11Genale(Chenemasa)

9,190.3 10.0 44.46 45.56 4.84/4.96Volcanic/basement

RG12 Mana 272.5 1.8 0.10 0.17 0.37/0.62 Volcanic

RG13 Shawe 193.7 15.9 1.12 1.99 5.78/10.27 Volcanic

RG14 Yadot 451.9 14.8 2.88 2.12 6.37/4.69 Volcanic

RG15 Halgol 182.1 4.1 0.32 0.46 1.76/2.53 Volcanic

RG16 Welmel 1,395.8 12.4 6.80 8.63 4.87/6.18 Volcanic

RG17 Deyiu 111.1 8.7 0.42 0.51 3.78/4.59 Volcanic

RG18 Logita 729.4 21.9 4.29 4.61 5.88/6.32 Volcanic

RG22 Gelana 376.2 18.8 2.62 2.59 6.96/6.88 Volcanic

RG23 Ererte 99.3 13.3 0.97 1.11 9.77/11.11 Volcanic

RG24 Morodo 85.9 10.1 0.29 0.32 3.38/3.72 Volcanic

RG25 Konkona 52.3 19.2 0.26 0.31 4.97/5.96 Volcanic

RG26 Gambetu 270.3 18.1 1.25 1.62 4.62/5.92 Basement

RG27U.Genale(Girja)

3,177.4 23.3 24.64 39.34 7.75/12.38Volcanic/basement

RG28 Dumel 207.5 No data Volcanic

RG29 Denka 89.6 9.4 0.21 0.98 2.34/10.88 Volcanic

RG31 Weyb 794.0 5.8 0.53 0.51 0.67/0.64 Volcanic

RG32 Weyb 1,048.0 3.9 0.44 0.49 0.42/0.47 Volcanic

RG33 Weyb 3,722.6 3.3 0.93 2.82 0.25/0.75 Volcanic

RG34 Shaya 450.9 9.6 0.74 0.88 1.64/1.95 Volcanic

RG35 Tegona 84.4 16.6 0.21 0.42 2.49/4.98 Volcanic

RG37 Weyb 4,546.3 5.2 2.69 6.11 0.59/1.34 Limestone

RG39/H1 Wabe 1,040.0 1.7 1.50 1.11 1.44/1.07 Volcanic

RG41/H3 Ukuma 137.0 11.0 0.11 0.5 0.80/5.84 Volcanic

RG45/H7 Herero 122.0 12.8 0.52 0.7 4.26/5.74 Volcanic

RG50/H12 Maribo 192.0 15.6 0.56 1.25 2.92/6.51 Volcanic

RG49/H11 Lelisso 120.0 11.7 0.30 0.66 2.50/5.50 Volcanic

RG42/H4 Fruna 86.0 36.9 0.20 0.26 2.33/3.02 Volcanic


2.5 Water BalancePrecipitation is partly evaporated, partly transpired and part of the water flows to rivers as runoff

(surface runoff and baseflow). The rest of the water infiltrates into aquifers. The balance was studied for Goro meteo-station, which is located on the volcanic plateau and river gauging stations on the surrounding rivers. The upper part of the aquifer developed in volcanic rocks is drained as shallow local baseflow on the plateau which is represented by the Robe river gauging stations on the Shaya River or the Goro river gauging station on the Tegona River. The aquifer developed in volcanic rocks is totally drained by deeper local drainage occurring below the escarpment and is represented either by the Delo Mena river gauging stations on the Yadot River, the Deyu Harewa river gauging station on the Deyiu River, or the Melke Amana river gauging station on the Welmel River. Deep regional drainage aquifers developed in volcanic and sedimentary rocks are totally drained by deep regional drainage which is measured between river gauging stations Chenemasa and Haloway. Data for assessment of water balance are shown in Tab. 2.10.

The water balance assessment is based on the following considerations:• The average monthly precipitation from Goro meteo-station (Tab. 2.11) represents the input

recharge for the whole plateau.• The average monthly evapotranspiration in Goba meteo-station is shown in Tab. 2.11.• Infiltration into the shallow local aquifer is represented by Shaya and Tegona baseflows.• The deficit in the water balance of Shaya or Tegona basins represents infiltration into deeper

aquifers and its value is manifested as deeper local and/or regional baseflow. Infiltration into deeper aquifers can be expressed by the equation I

deeper = P

precipitation – Et

evapotranspiration – TR

total runoff.

• Infiltration and formation of deep local baseflow was computed for the sub-basins of Yadot, Deyiu, Welmel and Halgol rivers from catchments located on the plateau and escarpment. The rate

Tab. 2.10 Water balance input data

RiverGauging station

Area[km2]

Base flow[m3/s]

Base flowtype

Infiltra-tion[mm/year]


Mean flow[m3/s]

Mean base-flow rate

Genale Chenemasa 9,190.3 44.46 deep local 152.7 4.8 92.12 0.48

YadotDeloMena

451.9 2.88 deep local 201.1 6.4 6.67 0.43

DeyiuDeyuHarewa

111.1 0.42 deep local 119.3 3.8 0.97 0.43

WelmelMelkaAmana

1,395.8 6.80 deep local 153.7 4.9 17.34 0.39

Shaya Robe 450.9 0.74 shallow local 51.8 1.6 4.31 0.17

Tegona Goba 84.4 0.21 shallow local 78.5 2.5 1.40 0.15

Genale

between Chenemasaand Haloway minus its si-nistral tribu-taries

47,392.6 13.30deep regional

8.9 0.3 65.40 -

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between surface and baseflow, which is higher compared to the river gauging stations represents shallow local baseflow and deep regional flow. This condition shows that the groundwater catchments of these rivers are possibly bigger than their surface catchments and groundwater infiltration on the highest parts of the plateau also participates in deep local and regional baseflows. This condition is also common in relatively homogeneous aquifers (e.g. aquifers in volcanic rocks) where the groundwater level gradient is not uniform in both directions from the groundwater divide which is caused by the steep gradient of the erosion escarpment (Harenna escarpment).

• Groundwater of deep regional baseflow is drained by the Genale River particularly in the segment between river gauging stations Chenemasa and Haloway and the difference in baseflows between both river gauging stations represents deep regional base flow (calculated deep local baseflow of Welmel River was subtracted from this difference as well as deep local baseflow of Dumal, Wabera and Wabe Mena rivers which was assessed by analogy with Welmel).

• Not all components of base flow fluctuate and are stable during the year.

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Tab. 2.11 Water balance of Shaya basin

Month/parameter

Units Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Precipitationmm/year

Goro 20 28 67 201 155 37 26 48 86 173 58 16 915

Evapotranspi-ration

mm/year

Goba 101 100 116 98 101 98 94 95 89 80 83 95 1,149

Total runoffmm/year

Shaya 7 7 5 27 26 11 30 51 38 61 28 13 302

Deep local and regional flow

mm/year

77 28 32 137

Tab. 2.12 Comparison of water losses in water balance with estimated deep base flow

Base flow Source of data Balanced value [mm/year]

Shallow local base flow (included in total runoff of Tegona)

Tegona 78.5

Deep local and regional flow Water balance for Shaya 137

Deep local and regional flow Water balance for Tegona 207

Deep local base flow Yadot, Welmel, Deyiu 157

Deep regional base flowGenale between Chenemasa and Haloway minus its sinistral tributaries

14

Difference between water balance of Shaya and deep base flows

Shaya -34

Difference between water balance of Tegona and deep base flows

Tegona 36


The highest monthly precipitation occurs in April, May and October. During these months not all water volume is consumed either by evapotranspiration or by run off and rest of the water can infiltrate into the first aquifer developed in volcanic rocks (shallow part of aquifer). Assessment of total volume of infiltration for the Shaya basin is 137 for mm/year (Tab. 2.11) and 207 mm/year for the Tegona basin.

Comparison of infiltration into deeper aquifers from the Tegona and Shaya basins with calculated deep local baseflow of 157 mm/year and deep regional base flow of 14 mm/year revealed a difference of -34 mm/year for the Shaya basin and 36 mm/year for Tegona basin (Tab. 2.12). Calculated deeper baseflows are more or less in equivalence with balanced infiltration from the Tegona and Shaya basins into deeper aquifers.

The presented water balance is calculated based on available data and demonstrates a system approach to assessment of hydrological and hydrogeological data and is in conformity with the conceptual hydrogeological model presented in Chapter 4.

2.6 Drought and Climate ChangesThe whole Ethiopian territory is often affected by reoccurring droughts causing famine. The impact

of drought is severe in both the arid lowlands as well as the highlands of Ethiopia. The existence of drought and desertification is well known from geological and archeological evidence as well as from historical documents and on-going measurements. It is matter of fact that the centre of the Ethiopian civilization was shifted about 1,000 km from Axum in the dry north to Addis Ababa located in the more humid centre of the current (modern) Ethiopia over the last 2,000 years. The northern and eastern parts of the country appeared to be highly vulnerable to reoccurring drought and famine. The most drought-prone regions of Ethiopia are shown in Fig. 2.49.

There are many causes of drought, starting with a local deficit of vapor and condensation nuclei and changes in land use causing changes in soil reflectivity etc., to global changes related to the greenhouse effect with the warming of the surface water of tropical seas. Climate change is dangerous because it can accelerate irregularities in the behavior of synoptic weather systems over the country which is one of the main reasons for the failure of the seasonal rains. Geological and historical evidence was described in detail by Brooks (draft, 2005) and Sima (2009).

The study of NMSA (1996) considers an occurrence of meteorological drought when seasonal rainfall over a region is less than 19 % of its mean. In addition, a drought is classified as moderate and severe if seasonal rainfall deficiency is between 21–25 % and more than 25 %, respectively. A year is considered to be a drought year for the country as a whole in the case the area affected by one of the above criteria for drought, either individually or collectively, is more than 20 % of the total area of the country. The study of drought incidence, intensity and frequency within the whole Ethiopian territory takes into consideration data from the period 1969 to 1987 resulting in the following:

1. Occurrence of drought in the Belg season in 1971, 1973, 1975, 1977, 1984 and 1986 affected more than half the regions. The year 1975 was the most serious, including in the Bale region. The impact is considered to be catastrophic if drought occurs continuously for three or more years.

2. Occurrence of drought in the Kiremt season has more of an effect because 95 % of crop production relies on these rains. Drought occurred in 1972, 1984 and 1987 of which the latter affected about 70 % of the country, including a part of the Sidamo region.

3. Occurrence of drought in both the Belg and Kiremt seasons (drought year) in 1973 and 1984 with failure of rain in 6 out of 14 regions.

The study revealed that Belg drought was serious in the Bale and Sidamo areas in (severe drought in bold italics) 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, and 1984 (Sidamo),

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which puts the Bale area in the third place in drought probability in Ethiopia after Tigray and Wollo araes. The Kiremt drought was serious in the Bale and Sidamo areas in 1969, 1970, 1971, 1972, 1974, 1976, 1977, 1979, 1980, 1984 (Sidamo) and 1987 (Sidamo), which puts the Bale area in first place in drought probability in Ethiopia during the Kiremt season, in front of the areas of Gonder and Haraghe. Drought was serious in the Bale and Sidamo areas in 1969 (Sidamo), 1973, and 1977 showing the Bale region as having the highest probability of drought during the whole year.

Climate ChangeCurrent climate change poses a significant challenge to Ethiopia by affecting food security, water

and energy supply, poverty reduction and sustainable development efforts, as well as by causing natural resource degradation and natural disasters. For example the impacts of past droughts such as those of 1972/73, 1984 and 2002/03 are still fresh in the memories of many Ethiopians. Floods in 2006 caused substantial loss to human life and property in many parts of the country. In this context, planning and implementing climate change adaptation polices, measures and strategies in Ethiopia will be necessary.

The agricultural sector is the most vulnerable to climate variability and change. In terms of livelihoods, small scale rain-fed subsistence farmers and pastoralists are the most vulnerable.

Fig. 2.49 The most drought prone areas of Ethiopia (source: RRC, 1985)

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The annual minimum temperature is expressed in terms of temperature differences from the mean and averaged for 40 stations. There has been a warming trend in the annual minimum temperature over the past 55 years. It has increasing by about 0.37 °C every ten years. The trend analysis of annual rainfall shows that precipitation remained more or less constant when averaged over the whole country.

For the IPCC mid-range (A1B) emission scenario, the mean annual temperature will increase in the range of 0.9–1.1 °C by 2030, in the range of 1.7–2.1 °C by 2050 and in the range of 2.7–3.4 °C by 2080 over Ethiopia compared to the 1961–1990 normal. A small increase in annual precipitation is also expected over the country.

The other climate related hazard that affects Ethiopia from time to time is flooding. Major floods occurred in different parts of the country in 1988, 1993, 1994, 1995, 1996 and 2006. All of them caused loss of life and property.

In recent years the environment has become a key issue in Ethiopia. The main environmental problems in the country include land degradation, soil erosion, and deforestation, loss of biodiversity, desertification, recurrent drought, flood and water and air pollution.

A large part of the country is dry sub-humid, semi-arid and arid, which is prone to desertification and drought. The country has also fragile highland ecosystems that are currently under stress due to population pressure and associated socio-economic practices. Ethiopia’s history is associated – more often than not – with major natural and manmade hazards that affect the population from time to time. Drought and famine, flood, malaria, land degradation, livestock disease, insect pests and earthquakes have been the main sources of risk and vulnerability in most parts of the country. Especially, recurrent drought, famine and recently floods are the main problems that affect millions of the country’s population almost every year. While the causes of most disasters are climate related, the deterioration of the natural environment due to unchecked human activities and poverty has further exacerbated the situation.

The major adverse impacts of climate variability in Ethiopia include: • Food insecurity arising from the occurrence of droughts and floods. • Outbreaks of diseases such as malaria, dengue fever, water borne diseases (such as cholera,

dysentery) associated with floods and respiratory diseases associated with droughts. • Heavy rainfalls which tend to accelerate land degradation. • Damage to communication, road and other infrastructure by floods.

For example in 2006 flooding in the main rainy season (June – September) caused the following disasters (NMA, 2006): • More than 250 fatalities and about 250 people unaccounted for in Dire Dawa flood. • More than 10,000 people in Dire Dawa became homeless. • More than 364 fatalities in Southern Omo and more than 6,000 (updated to 8,350 after August

15) people were displaced over Southern Omo, where around 14 villages were flooded. • More than 16,000 people over West Shewa were been displaced. • Similar situations also occurred over Afar, Western Tigray, Gambella Zuria and over the low lying

areas of Lake Tana.

In terms of loss in property and livestock • The DPPA estimate is about 199,000 critically affected people due to the flood in the country. • More than 900 livestock drowned over South Omo. In addition, 2,700 heads of cattle and 760

traditional silos were washed away (WFP). • About 10,000 livestock encircled by river floods in Afar. • Over Dire Dawa, the loss in property is estimated in the order of millions of dollars.

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The geology of the Dodola area is part of the Ogaden basin. The sheet area is a small portion of the southeastern Ethiopian plateau. Geologically, the Dodola map sheet is covered by Precambrian crystalline basement rocks, Mesozoic Sedimentary units at the middle, Cenozoic volcanic rocks at the top with a very recent sediment deposit.

Geologically, the Dodola map sheet is covered by Precambrian crystalline basement rocks, Mesozoic Sedimentary units in the middle, Cenozoic volcanic rocks at the top which are covered by Quarternary sediment in places. Eight Cenozoic volcanic map units were delineated in the area and seven of these are of Tertiary age while the eighth is of Quaternary age. Four Mesozoic sedimentary units were also identified in this area, consisting of different verities of both sandstone and limestone units. The Precambrian basement rocks covering the Dodola map sheet are understood to be low and high grade metamorphic rocks. Plutons of Precambrian age are also present.

3.1 Previous WorkNumerous geological investigations have been conducted in the Dodola map sheet area and

further to the southwest in Adola and its surroundings for the existence of gold mineralization. The earliest geological investigation started in the early 1940s after the discovery of placer gold by local people in the area of Adola. Some of the earlier works include: Jelenc (1966) in his compiled work, during his investigation for economic mineral deposits of Ethiopia, divided the rocks of the Adola region into high grade and low grade series. Subsequent works (Gilboy, 1970; Chater, 1971) modified this concept and proposed a threefold classification for the rocks of the Adola region and its surroundings i.e. Lower, Middle and Upper Groups.

Mohr (1962) grouped the Mesozoic Sedimentary rocks of the area into Adigrat sandstone and Antalo limestone formations. Belay (1978) produced a preliminary geological map of the Dodola sheet at 1:250,000 scale with an accompanying report and grouped the Precambrian rocks of the area into the Alghe and Bidimo gneiss and the lower Wadera and Adola groups. He correlated the Mesozoic rocks with the Hamanlei formation, while volcanic rocks were mapped as the Alaje formation, Arroresa trachyte, Nazereth group (Dodola and Damole Ignimbrite), Hora Bora Ignimbrite, Sanate Basalt and Sub-resent to resent basalts. BEICIP (1985) in their 1:1,000,000 scale photo geological map showed bioclastic limestone of the Upper Hamanlei formation and Tertiary to recent volcanic rocks in the Dodola map area. Kazmin (1972) grouped the Precambrian rocks of the area under the Alghe and Awata biotite and biotite-amphibole gneiss (lower complex) and the Wadera group metasediment (middle complex). He sub-divided the Mesozoic sedimentary rocks into Dibigia, and Genale Doria formations and the Cenozoic volcanic into the Ashangi (Paleocene – Miocene) and Shield (Miocene) group of the Trap series and the Magdala group (upper Miocene – Pleistocene).

3.

Geological Settings

3. Geological Settings

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The Genale-Dawa and Wabe Shebelle river basin integrated resources development master plan studies identified Urandab/Hamanlei limestone, Korahe gypsum and Metamorphic rocks which were later classified based on their hydrogeological characteristics.

Bosellini (1989) has exhaustively dealt with the lithostratigraphic sequence of the continental margin of Somalia and the surrounding regions. In his work, intermittent basin evolution is proposed to be caused by subsequent episodic subsidence and uplifts associated with the major catastrophic events in the Horn of Africa during the Mesozoic era. Hambisa et al. (1997) mapped four uninterrupted Mesozoic sedimentary successions with gradational contacts in the Dodola area.

Report on The Sothern Rangelands Livestock Development Project by Agrotec-C.R.G. – S.E.D.E.S. Ass. (1974) also describes basic data on the geology of the area.

3.2 StratigraphyThe geology of the Dodola area consists of a variety of litho-startgraphical units ranging from the

Precambrian metamorphic basement to Mesozoic sedimentary sequences and Cenozoic volcanic rocks. The Quaternary volcanic rocks and sediments also cover some areas on the sheet. The late Proterozoic basement rocks are overlaid by Jurassic to Cretaceous sedimentary sequences. The Jurassic limestone gradationally overlie the lower lying basal clastic succession and form a more than 700 m thick sequence of limestone, calcareous sandstones and intraformational conglomeratic breccia horizons. Cretaceous sandstone represents the final layer of Mesozoic

Geological Settings

Tab. 3.1 Litho stratigraphy of the mapped area

Age FormationAverage thickness [m]

Generalized lithological description

Quaternary

Alluvial and Eluvial deposits Gravel sand and clay

Volcanic rocks (V)Scoracious basalts and minor cinder cones and vitric tuff

Tertiary Plateau volcanic rocks various Basalt and trachyte

Me

sozo

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CretaceousSubarcose with calcareous sandstone (Jessoma sst)

Sandstone

Jurassic

Upper Hamanlei Jh2 Melmel limestone

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Pelletal oolitic grainstone chalky limestone

Lower Hamanlei Jh1Jerder limestone

Bioclastic limestone and dolomite

Sandstone with basal conglomerate (Adigrat sst)

Sandstone

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Low grade Metavolcano-sedimentaryrocks and mafic-ultramafic complexes

High grade Gneisses, migmatite and schist

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sedimentation. The Cenozoic volcanic flows overlie non-conformably both the Proterozoic basement rocks and the Jurassic limestone and Cretaceous sandstone.

A general stratigraphy scheme of the area with the age of the formations is shown in Tab. 3.1.

3.3 LithologyThe description of the lithological units is mainly taken from 1:250,000 geological mapping

of the Genale-Dawa river basin integrated resource development master plan study (Lahmeyer international, 2005) and the geological map of the Dodola sheet in 1:250,000 scale by Belay (1978).

3.3.1 Crystalline Basement Rocks and Associated Intrusive RocksPrecambrian metamorphic rocks cover about 25 % of the study area. These rocks occupy the

southwestern part of the map area Fig. 3.1. Towards the north and east they are covered by Tertiary volcanic and Mesozoic sedimentary units, respectively. The basement is composed of various gneisses as well as schists and felses of psammaitic to pelitic composition. These crystalline metamorphic rocks are intruded by gabbro (Pga), mafic-ultramafic rocks (Pum), foliated biotite granite (Pgtf) and massive biotite granite.

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Fig. 3.1 Granitic rock near Hora Kore

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Based on metamorphic grade the basement rocks can be divided into older and high grade metamorphic rocks comprising biotite gneiss (Pbg), hornblend-biotite gneiss (Phbg) quartzo feld sphatic schist, fels and gneiss (Pqfr), mylonitic quartezofeldsphatic gneiss (Pqfg) and slightly sheared biotitic gneiss and lesser mylonite (Pmg) and younger low grade metamorphic rocks. The low grade metamorphic rocks occupy a small area in the southwestern corner of the map sheet and are lithologically composed of mica schist and lesser chlorite schist (Pms), graphite schist and minor marble (Pgm) and metamorphic-ultramafic rocks (Pum).

3.3.1.1 High Grade Rocks

This is the predominant map unit in the basement terrain forming topographically low areas dissected by many streams and ridges in places. They are dark to light gray reddish, pinkish, rarely mottled, generally leucocratic to mesocratic (color index up to 40) and commonly medium grained with a few fine and coarse grained verities.

The rocks are strongly gneissose, the gneissosity being defined by the segregation of stretched felsic minerals on the one hand and mafic minerals on the other. The rocks are mostly weak to deeply weathered with occasionally friable exposures. The weathered friable quartzofeldsphatic gneiss outcrops along Gobele river valley.

This high grade metamorphic rock shows a fresh appearance in a few places and contains the different main rock units.

3.3.1.2 Low Grade Rocks

This unit is exposed at the extreme southwestern corner of the area and discontinuously appears at very few parts of the southern center. Precambrian low grade metamorphic rocks are broadly composed of meta sedimentary rocks and associated intrusive rocks. This Precambrian low grade meta sedimentary rock is composed of mica schist and lesser chlorite schist (Pms) and graphite schist and minor marble (Pgm).

3.3.1.3 Associated Intrusive Rocks

The intrusive rocks in the Dodola map sheet are dominantly granitic in composition. They are represented by foliated biotitic granite (Pgtf) and massive biotitic granite (Pgt). Precambrian intrusive rocks also contain meta-gabbro (Pga) and metamorphosed mafic-ultramafic rocks (Pum).

3.3.2. Mesozoic Sedimentary FormationsThis unit includes sandstone with lesser basal conglomerates (Jsst), sparry and microcrystalline

allochemical limestone with lesser microcrystalline limestone (Jlst1), microcrystalline and allochemical limestone (Jlst2) and subarkosic with lesser calcareous sandstone (Ksst). The formations lie one upon the other with gradational contacts indicating lateral facies migration and a sedimentation cycle starting from clastic to chemical sediments and ending again by clastic sediments.

In the literature, the Mesozoic sedimentary rocks have been divided into two different successions on the base of their presumable age (e.g. Kazmin, 1972). The lower carbonate succession is referred to be Jurassic while the upper carbonate succession is considered to be of Jurassic to Cretaceous age. The Dodola sheet consists of sedimentary rocks of the lower carbonate succession which is represented by the Hamanlei formation and has organogenic and oolitic limestone with shale and sandstone which grades southward (the present Somalia coast) into deeper water shale and limestone (BEICIP, 1985). According to Yihunie and Tesfaye (1998) this unit is further classified into two major sequences of limestone based on recognizable angular unconformity.

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3.3.3 Tertiary Volcanic Rocks The Bale massive volcanic succession attains its maximum thickness of 2,300–2,400 m in the

northern part of the map area (Fig 3.2). Generally the succession is divided in to eight major volcanic units consisting of aphyric to porphyritic basalts (Tapb), ankaramite and lesser olivine phyric basalt (Tak), alkali trachyte flows with some plugs (Ttr), aphyric basalt (Tab), trachytic tuffs with minor basalt and alkali trachyte flows, sandstone and limestone (Tps), interlayered alkali trachyte and basalt flows (Tbt), alkali trachyte flows and minor plugs (Tpt) and acoriacious aphanitic to slightly porphyritic basalt and associated rocks (Qsab). Syentic intrusions (Tsy) are also present in lava flows.

3.3.4 Quaternary Sediments and BasaltsThis unit includes river deposits along river valleys and wind deposits on the top of flat forming

volcanic highlands and a horizontal table forming limestone units. It was commonly mapped as Elluvial and Alluvial sediments (Qs).

Quaternary lava flows are found in river valleys and form patches within the mapped area.

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Fig 3.2 Bale basalt at the Sanetti Plateau

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3.4 StructureThe Phanesozoic marine record of East Africa and the surrounding region is mainly governed

by extensional deformation related to the break up of Gondawana land, starting at Permian. It produced a northeasterly trending rift and northwesterly trending transverse fault system. The main rift gave rise to the present Indian Ocean whose major faults run along the eastern coast of Africa (Kenya and Somalia). In association with this, a triradial system of E-W, NE-SW and NW-SE trending grabbers developed. As a result of the opening of the North Atlantic and Proto-Indian oceans the triple junction of these grabens has been identified in the Southern Ogaden, Calub area.

The development of various tectonic structures in the Precambrian gneisses and schist of Dodola map sheet were occurred when this unit subjected to several deformational episodes. According to structural map of Dodola map sheet planar and linear fabrics, folds, shear zones and faults were developed in the area. Generally, two main phases of deformation were identified in the Precambrian rocks on the basis of superimposition or cross cutting relationships.

The biotitic and quartezofeldsphatic gneiss (Pbg and Pqfg) of both sides of the Melka – Amana – Negele road show strong mylonitization. Some kinematic indicators like mesoscopic folds with northerly trending outer limbs which were recognized in the Pbg unit about 4 km NNE and 2.5 km WSW of Melka Amana and macroscopic z folds with rounded hinges which were seen in the Chemeri area suggest that the area has been affected by N-S dextral strike slip shearing. Similar indicators of N-S dextral shearing are present throughout the Precambrian high grade terrain from the area of Agangaro on the west to the area of Welmel River in the east.

Faults occur on a mesoscopic to macroscopic scale. They manifest themselves mainly in the basement and to a certain extent in the younger cover rocks showing displacement of centimeters to a few kilometers. Almost all the faults appear to be strike slip. Various sets of both definite and probable faults were recognized. Most are NW trending, with subsidiary NE, NNE, NNW and E-W oriented sets and control the flow directions of some rivers in the area.

Within the center of the Bidimo shear zone, the Hore-Kore hot spring (Hsp-1) with multiple eyes has developed as a result of this structure cutting the water table in the area. In addition, Garbigalo (Csp-17) and Lalafto (Csp-18) with a dry period discharge of 0.018 and 0.93 l/s, respectively are developed along this shear zone in the basement rock of the area. Hara haji (Csp-11) with 2 l/s and Serbo dug well (Dw-4) in turn developed along the Gobele shear zone while most shallow wells (Warie shw-1, Oborso shw-2 and Oda shallow well currently not functional) and dug wells (Erbaweni DW-5, Oborso DW-6) are developed within Oborso graben. In the NE part of the area Harawa (Csp-48) and Cheketa (Csp-47) with 90 and 16 l/s dry period discharge springs, respectively are also developed from visible local fractures. The hydrogeological map of the area illustrates where these water points overlap along the structures.

Therefore, the complex structures of the Dodola map sheet highly control the occurrence and movement of groundwater in volcanic and sedimentary aquifers in the area, as well as the occurrence and movement of groundwater in Basement rocks in the area.

3.5 Geological HistoryThe geological development of the area has built gradually over several geological cycles. The first

geological cycle is of the Precambrian age and includes metamorphic and ultrametamorphic rocks. The second cycle ranges from the Jurassic to the Cretaceous and is represented by sedimentary rocks which rest non-conformably on the Precambrian. This cycle is mainly composed of a clastic

Geological Settings

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to carbonate series with evaporate intercalations deposited in marine and lagoon environments. The third cycle is mainly composed of volcanic rocks connected to Tertiary volcanism. The Dodola sheet is covered by the Precambrian basement, Mesozoic sediments, Tertiary volcanic rocks, Quaternary volcanic rocks and sedimentary deposits.

Construction of the Proterozoic complex basement assemblage located in the south western part of the map area reflects a reworking of existing materials; accretion and collision events, and the addition of new lithosphere via magmatism associated with sea-floor spreading and continental rifting. The approximate N-S structural grain elements imposed by this evolution has controlled the deposition of phanerozoic materials and most importantly the location of widespread Cenozoic volcanism which is almost exclusively restricted to areas affected by pan-African events (Kazmin et al., 1978).

The basement complexes (gneissic terrain and narrow low grade belts) are designated as parts of the Mozambique belt and the Arabian-Nubian Shield, respectively. The crystalline basement rocks are suggested to be affected by the late Proterozoic (Pan-African) deformation, metamorphism, and magmatism and are contemporaneously intruded by syn and post tectonic basic to acid intrusive rocks. Thrust contacts between the gneissic terrain and low-grade belts, are often marked by N-S trending regional lineaments and associated shear zones accompanied with an easterly and westerly dipping mylonitic foliation and reverse drag folds. Following this line of interpretation W/ Haimanot and Behrmann (1995) and Worku (1996) suggested the Precambrian rocks of Southern Ethiopia to constitute a transitional zone between the low grade volcano-sedimentary

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Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after Gani et al., 2008)

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rocks and the mafic-ultramafic complexes of the Arabian-Nubian Shield, and high grade gneisses, migmatite and intercalated schists of the Mozambique Belt.

The late Triassic is a time of regional subsidence during which rifting begins. During this period the progression in the Karroo rifting allowed the deposition of thick clastic rocks of continental origin which become thicker towards the central part of the rift. Sea floor spreading (separation of east Gondwana from west Gondwana) began after a long period of subsidence in the Callovian and early Oxfordian. The floor spreading ended in the early Hauterivian (121-120 Ma). The Jurassic transgression came from the southeast, reaching its maximum limit in Western Ethiopia and Eritrea during the Kimmeridgian. This transgression deposited a sandy formation (Adigrat Sandstone), followed by neritic sediments composed mainly of thick limestone. From the Hauterivian to the early Tertiary, is a time of crustal uplifting and consecutive formation of the Upper Sandstone due to a forced regression of the sea. Tertiary uplift of the Arabian-Ethiopian swell was accompanied by laterization processes and followed by eruption of volcanic trap rocks.

A continental environment prevailed during the late Tertiary and also in the present and leads to the formation of alluvial and eluvial deposits in the area.

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Hydrogeology of the Dodola area is based on the assessment of a large amount of data collected from existing reports and maps and during field work. There is no previous hydrogeological work at a scale of 1:250,000 and full data sets required for geometrical aquifer configuration are scarce. Analogy is used to assess the groundwater potential of units in the study area where no data was found in the field mapping because the map sheets of the Ginnir, Megalo, Filtu, Negele and Sede have been compiled by GSE at the same time and findings in other areas were also used for hydrogeological assessment of the Dodola area.

4.1 Water Point InventoryThe field water point inventory was based on a desk study, during which the relevant

materials like geological and drilling reports and maps and aerial photographs were collected from the regional geology department of GSE. Important climatic and gauging station data and topographic maps were obtained from various offices. The desk study also included preliminary data interpretation and preparation of field maps using satellite images, aerial photographs and a digital elevation model (DEM) of the terrain with the geology as a background.

The hydrogeological map of Ethiopia at a scale of 1:2,000,000 was published by Tesfaye Chernet (1993). He classified the geological units of Ethiopia into four major groups depending on the type of permeability and the extent of the aquifer. This hydrogeological map was the basic document for preparation of the field work. Tesfaye (1993) identified the following units:• Mesozoic limestone (Hamanlei limestone) with fissured and/or karst permeability was classified

as a highly productive aquifer; the specific yield of wells was estimated to be in the interval 0.2 –7.6 l/s.m and total yield of wells with 20 m of drawdown varies in the interval 1.8 –68.4 l/s in highly productive aquifers.

• Volcanic rocks of the highlands and other sedimentary rocks along rivers and plain areas with fissured porosity were classified as moderately or low productive (Warandab series) aquifers; the specific yield of wells was estimated to be in the interval 0.05–1.1 l/s.m and total yield of wells with 20 m of drawdown varies in the interval 0.45 –9.9 l/s in moderately productive aquifers.

• Basement rocks are described as localized aquifers with fracture and intergranular porosity and are characterized as a regional low productive aquiclude.

• Recharge and discharge characteristics were derived for 50 –150 mm/year for the lowlands and 150 –250 mm for the highlands.

• The highlands (in the northwest) were classified as an area with major water resources. These were assessed to be widespread and moderate to large in quantity. Groundwater and surface water are of good chemical quality (TDS less than 500 mg/l). Most of the streams are perennial; there are many cold springs, and the groundwater level is between 0 and 100 m and can be exploited in low relief areas (valleys).

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• The lowlands (in the southeast) were classified as an area with major water resources. These were assessed to be widespread and moderate to large in quantity. Groundwater and surface water have variable chemical quality (TDS 500– 1,500 mg/l), with most of the perennial streams, and with a groundwater level of below 150 m.

• Groundwater chemistry is characterized as being bicarbonate (HCO3) in the highlands and

sulphate in the lowlands.

A complex assessment of hydrogeological data, including water point inventory, hydrological and climatic characterization was carried out by Lahmeyer (2005) in “Genale-Dawa River Basin Integrated Master Water Plan Study Project”, which provided a statistic assessment of borehole yield (Tab. 4.1).

The authors classified geological formations of the Genale-Dawa basin based on observations made in the field and existing data. Examples of the different levels of productivity are given in Tab. 4.2.

Topographic maps of 1:50,000 scale were used during the field work as a base map in addition to 1:60,000 aerial photographs. Existing reports about borehole data were collected from regional water bureaus, NGOs and private drilling companies as well as from direct contact with drillers and geologists in the field. A compass and a GPS were used for navigation and locating the water points. The water points were characterized by location, lithology, and topography, and field measurements of pH, temperature and EC were taken. Pictures and video sequences were captured for documentation and interpretation. Discharge of springs and rivers was measured by

Tab. 4.1 Aquifer classification based on well yield for Genale-Dawa basin

Formation (symbol)

Yield [l/s] Specific capacity [l/s.m] Number of wellsRange Mean Median Range Mean Median

Alluvium 0.50–3.75 2.01 1.47 0.02–8.92 1.657 0.23 7

Basalt (Q) 1.70 2.39 2.00 0.05–0.38 0.160 0.12 7

Basalt (T) 1.50–4.40 3.15 3.15 0.01–1.22 0.339 0.12 10

Ju + Jh 0.83–7.00 2.58 1.50 0.01–35.00 0.04 7

Gt + Qa 0.13–6.50 2.18 1.76 0.02–0.87 0.268 0.10 6

Hm + Qa 0.20–4.67 1.56 0.93 0.02–1.33 0.232 0.08 9

Lm + Qa 1.40–5.00 2.80 2.00 0.11–36.00 0.253 0.29 3

Remark: Gt–granite, Hm–gneiss, migmatite, Ju–Urandab f., Jh–Hamanlei f., Qa–Quaternary alluvium, Lm–limestone

Tab. 4.2 Aquifer classification by Lahmeyer (2005)

Classification Formation name

Low Kohare, basement

Moderate/Low Urandab/Hamanlei

Moderate Gabredare

High/Moderate Volcanic rocks (Basalt)

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the floating, volumetric method and by visual assessment. The static water levels of boreholes with piezometers and open hand dug wells were measured using an electrical sounding deeper. A summary of the field inventory is shown in Tab. 4.3 and an extract from the water point inventory database is shown in Annex 1. Groundwater from water points representing important parts of the area´s hydrogeological system was sampled for chemical analysis (see Chapter 6 and Annex 2). Well logs of selected borehole are shown in Annex 3.

Data Assessment was mainly dedicated to data organization, processing, and interpretation in the form of maps and the text of the presented explanatory notes. Aquifers are classified according to their productivity based on the yield measured in the field and hydraulic properties like transmissivity obtained from pumping test data together with topographic settings and recharge conditions. The geographic information system (GIS) ArcGis was used for compilation of the maps.

4.2 Hydrogeological Classification/CharacterizationThe qualitative division of lithological units is based on the hydrogeological characteristics of

various rock types using water point inventory data. The lithological units were divided into groups with dominant porous and fissured permeability and impermeable rocks. This division served for definition of the area´s aquifer/aquitard system. Since quantitative data such as permeability, aquifer thickness and yield are not adequate or evenly distributed enough to make a detailed quantitative potential classification; analogy was used for characterization of rocks without the adequate number of water points. Hence, the hydrogeological characterization of the study area reveals the following aquifer/aquitard systems:

Units with porous permeability where the groundwater is flowing through and is accumulated in pores of an unconsolidated or semi-consolidated material. Porous materials of Quaternary age are represented by fluvial and colluvial sediments developed in depressions and/or along valleys of former and existing rivers. The porous aquifers are only locally developed and scattered over the study area. The aquifer with porous permeability forming aquifers is expressed on the hydrogeological map in blue.

Units with fissured and karst permeability where the groundwater is flowing through and is stored in fissures developed in limestone and the permeability can be enhanced by karstification along some fissures. Solution phenomena and karstification in the underground drainage of carbonate rocks are controlled by the drainage base level, which may be represented by a perennially draining stream and/or an impervious formation inside the limestone (marlstone, gypsum) and/or by rocks underlying the carbonate aquifer. A carbonate rock surface, with soil or

Tab. 4.3 Summary of field inventory

Water point type Number of inventory Sampled

Borehole (BH) 11 6

Cold spring (CS), Hot spring (HS) 65 63

Dug well (DW) 17 17

River water (Riv) and Lake water (Lk)

4 4

Rain water (RW) 2 2

Total 99 92

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a relatively permeable, less soluble cover is more favorable for initiation of karstification than bare rock. The rock is presumably dissolved most rapidly in the zone between the highest and lowest positions occupied by the watertable. The units with fissured and karst permeability forming productive aquifers are expressed on the hydrogeological map in green.

Units with fissured permeability, where groundwater is accumulating in and is flowing through the weathered and fractured part of volcanic and non-carbonate sedimentary rocks (sandstone). The porosity of lava flows may be high but the permeability is largely a function of a combination of the primary and secondary structures (joints and fissures) within the rock. In addition, the permeability of lava flows tends to decrease with geological time. The pyroclastic rocks between lava flows are generally porous but usually less permeable due to poor sorting. Layers of paleosoil of various thicknesses in between lava flows are also less permeable and consist usually of clay material on the one hand, whereas layers of fluvial and lake sediments between various lava flows can enhance well yield on the other hand. Hence, extensive volcanic ash beds may form semi-horizontal barriers to water movement (infiltration) resulting in lower productivity of basaltic units located at greater depth.

Mesozoic sediments represented by sandstone and Tertiary and Quaternary volcanic formations represented mainly by basalts form aquifers with good fissured porosity. The units with fissured permeability forming productive aquifers are expressed on the hydrogeological map in green. The units with fissured permeability forming only minor aquifers with low productivity are expressed on the hydrogeological map in light brown.

Basement rocks represent fissured aquifers of low potential. The groundwater in the hard rock is practically all stored in the fractured zones and the weathered mantle called overburden or regolith. The depth of fractured aquifer zones is generally no more than 50–70 m below the surface. The fractures will tend to close at depth. The faults and joints in igneous rocks are nearly vertical, except for narrow fractures, which are more or less parallel to the rock surface, sheeting and exfoliation. The greatest permeability is found in the sub-soil zone within the partly decomposed rock. Wells tapping this zone have yields roughly an order of magnitude greater than in the fresh rock. The aquifers are expressed in the model of the hydrogeological map in brown/red.

4.3 Elements of the Hydrogeological System of the Area (Aquifers)

Geological description and qualitative division of various geological units together with their topographical position within the area lead to a definition of elements of the hydrogeological system and its conceptual hydrogeological model. The system consists of the following elements:• Porous aquifer developed in alluvial and colluvial sediments of Quaternary age on the plateau,

along rivers and plains of the lowlands.• Fissured aquifer developed in Tertiary and Quaternary basalts and Mesozoic sandstones on

highlands (plateau) and flat lands with deep valleys of rivers.• Fissured and karst aquifer developed in Mesozoic limestone.• An aquifer developed in fractured zones and the weathered mantle of basement rocks.

The hydrogeological map shows aquifers and aquitards defined based on the character of the groundwater flow (pores, fissures), the yield of springs and the hydraulic characteristics of boreholes. The following aquifers and aquitards were defined:

1. Extensive (173 km2) and moderately productive or locally developed and highly productive porous aquifers (T = 1.1 –10 m2/d, q = 0.011– 0.1 l/s.m, with spring and well yield Q = 0.51– 5 l/s). The aquifers are shown in light blue.

Aquifers consist of Quaternary unconsolidated deposits.

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2. Extensive (13,675 km2) and moderately productive fissured / karst aquifer (T = 1.1–10 m2/d,q = 0.011– 0.1 l/s.m, with spring and well yield Q = 0.51 –5 l/s). The aquifers are shown in light green.

The aquifers consist of Hamanlei limestone and Adigrad sandstone in the lower reaches of rivers, calcareous sandstone of the plateau area and Tertiary volcanic rocks (not forming plugs).

3. Extensive (4,167 km2) low productive fissured aquifers (T = 0.11–1 m2/d, q = 0.0011–0.01 l/s.m, with spring and well yield Q = 0.051–0.5 l/s). The aquifers are shown in brown/red.

The aquifer consists of basement rocks. 4. Minor formation (340 km2) consisting of minor fissured aquifers with local and limited groundwater

resources – Aquitard (T < 0.1 m2/d, q < 0.001 l/s.m, Q < 0.05 l/s). The formation consists of plugs forming trachyte at plateau area (Tpt) and is shown in light brown.

The following detailed hydrogeological characteristics of the aquifers and hydrogeological characteristics of the individual lithological units are described based on field observation in which 99 water points consisting of boreholes, springs and dug wells were inventoried during field seasons of 2010. Unfortunately, only a limited number of wells had drilling reports with hydraulic data (Tab. 4.4).

4.3.1 Local and Moderately Productive Porous AquifersThe porous aquifers altogether make up 173 km2, accounting for less than 1 % of the area and

consist of alluvial, colluvial and elluvial sediments of the Quaternary age. These aquifers are shown on the map in light blue.

Tab. 4.4 Basic hydraulic characteristics of wells in the Dodola sheet

Well ID Depth [m] Aquifer Specific yield [l/s.m] T [m2/d]

BH-2 80.00 Basalt/limestone 0.14 30.24

Shw-1 60.02 Basement 0.02 2.25

Shw-2 44.02 Basement 0.14 2.97

Fig. 4.1 Marsh developed on elluvial deposits in the Meleyu area

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Elluvium and Alluvium (Qa)

The mappable alluvial and elluvial deposits were observed in various parts of the map sheet. Small unmappable deposits occur throughout the area, often along stream and river courses. This unit as a whole consists of unsorted materials like loose, brown to gray sand, silt and clay bearing material. The material is grayish yellow and angular to sub-rounded, mostly in pebble to boulder size. This unsorted character of the material enables the unit to be a good aquifer. The clay and silty materials fill the pore spaces between the sand and boulders to minimize infiltration of water to the aquifer so the elluvial and alluvial sediments usually form flat-lying marshy areas (see Fig. 4.1).

The Quaternary sediments of the sheet are classified as a moderately productive aquifer considering their position at the bottom of valleys along stream channels, flat lands and narrow valleys which are convenient for water storage and water well siting. The thicker cover of Quaternary sediments can be located using simple geophysical measurements, e.g. VES.

4.3.2 Extensive and Moderately Productive Fissured and Karstic AquifersThe fissured and karst aquifers altogether make up 13,675 km2 accounting for 74 % of the

area and consist of fissured and karstic aquifers developed in the Lower and Upper Hamanlei limestone of the Mesozoic age. Fissured aquifers developed in sandstones of the Mesozoic age and aquifers developed in volcanic rocks of Tertiary and Quaternary age. These aquifers are shown on the map in light green. The extent and location of the fissured and karstic aquifers are shown in Fig. 4.2.

The Hamanlei limestone is exposed in the southeastern part of the map sheet where it directly overlays the Lower sandstone. Topographically it forms flat plains, cliffs, and canyons of rivers. Cave-like structures have openings 50 to 100 cm in height and 1 to 2 m in width, and include the development of small stalagmites and stalactites from the floor and roofs of the caves. The

Fig. 4.2 Extent and location of moderately productive fissured and karst aquifers

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limestone forming flat topography is highly fractured and this together with the karst phenomena makes this unit into a good groundwater transmitting and storing unit. The groundwater mostly appears as springs in deep gorges which show high limestone permeability. Springs with high discharge also confirm development of karstification within limestone units.

Springs often emerge along local fractures at the contact between limestone and intercalated shales. The discharge of springs was measured during the dry period and it varies in intervals from 0.5 l/s to 90 l/s with an average discharge of 17.29 l/s. The highest discharge was measured at the Harawa spring which is shown in Fig 4.3 (Csp-48) and in Hatebela spring (Csp-46) which is located nearby. Both springs are near the Harenna erosional escarpment and have contact with an overlying sandwich of aquifers developed in various volcanic rocks. The limestone forms a relatively small outcrop here and direct infiltration from precipitation cannot provide an adequate volume of water for the spring. This fact is supported by a conceptual hydrogeological model (Chapter 4.4) and its idea about substantial recharge of aquifers developed in sedimentary rocks from overlying aquifers developed in volcanic rocks.

As discussed in the conceptual hydrogeological model, direct infiltration into fissured and karst aquifers developed in limestone is limited in the lowlands. The Dodola area, and particularly its northern part, receives an adequate amount of precipitation allowing direct infiltration into the aquifer. This fact is confirmed by the development of cave-like structures with small stalagmites and stalactites.

The Lower sandstone with basal conglomerates is exposed along the Dumal river valley to the southeast and west of Angetu. It is also exposed in a narrow strip along the NNW-SSE oriented Oborso valley (graben) forming an adjacent low-lying, gently undulating area. The sandstone is recharged from hill-forming, mostly highly fractured and foliated basement rock in west and fractured with karst Carbonate in the south east. To the north of Oborso town and Buluk this unit

Fig. 4.3 The Harawa spring Csp-48

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is fully saturated with shallow groundwater. This sandstone covers about 231 km2 and known measurements of discharge show an average of 5.33 l/s for 4 water points.

Subarcose with some calcareous sandstone outcrops in the northeastern part of the map sheet in an area of about 88 km2. It locally forms cliffs and the rocks are moderately to well sorted, non friable to moderately friable with rounded to sub rounded framework grains. Its thickness varies from 60 m to 100 m and few small discharge water points were observed during the field survey but they were difficult to measure and estimate the discharge because of their exposure in the river valley at surface water level. Water points are not perennial and intermittent or no water exposure was found and local settlers in this unit are subjected to a shortage of water even for household consumption, hence it is grouped as a low productive local aquifer.

Data about the yield of various water points from aquifers developed in sedimentary rocks from the Dodola sheet were combined with data from neighboring Ginnir, Negele and Filtu sheets and the frequency of water point yield was plotted in Fig. 4.4.

The springs and wells have an average discharge of 9.9 l/s and a median discharge of 2.7 l/s for the limestone and sandstone of the Ginnir, Dodola, Negele and Filtu sheets. The lowest discharge was measured to be 0.5 l/s and the highest discharge was measured to be 90 l/s. The large differences in the discharge of springs are given by the character of the aquifers developed in limestone of which permeability can be increased by karstification.

Volcanic Rocks

Fractured volcanic rocks are mostly exposed in the highlands forming the Sanetti Plateau, the Harenna erosional escarpment and a relatively flat central part of the sheet between Welmel and Dumal rivers. The volcanic rocks form a thick multilayered aquifer system. This aquifer system includes basalt often scoracious with minor cinder cones and vitric tuffs (Qsab), interlayered alkali trachyte and basalt flows (Tbt), trachytic tuffs with minor basalt and alkali flows and sediments (Tps), aphyric basalt (Tab), alkali trachyte flows with some plugs, minor alkali trachyte tuff and

Fig. 4.4 Frequency of yield of springs and wells in fissured aquifer developed in sedimentary rocks

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basalt (Ttr) and aphyric and porphyritic basalt (Tapb). The extent and location of the fissured aquifers developed in volcanic rocks are shown in Fig. 4.2.

The quaternary basalt has often scoracious layers; it forms minor cinder cones and consists of vitric tuffs (Qsab). The basalt is vesicular and mostly has bolder forming exposures in the highlands and thick massive exposures in the low-lying areas, specifically when filling valleys of former or recent rivers. Seepage and springs with low discharge emerge at the contact between basalt and the underlying rocks. The spring (Csp-43) merging from the contact between bolder forming basalt and the tuff containing underlying formation (Tbt) is a typical example of a contact spring.

Interlayered alkali trachyte and basalt flows (Tbt) cover an extensive flat-topped area in the northern part of the map sheet. Swamps and numerous small lakes are associated with this unit. Hydrogeological characterization is based on 10 water points of which the mean discharge is 1.7 l/s. In general, the unit transmits and stores a considerable amount of groundwater due to layers of slightly weathered, well-fractured and jointed aphanitic basalt and alkali trachyte with interlayered tuffs and ash.

Trachytic tuffs (ignimbrite) with minor basalt and alkali trachyte flows and intercalated sediments (Tps) are exposed over a large area in the west from Serofta village, eastward from the Furuna River and around Mt. Gura Damole, mainly covering relative low-lying areas. The ignimbrite mainly develops hexagonal joints and sometimes sheet joints. Groundwater of the unit is developed by deep wells and some springs emerge from ignimbrite. The unit is characterized based on the discharge of the Serofta spring (Csp-35) with a discharge of about 30 l/s, and together with another 8 springs the unit has an average discharge of 7.8 l/s.

Aphyric basalt (Tab) is exposed along the upper part of the Harenna escarpment. The aquifer is mainly recharged from overlying aquifers because of its topographical position and one inventoried spring has a discharge of 3 l/s.

Alkali trachyte flows with some plugs (Ttr) to form an extensive volcanic unit. Four springs discharge groundwater accumulated in this unit were inventoried with an average discharge of 4.23 l/s.

Fig. 4.5 Forest nearby Alabada spring

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Ankaramite and lesser olivine phyric basalt (Tak) is exposed along the lower part of the Harenna erosional escarpment and extends to the east starting from about 2.5 km south of Rira village. It forms steep slopes, cliffs and very rugged topography in places with numerous small ridges and peaks developed between valleys. Ankaramite is a dark grey massive, usually fresh with shiny surfaces and is hard to break. No water points were observed from this unit.

Aphyric to porphyritic basalts (Tapb) are the oldest and most extensive of the volcanic units in the area. They cover central part of the sheet. The basalts have well developed columnar jointing with variable joint spaces and locally they are moderately vesiculated, with the vesicles being partially to completely filled with secondary minerals. There are many springs and groundwater is also developed by dug wells. Discharge of springs is from seepages to springs with large yields like Albada (Csp-2) and Lancha (Csp-6) springs with a yield of 60 l/s each. These springs are used for public water supply (Delo Mena town). Basalts at the foot of escarpment form deep local drainage for the whole of the Sanetti Plateau. Groundwater discharge in this area also contributes to the development of forests (Fig. 4.5).

Groundwater in aquifers developed in volcanic rocks is mostly under water table conditions; however, semi-confined conditions can be found. These semi-confined conditions are a result of the vertical as well as lateral inhomogeneity of volcanic rocks partly intercalated by various impermeable (tuff, clay and clayey paleo-soil) and permeable (gravel and sand) sediments. In general, the volcanic rocks of the area act as a recharge area for underlying Mesozoic sediments.

Data from the Dodola sheet were put together with data from the neighboring Ginnir sheet and the frequency of their yield was plotted in Fig. 4.6.

The springs and wells have an average discharge of 5.1 l/s and a median discharge of 0.6 l/s for aquifers developed in the volcanic rocks of the Ginnir, Dodola, Negele and Filtu sheets. The lowest discharge was measured to be 0.01 l/s and the highest discharge was measured to be 80 l/s. Large differences in the discharge of springs is given by the character of the aquifer of which the permeability can be increased by tectonic structures.

Fig. 4.6 Frequency of yield of springs and wells in fissured aquifer developed in volcanic rocks

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The Mesozoic sedimentary rocks of the sheet are classified as moderately productive fissured and karstic aquifers considering their position at the bottom of valleys, along stream channels and in flat lands. These aquifers are convenient for groundwater storage and their resources can be developed by wells. The well sites can be located using simple geophysical measurements, e.g. VES. The measurement is important in karst aquifers developed in limestone for estimation of depth to groundwater level (which is usually deep) and in fissured aquifers developed in sandstones and volcanic rocks for the location of zones with higher frequency and openness of fissures additional to estimation of the depth of the groundwater level.

4.3.3. Extensive and Low Productive Fissured AquifersThe basement rocks are classified as a low productive fissured aquifer which makes up 4,167 km2,

accounting for 23 % of the area and consists of various crystalline (metamorphosed and igneous) rocks of Precambrian age. The basement rocks occupy large areas in the southwestern part of the sheet. Aquifers with fissured permeability are shown on the hydrogeological map in brown/red. The extent and location of the low productive fissured aquifer developed in basement rocks are shown in Fig. 4.7

The biotite gneiss sometimes appears massive while quartzo feldsphatic gneiss are highly sheared, fractured, and weathered, but weathering resistance quartz and feldspars leads to the development of coarse residual material. The groundwater emerging from high grade metamorphosed gneisses has low discharge and the water points were mostly observed along streams where this unit is subjected to local fractures. Discharge of springs varies from 0.018 l/s to 4 l/s during the dry period when they were measured in the field.

Low grade metamorphic rock composed of metasedimentary and mainly intrusive rock forms a rugged terrain in the extreme southwestern corner of the area.

Fig. 4.7 Extent and location of the low productive fissured aquifer developed in basement rocks

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Data about the yield of water points from aquifers developed in basement rocks from the Dodola sheet (including Wora Kora hot spring) were combined with data from the neighboring Negele sheet and the frequency of their yield was plotted in Fig. 4.8.

The basement rocks of the sheet are classified as a low productive fissured aquifer considering their position at the bottom of valleys along stream channels, and in flat lands. These aquifers are convenient for groundwater storage and their groundwater resources can be developed by wells. The well sites can be located using simple geophysical measurements, e.g. VES. The measurement is important in fissured aquifers developed in crystalline rocks for location of zones with a higher frequency and openness of fissures additional to the estimation of the depth of the groundwater level.

4.3.4. Extensive Formation Consisting of a Minor Fissured Aquifer with Local and Limited Groundwater Resources – Aquitard

A minor formation (340 km2) consisting of minor fissured aquifers with local and limited groundwater resources or aquitards of the Dodola sheet consist of plugs forming trachyte in the plateau area (Tpt). The formation is shown in light brown. The extent and location of the aquitard is shown in Fig. 4.9.

This unit is exposed in the north central part of the plateau area. The rock near the edges of flows and plugs shows well developed vertical columnar jointing. The joints are usually open and not filled with clay, which leads to rapid infiltration of precipitation and release of infiltrating groundwater into the underlying alkali trachyte flows with tuffs (Tbt). In places where the underlying unit is composed with thick layers of tuffs infiltrated groundwater flows along this contact and emerges as intermittent springs. The springs disappear a few weeks after the rain stops (see Fig 4.10). In places where the trachyte flows are thick with joints filled with silts or/and clay, the unit develops long lasting seasonal and perennial springs.

The plugs form the highest peaks of the plateau, the runoff is relatively fast and infiltration is limited.

Fig. 4.8 Frequency of yield of springs and wells in fissured aquifers developed in basement rocks

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Fig. 4.9 Extent and location of aquitard

Fig. 4.10 Intermittent springs at the contact of Alkali trachyte flows (Tpt) and tuff (Tbt)

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Due to the limited recharge and low capacity to store groundwater the plugs forming trachyte in the plateau area (Tpt) are considered as the aquitards of the sheet.

Infiltrating rain water penetrates to greater and greater depths and saturates the aquifer. Limited recharge is also supposed directly through the outcrops of the aquifer.

4.4 Hydrogeological Conceptual Model The general concept of infiltration and groundwater circulation in southeastern highlands and

adjacent lowlands is shown in Fig. 4.11.

Precipitation infiltrates in the highlands into aquifers developed in outcropping volcanic rocks. Infiltrated groundwater forms shallow local groundwater flow which is drained by the local perennial and/or intermittent rivers of the plateau area. Some of the groundwater infiltrates to deeper aquifers developed by deeper volcanic as well as sedimentary rocks. This deep groundwater flows northwest to the rift valley and to the southeast to the adjacent lowlands. The groundwater that forms deep

local groundwater flow is drained by large springs in deep valleys and/or the foot of the erosional (Harenna) escarpment as big springs and feeds perennial rivers (e.g. the Yadot River). The remaining groundwater penetrates even deeper and forms deep regional groundwater flow that recharges aquifers in sedimentary rocks. The deep regional groundwater flow is drained by the main perennial rivers of the lowlands (Genale, Dawa, Wabe Shebelle rivers) and their main tributaries. Direct infiltration into aquifers developed in sedimentary rocks in the lowlands is limited because of limited precipitation in this area; however it contributes to the development of shallow local groundwater flow that is drained by intermittent rivers of the lowlands and also contributes to deep local and deep regional circulation that is drained by the rivers of the lowlands mentioned above. Limited direct infiltration into the aquifers of lowlands was confirmed during inspection of the Sof Omar cave,

Fig. 4.11 Conceptual hydrogeological model of southeastern highlands and lowlands

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whose roof is without any stalactical features. Local people also reported that no water is dripping from the clearly visible and relatively open fractures developed in the roof of the cave during the rainy season (Fig. 4.12). Cave-like structures found on the Dodola sheet have openings 50 to 100 cm in height and 1 to 2 m in width, including the development of small stalagmites and stalactites from the floor and roofs of these caves. These karst features confirm the existence of direct infiltration from precipitation in areas receiving an adequate volume of precipitation.

Basement rocks outcropping in valleys of the Genale, Welemel and Dawa rivers form the total drainage level of the area. Large outcrops of basement rocks on the Negele and Dodola sheets form separate low productive fissured aquifers recharged directly by enough precipitation to form good groundwater resources. This direct infiltration forms shallow local groundwater flow which is drained by local permanent rivers (e.g. the Upper Genale, Awata and Mormora rivers). The aquifer developed in basement rocks is also recharged by deep regional groundwater flow. This deep groundwater circulation leads to the formation of Wora Kora hot springs.

The groundwater divide between the main Genale-Dawa and Wabe Shebelle catchments is difficult to define because there is not enough data and the surface water divide should not conform to the groundwater divide. It is necessary to consider that the deep regional groundwater flow in the Dodola area follows the general dip of the whole Ogaden basin to the east (southeast).

The principles of the general conceptual model of the southeastern highlands and adjacent lowlands can be applied to the area of the Dodola sheet. There can be three main mechanisms of recharge in the Dodola area as follows:• direct recharge to outcropping aquifers, • recharge from rivers during high waters,• transfer of groundwater by deep regional groundwater flow from areas northwest of the sheet

with better infiltration potential (Sanetti Plateau).

Fig. 4.12 Fissures in the roof of the Sof Omar cave

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Recharge to aquifers is mainly direct through overlying soil and elluvial cover; however, the position of the aquifer in the lowlands with a low precipitation depth and limited surplus of water for infiltration also causes limited direct recharge of aquifers. There is also the possibility of recharge from areas with higher precipitation in western and northern parts of the sheet. Infiltrated water flows in the aquifer from the northwest to the southeast following the general dip of the basin and the hydraulic gradient towards the east. Local recharge of aquifers is also possible from rivers during high waters. Infiltrated groundwater mainly forms deep regional groundwater flow and is discharged in the dry period directly to the Melmel and Genale rivers and their main tributaries. Discharge of groundwater by springs is common at the foot of the Harenna escarpment and is also common in the lowlands. Intermittent rivers receive a small amount of groundwater from shallow local groundwater flow in short periods after rainy seasons.

The southeastern part has less favorable conditions for the formation of groundwater resources and groundwater circulation. The precipitation is less than in the northwestern part and the main aquifers developed in limestone receive less direct recharge. Recharge of aquifers in the lowlands is also possible from areas with higher precipitation located in the northwestern highlands. This mechanism is well known at the foot of the Harenna escarpment where large karst springs exist. Local recharge of aquifers, particularly porous aquifers along rivers, is possible from local rivers during high waters.

Infiltrated groundwater mainly forms shallow and deep local groundwater flows which are drained by intermittent rivers in short periods after rainy seasons. Infiltrated groundwater can also contribute to the deep regional groundwater flow which flows to the southeast to an area on the Filtu sheet and is drained by the Genale River. However, it can flow to more distant areas in the centre of the Ogaden basin. Deep groundwater in the southeast can be under artesian conditions because of the less permeable or impermeable lithological units.

The groundwater flow direction in the plateau area and adjacent lowlands in general coincides with the topography following the surface water flow direction because small intermittent and particularly perennial rivers form local drainage levels for shallow and deep aquifers, and because the main rivers of the area form deep valleys (canyons) and cut one or more overlying aquifers. The flow is partly controlled by the structure but mainly by the geomorphology of the area. Most of the springs are topographically controlled and others emerge along structures indicating that the groundwater flow is controlled by both factors. Local groundwater flow directions vary from place to place according to the local topography. An important phenomenon for both surface and groundwater flow direction is the inclination of the whole eastern plateau to the southeast.

4.5 Annual Recharge in the Area There is not enough data for direct assessment of recharge. The regional mechanism of recharge

of aquifers in the area was described above. Like in other areas, the groundwater recharge is mainly from precipitation depending on its intensity and annual distribution, topographical gradient of the area, lithological composition of aquifers and their tectonic disturbance. The groundwater of the highlands is generally recharged from direct precipitation. There is also a seasonal but significant amount of recharge to localized aquifers from most of the permanent as well as intermittent streams after the Kiremt rains when the level of rivers is above the groundwater level. Aquifers along the rivers are recharged by the surface water of streams. This type of recharge is important in the lowlands where evapotranspiration is higher and precipitation is lower than in the highlands. This type of bank infiltration is very important for local alluvial aquifers and where most of the water well sites are located.

Lahmeyer (2005) in the study of the Genale-Dawa basin considered the infiltration depth shown in Tab. 4.5.

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The map of infiltration shows an infiltration depth of about 20 mm/year.

Recharge assessment is based on rainfall infiltration (recharge from rainfall) according to the rainfall infiltration factor (RIF). The criteria used by WWDST (2003) are shown in Tab. 4.6. The recharge area of outcrops of lithological units was considered only if the slope of terrain is less than 20 %.

WWDST (2003) described the recharge of the whole Wabe Shebelle basin to be 1,500 Mm3/ year. This estimation was done based on different approaches that are described as follows:• Subsurface drainage approach 2,046 Mm3/year• Recharge area approach 397 Mm3/year• Baseflow approach 1,128.6 Mm3/year• Rainfall infiltration 2,311 Mm3/year

The baseflow approach was based on the baseflow separation method where separation of the direct runoff and groundwater components was performed using a runoff hydrograph. However, baseflow separation can only be considered when the groundwater level is always above the water level of the surface water. The method can be applied for the upper Wabe Shebelle catchment where all of the groundwater is discharged to rivers. In the lower part from Imi to Mustahil, water level monitoring for one hydrologic year showed that the water level is always lower that the Wabe Shebelle river bed. When WWDST (2003) calculated renewable groundwater resources of the Wabe Shebelle river basin the safe yield (dynamic groundwater resources) was assessed to

Tab. 4.5 Estimated minimum recharge to groundwater from stations of the Genale-Dawa basin

StationRecorded period

Area [km2]

Min[m3/s]

Approximate minimum recharge [mm/year]

Welmel at Melka Amana 1988–1996 1,396 6.25 141.2

Weyb at Sof Omar 1973–2002 4,546 1.86 12.9

Genale at Chenemasa 1989–1997 9,190 10.80 37.1

Genale at Kole* 1989–1998 56,234 439.90 246.7

Remark: * it seems that mean flow has an erroneous value in the order of magnitude – the total runoff is referred in the report to be 84 mm/year

Tab. 4.6 Rainfall infiltration factor for Wabe Shebelle basin by WWDST (2003)

Lithostratigrapfical unit Rainfall infiltration factor [%]

Alluvium 6

Basement rocks 5

Basaltic rocks 6

Sandstone and siltstone 5

Limestone 6

Gypsum beds 3

Shale / siltstone 2

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be 2,294.7 Mm3/year and 2,228 Mm3/year after subtracting the present use which represents 56.34 mm/year.

Tesfaye (1993) characterized recharge to be 150 –250 mm/year in the northwestern part of the sheet and 50 –150 mm in the southeastern part of the sheet.

Separation of baseflow and water balance presented in Chapter 2 revealed a value of recharge of about 150 –200 mm/year for the northwestern part of the sheet and 50 –12 mm/year for the southeastern part of the sheet.

Recharge calculated from mean values of baseflow shows recharge variability from 0 mm/year to about 150 mm/year depending on the depth variation of precipitation in different years.

The adopted value of recharge for the Dodola area is 175 mm/year for the plateau area, 80 mm/year for the lowland areas including the Harenna escarpment of the Dodola sheet.

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One of the important tasks of the water point inventory and data collection was to survey the groundwater chemistry and to assess the groundwater quality for its use within the mapped area. Therefore, a study of the groundwater quality was carried on the different aquifers (geological formations) of the area as well as various parts of the water circulation system. The results of the hydrochemical study can help to understand the groundwater circulation within the aquifers in addition to comparing the water quality with various standards.

Tesfaye Chernet (1993) identified the hydrochemical characteristics of the natural waters which were collected from different sources and the recharge/discharge conditions of the groundwater. According to Tesfaye Chernet: • the water resources of the highlands in the northwestern part of the sheet are classified as

being water with good chemical quality with TDS less than 500 mg/l,• the water resources of the lowlands in the southeastern part of the sheet are classified as being

water with good chemical quality with TDS 500–1,500 mg/l, • the groundwater chemistry is characterized as being bicarbonate (HCO

3) type.

Lahmeyer (2005) performed an assessment of water quality and described the area where groundwater TDS is less than 500 mg/l in the northwestern highlands and is between 500 mg/l and 1,500 mg/l when groundwater circulates in the limestone of the southeastern lowlands. The study concluded that the general trend in the increase of TDS is from the northwest highlands to the southeast lowlands within the Genale-Dawa basin. Associated with a general increase in salinity of bicarbonate type water is a change to chloride type and sulphate type is envisaged to occur where groundwater circulates in gypsiferous formations. Water samples from groundwater of volcanic, metamorphic and intrusive rocks show calcium sodium bicarbonate type in low salinities and sodium chloride type in high salinities.

Results of the chemical analyses were interpreted graphically and are shown on the hydrochemical map of the area.

5.1 Sampling and Analysis A total of ninety two (92) water samples were collected from boreholes, dug wells, springs,

river water, lakes and precipitation water in the study area. All of the water samples collected for laboratory analysis were submitted to the central laboratory of GSE and analyzed for chemical composition. The chemistry of the water obtained from the samples is shown in Annex 2. Chemical analysis of the major constituents (Mg, Ca, Na, HCO

3, SO

4, Cl) and secondary constituents (K, NO

3,

F, HBO2, CO

2, SiO

2), and measurements of electrical conductivity (EC) and pH at room temperature

were performed in the laboratory. Field measurements of pH, temperature and electrical

5. Hydrogeochemistry5.

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conductivity were made at the time of sampling. The analytical results were presented graphically on a hydrochemical map to facilitate visualization of the water type and their relationships. Suitability of groundwater for drinking, industrial and agricultural purposes is assessed based on the pertinent quality standards.

Reliability of the analyses was assessed using the cation-anion balance. The assessment showed that only 7 out of 97 (4 %) significantly exceeded the reliability level of 10 %. The frequency of the level of balance is shown in Fig. 5.1 and Tab. 5.1.

5.2 Classification of Natural WatersClassification of natural water was used to express the groundwater chemistry on the

hydrochemical map. Hydrochemical types are classified based on the Meq% representation of the main cations and anions by implementing the following scheme: • Basic hydrochemical type, where the content of the main cation and anion is higher than

50 Meq%. This chemical type is expressed on the hydrochemical map by a solid color.• Transitional hydrochemical type, where the content of the main cation and anion ranges

between 35 and 50 Meq%, or exceeds 50 % for one ion only. A dominant ion combination

Tab. 5.1 Level of balance

Level of balance [%] Frequency Cumulative frequency [%]

5 85 87.6

10 5 92.8

15 3 95.9

20 and more 4 100.0

Fig. 5.1 Level of cation-anion balance

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is expressed on the hydrogeological map by the relevant colored horizontal hatching. The secondary ion within the type is expressed by an index (e.g. Mg2+).

• Mixed hydrochemical type, where the content of cations and anions is not above 50 Meq% and only one ion has a concentration over 35 Meq%. This type is expressed on the hydrogeological map by the relevant colored vertical hatching.

Chemistry of groundwater in the Dodola area is relatively uniform reflecting uniformity of the hydrological system and system of groundwater circulation despite the variability in geology and hydrogeology of the area consisting of volcanic, basement and sedimentary rocks. The dominant hydrochemical type of groundwater of the Dodola area is bicarbonate type. The transitional Ca –HCO

3 type dominates in the northwestern and southeastern part of the Dodola sheet. The

basic Ca –HCO3 type is developed as a strip in the centre of the sheet partly copying the Harenna

erosional escarpment. The basic and transitional Na– HCO3 types are also found within the sheet

but they do not form a mapable occurrence.

The secondary ion constituent in transitional bicarbonate type of groundwater in the Dodola area can be any of cations (Mg, Na) and/or anions (SO

4, Cl).

The gradual development in TDS and relatively uniform hydrochemistry of groundwater and dominant bicarbonate groundwater indicates the dynamic hydrogeological regime with fresh water. Rain water mainly infiltrates into aquifers in the highlands with a cold climate, receiving

Tab. 5.2 Summary of hydrochemical types

Hydrochemistry Type Number of cases Percentage

Ca–HCO3

Basic 40 41.2

Ca–Mg–HCO3

Basic 2 2.1

Mg–Ca–HCO3

Basic 1 1.0

Mg–HCO3

Basic 1 1.0

Na–HCO3

Basic 6 6.2

Ca–HCO3

Trans 25 25.8

Ca–Mg–HCO3

Trans 5 5.2

Ca–Na–HCO3

Trans 5 5.2

Mg–Ca–HCO3

Trans 2 2.1

Mg–HCO3

Trans 2 2.1

Na–Cl–HCO3

Trans 1 1.0

Ca–HCO3

Mixed 1 1.0

Ca–HCO3–SO

4Basic 1 1.0

Na–Ca–HCO3

Trans 3 3.1

Na–Cl–SO4

Trans 1 1.0

Ca–Mg–HCO3–Cl Trans 1 1.0

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a high volume of precipitation in the west and northwest. Groundwater forms local and regional circulation and later flows to the east and southeast to the lowlands with a hotter climate, receiving less precipitation. Groundwater flows in lithologicaly homogeneous fissured aquifers developed in various Tertiary volcanic rocks in the highlands and flows in lithologicaly inhomogeneous fissured aquifers developed in various Mesozoic sedimentary rocks (mainly limestone), basement rocks and unconsolidated Quaternary sediments in the lowlands. In general, the TDS increases from the northwest from the infiltration area to the southeast to the drainage area formed by the valleys of the Welmel and Genale rivers and their tributaries. This trend in TDS is shown by idealized isosalinity lines on the hydrochemical map. The general trend in TDS as well as in groundwater hydrochemistry is highly affected by soluble gypsum and even rock salt which is common in some sedimentary units. It may be also be affected by various sulphidic mineralizations inside the basement rocks.

The hydrochemistry of groundwater of the area is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional types).

A general overview of the hydrochemistry of the natural water of the study area is shown in Tab. 5.2. To facilitate visualization of the classification of water types, the percentage of major cations and anions of the analyzed samples is plotted on the Piper diagram as shown in Fig. 5.2.

Fig. 5.2 Piper diagram for classification of natural waters

80 60 40 20 20 40 60 80

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Ca Na HCO 3 Cl

M g SO 4

LimestoneSoilPrecipitationRiver waterBasaltBasement

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The basic statistical data for values of electric conductivity (EC), total dissolved solids (TDS) and concentration of chloride (Cl) are shown in Tab. 5.3.

5.2.1 PrecipitationHydrochemistry of rain water of the area is not known in detail; however, chemical composition

of two samples taken in Goba town is shown in Tab. 5.4. A difference in the ion (cation and anion) balance of 38 and 44 % shows that the reliability of the analysis is very low. The water chemistry can be classified as transitional (Ca– Na) –HCO

3 type. Field measurements of pH showed values

less than 5 (4.85 and 4.88) which is uncommon in other rain water samples.

The hydrochemistry of rain water is shown on the hydrochemical map by a pie chart.

5.2.2 Surface Water Hydrochemistry of surface water is represented by 4 samples from Elgol (Rw-1), Yadot

(Rw-2) and Welemel (Rw-3) rivers and from the high altitude (3,934 m a.s.l.) Lake Gebreguracha at Shedem (Lk-1).

The chemistry of water in rivers is of basic and transitional Ca –HCO3 type with TDS of about

120 mg/l for Yadot and Welmel rivers and 464 mg/l for Elgol River. Samples taken from Lake Gebreguracha show transitional Ca –Na–HCO

3 type with TDS about 80 mg/l.

The hydrochemistry of surface water is shown on the hydrochemical map by a pie chart.

5.2.3 Groundwater in Tertiary Volcanic RocksRain water infiltrates in outcrops of volcanic rocks and flows within fissured aquifers from

recharge areas into discharge areas in shallow and deeper valleys and at the Harenna erosional

Tab. 5.3 Groundwater descriptive statistics of TDS, EC and Cl values

TDS [mg/l] EC [μS/cm] Cl [mg/l]

Average 512 529 23

Median 386 383 5

Minimum 35 32 1

Maximum 3,256 3,250 279

Count* 95 95 95

Remark: * analyses of precipitation are not considered in the statistic assessment

Tab. 5.4 Chemical composition of rain water

Na

[m

g/l

]

K [

mg

/l]

Ca

[m

g/l

]

Mg

[m

g/l

]

SiO

2 [m

g/l

]

HC

O3 [

mg

/l]

Cl [

mg

/l]

SO

4 [m

g/l

]

F [m

g/l

]

NO

3 [m

g/l

]

EC

[μS

/cm

]

pH

TD

S [

mg

/l]

0.5 0.1 0.4 0.07 < 0.4 5.0 < 1.0 1.0 0.02 0.4 9.0 6.15 9.29

0.7 0.4 1.0 0.08 < 0.4 6.0 1.0 1.0 0.04 6.2 19.0 5.58 17.22

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escarpment. It can infiltrate also in great depths and recharge deeper aquifers developed in sedimentary rocks and it appears as thermal water in the Wora Kora hot spring. The bicarbonate type is typical for groundwater circulating in various volcanic rocks. There are no differences in groundwater types related to different types of basalt or volcanic formations. Groundwater dissolves volcanic rock minerals along its flow and is enriched by various chemical compounds. The dominant water types of these volcanic rocks are basic and transitional Ca –HCO

3 types. The

secondary constituent cations in the transitional bicarbonate type of groundwater in volcanic rocks are Mg and Na.

Groundwater from springs Csp-33 and Csp-37 represents the drainage of water of trachyte in the highest part of the Sanetti Plateau and the low TDS of 116 and 85 mg/l shows very short water circulation in relative impermeable trachyte.

The total content of dissolved solids varies between 35 mg/l and 3,257 mg/l for cold water with an average 462 mg/l and mean of 342 mg/l.

The hydrochemistry of groundwater discharged from volcanic rocks is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional types). Chemistry of the thermal water of the Wora Kora hot spring is described and documented in Chapter 5.4.

5.2.4 Groundwater in Mesozoic and Quaternary SedimentsGroundwater from aquifers hosted in Mesozoic sediments represented by limestone (Hamanlei

formation) and sandstone, and in Quaternary sediments represented by alluvial and elluvial sediments occurs over about 25 % of the area in the southeastern part of the sheet. Rain water infiltrates in outcrops of sedimentary rocks and flows through pores, fissures and karst opening aquifers from recharge areas into discharge areas and appears as springs particularly in deeper valleys along perennial rivers or is directly drained by the Genale and Welmel rivers. The groundwater is also developed by dug wells and boreholes. The aquifers in limestone can be also recharged from highland areas in the northwest part of the Dodola sheet and from rivers during high waters.

The groundwater of sedimentary units on the Dodola map sheet is represented by 18 water samples. These samples were collected form 1 borehole, 3 dug wells and 14 springs. The dominant chemistry is mainly of basic Ca –HCO

3 type. The transitional types are rare as well as participation

of sulphate as the secondary constituent anion in the transitional type. This fact shows that there is relatively small amount of gypsum material in this part of the Hamanlei limestone and sandstones. The sampled water points are close to outcrops of overlying basalt and the small amount of sulphate anions confirms the idea about recharge of aquifers developed in sedimentary units by aquifers developed in volcanic rocks with dominant bicarbonate chemistry.

Groundwater TDS in sedimentary rocks varies from 138 mg/l to 1,629 mg/l with an average value of 684 mg/l and the sulphate content varies from 1 mg/l to 178 mg/l with an average concentration of 40 mg/l. The extreme content of sulphate in the groundwater of 332 mg/l, found in DW-14 tapping water from colluvial sediment, indicates that groundwater circulates in gypsum-containing material.

The hydrochemistry of groundwater discharged from sedimentary rocks is expressed on the hydrochemical map by the relevant colors.

5.2.5 Groundwater in Basement RockGroundwater from aquifers hosted in basement rock represented by low and high metamorphosed

rocks occurs over about 25 % of the area. Rain water infiltrates in outcrops of crystalline rocks

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and forms shallow local groundwater flow and circulates through pores, fissures of aquifers from recharge areas into discharge areas and appears as springs particularly in depressions and in valleys along perennial and intermittent rivers or is directly drained by bigger rivers like the Genale. The groundwater is also developed by dug wells and boreholes. The aquifers in the basement rock can be also recharged by deep regional groundwater circulation (Wora Kora hot spring) from a highland area located in the northwest and from rivers during high waters.

The groundwater accumulated in basement rocks of the Dodola map sheet is represented by 13 water samples. These samples were collected form 5 wells, 4 dug wells and 5 springs. The chemistry of groundwater is highly variable reflecting local characteristics of a shallow aquifer. Dominant chemistry is mainly of basic and transitional Ca– HCO

3 and Na– HCO

3 types but even

a chloride (Na–Cl– HCO3 –SO

4) type occurs in the area covered with basement rocks. Na, Mg, ions

are also present as the secondary constituents in transitional types of groundwater. Groundwater TDS in basement rock varies from 208 mg/l to 1,481 mg/l with average value of 620 mg/l. A relatively high TDS for shallow local groundwater flow typical for water resources developed in weathered and fissured part of crystalline rocks shows mixing processes of infiltrating water with some highly mineralized water that can originate when groundwater is in contact with various sulphidic mineralizations which is typical for basement rock in this area.

The hydrochemistry of groundwater discharged from basement rock is expressed on the hydrochemical map by the relevant colors.

5.3 Water QualityWater quality of the mapped area was assessed from the point of view of drinking, agriculture

and industrial use.

5.3.1 Domestic UseTo assess the suitability of water for drinking purposes, the results of the chemical analyses were

compared with the Ethiopian standards for drinking water (see Tab. 5.5.) published in the Negarit Gazeta No. 12/1990 and The Guidelines of Ministry of Water Resources (MoWR, 2002).

Tab. 5.5 shows that groundwater of the mapped area is convenient for drinking in more than 80 % of sampled points. Only about 4 % of water samples exceeds the maximum permissible level of concentration of nitrates and/or TDS. This situation reflects the fact that the majority of the groundwater circulates in aquifers developed in volcanic, basement rocks and limestone and sandstone with minimum gypsum and even rock salt occurring within sedimentary formations.

The content of calcium, sulphate and nitrate and particularly TDS in 16 % of cases exceeds the highest desirable level and represents the main threats to the groundwater quality. Deterioration of groundwater quality by a high content of calcium, TDS and sulphate is caused by the natural character of the aquifers and results from dissolution of gypsum and possible sulphidic minerals through which the groundwater is circulating. The high content of nitrates is caused by human factors (pollution) that add allochthonous material to the groundwater in the aquifer (human and animal waste).

Particular interest was paid to the content of nitrates in groundwater. The content of nitrates is not related to the rock composition (type) but it reflects pollution of groundwater by human and/or animal waste. The background content of nitrates in groundwater is about 5 to 10 mg/l depending on the relevant land cover. In forest areas it can be even higher because of decomposition of various plants and other organic material. The nitrate content varies in the Dodola area from 0.4 mg/l to 142 mg/l with a mean value of 2.2 mg/l (Fig. 5.3).

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Water samples (16 out of 92 or about 16 %) with a nitrate content of above 10 mg/l show that the first (shallow) aquifers are partly polluted by human activity. The value of 10 mg/l is considered as the natural content of nitrates in the groundwater. The more important finding is that 4 out of the 92 samples had concentrations of nitrates exceeding the maximum permissible level. This pollution is an important factor particularly in highly vulnerable groundwater resources in shallow aquifers developed in crystalline rocks and karst aquifers developed in limestone. This fact also has to be considered when planning for the future development and protection of groundwater resources in the area. Proper location of water points and suitable protective measures should be applied to boreholes, springs and dug wells used for human water supply. Fig. 5.3 shows the content of nitrates in the analysis of water in the study area.

Tab. 5.5 Groundwater chemistry compared to drinking water standards and guidelines

PropertyRange(min–max)[mg/l]

Ethiopian standards (1) and MoWR Guidelines (2) [mg/l]

Number of exceeding values

Highest desirable level

Maximum permissible level

Highest desirable level

Maximum permissible level

Na (2) 2–733 358 1

Ca (1) 0.4–229 75 200 18 1

Cl (1) 1–279 200 600 2 0

Cl (2) 1–279 533 0

HBO2

0–0 0.3 0

(free) ammonia

0.05 0.1

Fe (1) 0.1 1

Fe (2) 0.4

Mg (1) 0.07–239 50 150 5 2

Mn (1) 0.05 0.5

Mn (2) 0.5

SO4 (1) 1–332 200 400 2 0

SO4 (2) 1–332 483 0

TDS (1) 9.29–3,257 500 1500 34 3

pH (1) 5.58–8.04 7.0–8.5 6.5–9.2 14 5

pH (2) 5.58–8.04 6.5–8.5 5

NO3 (1) 0.4–142 10 45 16 4

NO3 (2) 0.4–142 50 4

F (1) 0.02–8.3 1 1.5 2 2

F (2) 0.02–8.3 3 1

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5.3.2 Irrigation UseAgricultural standards for the quality of groundwater used for irrigation purposes are determined

based on the Sodium Adsorption Ratio (SAR), total dissolved solids and United States Salinity Criteria (USSC). The Sodium Adsorption Ratio (SAR) is used to study the suitability of groundwater for irrigation purposes. It is defined by SAR = Na/[(Ca+Mg)/2] where all concentrations are expressed in mg/l.

Most of the water samples (see Tab. 5.6) from the study area are found to be suitable for irrigation since they show the SAR value within the water quality class of excellent for agricultural purposes. Groundwater classified as fair and/or poor quality water for irrigation representing only 5 % of samples corresponds with water points yielding Na– HCO

3 and Na –Cl types of water.

5.3.3 Industrial UseIndustrial water criteria establish the requirements of water quality to be used for different

industrial processes that vary widely. Thus, the composition water for high pressure boilers must meet extremely strict criteria whereas water of low quality can be used for cooling of condensers. The suitability of water for use in industry is shown in Tab. 5.7.

Fig. 5.3 Content of nitrate in analysis of water in the study area

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100

water points

nitr

ates

[mg/

l]

Shw-4

DW-11

Csp-39Csp-13

Tab. 5.6 Suitability of water for irrigation

Value of SAR Water class Number of samples in the range

<10 Excellent 86

10–18 Good 6

18–26 Fair 2

>26 Poor 3

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Of almost equal importance for industry as quality of used water is the relative time constancy in concentration of various components. As a result, an adequate groundwater quality often becomes a primary consideration in selecting a new industrial plant location. Groundwater from the mapped area can be used for industry in general, but some specific technologies require water treatment before the water is used in the technology.

Tab. 5.7 Suitability of water for use in industry

Industry or useSolids (TDS)[mg/l]

pHChlorides as Cl [mg/l]

Sulfates as SO

4[mg/l]

Number of samples in the range

Brewing 500–1,500 6.5–7.0 60–100 0

Carbonated beverages < 850 < 250 < 250 69

Confectionary 50–100 > 7.0 1

Dairy < 500 < 30 < 60 49

Food canning and freezing

< 850 > 7.0 59

Food equipment washing

< 850 < 250 69

Food processing general

< 850 69

Ice manufacture 170–1,300 64

Laundering 6.0–6.5 1

Paper and pulp fine < 200 16

Paper groundwood < 500 < 75 51

Paper bleached cardboard

< 300 < 200 28

Paper unbleached cardboard

< 500 < 200 51

Paper soda and sulfate pulps

< 250 < 75 23

Rayon and acetate fiber pulp production

< 100 5

Rayon manufacture 7.8–8.3 6

Sugar < 100 < 20 < 20 5

Tanning 6.0–8.0 77

Textile < 100 < 100 71

Remark: Sugar requirements for TDS are in general low

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Incrustation hazard is important for the design of various pipes as well as technological processes. Incrustation occurs if concentrations exceed the limits shown in Tab. 5.8. Corrosion hazard occurs if concentrations exceed the limits shown in Tab. 5.9.

There is low threat of incrustation as well as corrosion because 70 % and 90 % of the samples is in required range. Some groundwater circulates in carbonate rocks with gypsum and rock salt causing intercalations or corrosion when the groundwater is used in pipes for public water supply or for delivery of water for industry or agriculture.

5.4 Mineral and Thermal WaterThermal water was encountered during the water point inventory as a spring emerging from

basement rock. A single hot spring (HSP-1) with the name Hora Kore, discharging water with temperatures about 45 °C and yield of 4 l/s is found to the south of Malka Amana town at an elevation of 900 m a.s.l. This spring provides groundwater from Precambrian quartezo feldsphatic gneiss (Pqfg) and the temperature is assumed to be related to deep circulation. The Hora Kore hot spring emerges where joints or fractures around the springs cross cut each other. Since it appears from many fractures it is considered as a multi-eyed spring.

Tab. 5.8 Concentration limits for incrustation

Component Concentration [mg/l] Number of sample in the range

Bicarbonates (HCO–3) > 400 78

Sulfates (SO–4) > 100 76

Silicon (Si) > 40 58

Iron (total) > 2 Not analyzed

Manganese (total) > 1 Not analyzed

Hydrogen sulfide (H2S) > 1 Not analyzed

Total hardness (TH as CaCO3) > 200 55

Tab. 5.9 Concentration limits for corrosion

Component Concentration and/or value Number of sample in the range

pH < 7 83

EC > 1,500 μS/cm 91

Chloride (Cl–) > 500 mg/l 92

Hydrogen sulfide (H2S) > 1 mg/l Not analyzed

CO2

> 50 mg/l Not analyzed

Dissolved oxygen (O2) > 2 mg/l Not analyzed

Total hardness (TH as CaCO3) < 100 mg/l 60

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These springs were collected together in one collecting chamber and used as holy water. The hot spring was developed in 2006 in collaboration with Mekane Eyesus church and Medawelabu Wereda Water Bureau. The local peoples are currently using this spring for curative treatment.

The Hora Kore hot spring was sampled twice. The results of chemical analyses are shown in Tab 5.10.

Fig. 5.4 Multiple eyes of Hora Kore hot spring

Tab 5.10 Chemical composition of the Hora Kore hot spring

Na

[m

g/l

]

K [

mg

/l]

Ca

[m

g/l

]

Mg

[m

g/l

]

SiO

2 [

mg

/l]

HC

O3 [

mg

/l]

Cl [

mg

/l]

SO

4 [m

g/l

]

F [

mg

/l]

NO

3 [m

g/l

]

EC

[μS

/cm

]

pH

TD

S [

mg

/l]

Tota

l ha

rdn

es

as

Ca

CO

3 [

mg

/l]

Alk

alin

ity

[mg

/l]

283.0 6.4 52.0 9.0 47.0 94.0 265.0 294.0 8.3 < 0.4 1,686.0 7.72 1,106.0 166.93 77.1

285.0 7.0 56.0 4.0 48.0 90.0 284.0 282.0 7.4 0.399 1,705.0 7.87 1,063.8

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Natural resources of the Dodola area vary in origin relating to the geological composition, soil conditions, water, wind and solar radiation, as well as human resources.

6.1 Economic GeologyVolcanic and basement rocks cover a large part of the map area. They outcrop in the

northwestern part of the sheet and form the Sanetti Plateau. Ignimbrite, some trachyte and ash flow-tuffs can also be easily cut and are important building materials. Ignimbrites can be used for masonry and can be found as an intercalation between basalt and alkali trachytes (Tps). There are a few existing quarries exploiting ignimbrite rock for masonry used in the construction of buildings, bridges, culverts and other engineering structures. The quarries are not sufficient; although the ignimbrite is of good, workable quality. Coarse crushing of basalts and trachytes to the size of gravel is needed for raw materials for road construction. Larger sized blocks of rock are also important for foundations. The local people use them for local house construction and fencing in the highlands. Different basement rocks are used as a road construction material where volcanic rocks are not available.

The reddish brown, sandy silty, residual soil and fluvial deposits can be a good source of burrow material. The soil (vertisol) occurs on rounded, low relief hills composed of limestone. The soil can serve as an impervious blanket in the construction of dams and other water retaining structures. As it can be seen from field observation the local people make pottery products from the residual soil.

Sand and gravel naturally occurring along the main rivers can be used for preparation of concrete and gravel for water well development (gravel packing). There are many existing quarries of sand and gravel in the project area especially in the river valleys mentioned.

Limestone, gypsum and mudstone provide potential resources for development of cement and lime as well as for the development of various products in the chemical industry (paint production, plaster of Paris, dimension stones, etc.). There are no cement factories in the area and the potential has yet to be developed on the Dodola sheet. There are many existing quarries of limestone especially along roads and crushed limestone is used for road construction.

Metallic minerals resources occur in crystalline rocks of the southwestern part of the sheet. This area has a potential of mining e.g. iron occurrence which is observed around Angetu and gold which is currently mined by local people near Bedessa and Aramfama.

6. 6. Natural Resources

of the Area

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Natural Resources of the Area

116 Natural Resources of the Area

6.2 Water ResourcesWater resources of the area depend mainly on rainfall and other climatic characteristics, as well

as the hydrological, geological and topographical settings of the study area. Detailed assessment of water resources in the area is difficult because both climatic and water flow data are scarce and the existing data series are short and incomplete or inaccessible.

There are 12 meteorological stations operated by the Meteorological Institute in the mapped area. The Goro, Rira, Delo Mena, Melka Amana and Dodola stations have long-term measurements. The long-term mean annual rainfall of the area has been assessed to be about 850 mm/year i.e. about 800 mm/year for the lowlands represented by Sirima and Somaya plains (50 % of the area) and about 900 mm/year for Sanetti Plateau, Warka-Aramfama hills and the Harenna escarpment (50 % of the area).

The area of the map was calculated from the 1:250,000 hydrogeological map and an area of 18,356 km2 is used for further calculation.

The area of active aquifers that store and transmit water was calculated based on the hydrogeological map. The active aquifers (Tab. 6.1) of porous, karst and fissured permeability in the highlands and lowlands cover an area of 18,016 km2.

The runoff characteristics vary widely because of variability in climatic conditions and hydrogeological characteristics between different observation points.

The surface river flow measurements are performed on small rivers in the highlands at six gauging stations within the Dodola sheet. The measurements of the highland rivers provide some data about water resources of aquifers located on the plateau, but are not enough for assessment of groundwater resources of the whole area. Data from other river gauging stations within both the Genale-Dawa and Wabe Shebelle basins were considered in the assessment of surface and baseflow values. The surface flow–baseflow assessment is highly affected by the short and/or incomplete series of data and the intermittent character of rivers in some years. Data can also be highly influenced by the effect of bank groundwater storage, difficulties in flow measurements in wide and unstable river channels and unknown groundwater flow beneath gauging stations. For further calculations, the value of specific surface runoff of 15.0 l/s.km2 for areas covered by volcanic rocks, 10.0 l/s.km2 for areas covered by basement rocks and 2.0 l/s.km2 for areas covered by sedimentary rocks and a specific baseflow of 5.0 l/s.km2 for areas consisting of volcanic rocks, 1.0 l/s.km2 for areas consisting of basement rocks and 0.14 l/s.km2 for areas covered by sedimentary rocks have been adopted for the Dodola area. The assessed water resources of the Dodola area are shown in Tab. 6.2. Based on the adopted map area values of specific runoff are 10.5 l/s.km2 and specific baseflow are 2.8 l/s.km2.

Tab. 6.1 Aquifers of the area

Aquifers Area [km2]

Porous 173

Fissured and karst in sedimentary and volcanic rocks

13,676

Fissured in basement rocks 4,167

Total of the area 18,016

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117Natural Resources of the Area

6.2.1 Surface Water Resources DevelopmentDespite of the fact that river gauge measurements show relatively low, but logical,

evapotranspiration when nearly 78 % of precipitation is drained as total runoff from the area, there are good water resources to be used for irrigation, electricity generation as well as for drinking water supply of people living within the area. The total water resources of the area have been assessed to be 6,087 Mm3/year.

The surface water of the area should be primarily used for irrigation as well as for small scale electricity generation; however, construction of dams should not be easy in areas covered by karstified limestone. The irrigation should be preferably applied on rivers of the central and southern parts of the sheet where plateau topography dominates together with well developed alluvial plains. Irrigation dams in the lowlands should be designed in a different way respecting the intermittent character of rivers and topography of the area.

Dams for small scale electricity generation should be constructed on rivers in the highlands and possibly in the escarpment area where these rivers enter their gorges. Locations where rivers form waterfalls are particularly suitable.

Considering the fact that the use of surface water for irrigation is the most important development factor for food security in the area, we can recommend about 80 % of available surface water resources to be used for irrigation. This portion represents 4,870 Mm3/year. Considering about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 487,000 ha. This area represents 4,870 km2 which is about 26 % of the Dodola sheet.

It is known that the area can often be affected by drought periods and during some years irrigation dams will not be refilled by Kiremt rainfall. When this will happen over several years irrigation cannot be practiced in drought stricken areas. The meteorological observations and experience from the Dodola area as well as other areas shows that the occurrence of drought periods is not uniformly distributed over large areas and in the case of drought in one part of the area (sheet) other areas (or adjacent sheets) can gain a volume of precipitation sufficient for filling irrigation dams. This analysis results in the recommendation that irrigation dams are highly important for agricultural development of the area. Drought periods and their spatial distribution show that agricultural production in areas

Tab. 6.2 Assessment of water resources of the Dodola area

Input Area [km2] Resources total Remark

Precipitation 850 mm 18,356 15,603 Mm3/year

Total water resources – map

10.5 l/s.km2 18,356 6,087 Mm3/year 78 % rainfall

Renewable groundwater resources of active aquifers

2.8 l/s.km2 18,016 1,591 Mm3/year 21 % rainfall

Static groundwater resources of karst and fissured aquifers

5 % porosity100 m thickness

13,676 68,380 Mm3

Static groundwater resources of porous aquifers

15 % porosity3 m thickness

173 779 Mm3

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of adequate rainfall can support areas stricken by drought within the region without the requirement for long distance transport of food aid. It also shows that basic decisions can be made on a regional level. This decision will be quicker than one adopted at a federal level.

The irrigation as well as energy potential of the area has been known for a long time. It was assessed in the framework of the Lahmeyer (2005) Genale-Dawa Water Master Plans and by various specific studies.

The study conducted by Nehmia and Raghuvanshi (2005) for the Jemma basin in the central part of the country developed methodology that can be used for assessment of sites for construction of Small Hydropower Schemes in the Dodola area (particularly in the Harenna erosional escarpment). The site selection is based on the use of the main channel´s nature and length; the headrace channel length; the availability of sufficient head in excess of 60 m; accessibility of sites for proper utilization of energy, and the catchment area. These geospatial database criteria can be built by GIS. The site selection was made based on the following criteria:

1. The channel should be perennial in nature; in EMA topo-maps 1:50,000; perennial streams are marked with blue color and the stream name is written in capital letters.

2. The catchment should have an area of at least 25 km2. This will ensure a sufficient amount of discharge for power generation.

3. The main channel should have a minimum length of about 10 km. 4. The availability of head should be at least 60 m as lower head does not have a significant

power potential in small river yields. 5. The headrace channel must be less than 5 km in length. 6. The potential site should be easily accessible and should be located near population clusters

to ensure proper power utilization.

6.2.2 Groundwater Resources DevelopmentThe Dodola area has good groundwater resources to be used for the supply of drinking water to

people living within the area because it shows extremely high runoff when 79 % of precipitation is drained as total runoff from the area and 20 % of precipitation infiltrates and appears as baseflow based on gauge measurements. There is also the potential to use groundwater of the area to support irrigation as well as drinking water of people living outside of the mapped area. The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 1,591 Mm3/year.

Considering the total number of people living within the area is 778,928 (Tab. 1.1) the need for water supply can be nearly 5.7 Mm3/year. Assessment of drinking water demand was based on a calculation of 20 l/c.d (15 l/c.d rural and 22.5 l/c.d for towns with less than 15,000 inhabitants). The figure shows that recent demand represents about 2 % of renewable groundwater resources of active aquifers i.e. aquifers can provide adequate drinking water even in the future considering the trends in population growth.

Tesfay (2001) describes water supply issues and predicts that a large number of areas fall into the category of “water scarcity” areas because of an increase in population and in demands for more water for agriculture, industry and the community. This situation will be even worse in 2025 based on the trends in population growth. He defined “water scarcity” and “water stress” as cases where less than 1,000 m3/year and less than 500 m3/year are available annually per capita, respectively. These limits represent about 1,000 and 500 Mm3/year; however, they are not supposed to be covered only from groundwater. Comparing these limits to the overall water resources of the area of 6,086 Mm3/year, the scarcity limit represents about 8 % and the stress limit about 4 % of the overall water resources of the sheet. It is necessary to state that the limits are based on the idea of massive human, agriculture and industrial development of the area in the next 15 years.

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Most of the people within the area live in small towns and villages. There is a good practice to develop big springs which form regional drainage of aquifers developed in volcanic rocks to supply towns with drinking water. In addition to the development of big springs, water supply based on drilled wells represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells as follows:a) In aquifers developed in Hamanlei limestone with a depth of about 150 –250 m. Each of the wells

can yield about 2 l/s (recent average). The recent average depth of wells is 130 m with an average groundwater level at 100 m below the surface (maximum depth to groundwater level is 40 m below the surface). Each of these wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,640 inhabitants considering a daily consumption of 20 l/c.d.

b) In aquifers developed in volcanic rocks with a depth of about 50 –200 m. Each of the wells can yield about 2 l/s (recent average). The recent average depth of wells is 130 m with an average groundwater level at 100 m below the surface (maximum depth to groundwater level is 40 m below the surface). Each of these wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,640 inhabitants considering a daily consumption of 20 l/c.d.

c) In aquifers developed in basement rocks with a depth of about 30 –70 m. Each of the wells can yield about 1 l/s (recent average). The recent average depth of wells is 30 m with an average

Tab. 6.3 Sites proposed geophysical investigation and drilling

Site ID Wereda Kebelle Locality X UTM Y UTM Lithology Remark

gs_1DeloMena

Wabero Near Rira 583985 739284 TapbOnly single VES

gs_2Delo Mena

Melka Amana

M.Amana 588946 692394 Pbg

gs_3Mada Walabu

Laki Erba Weni 55898 768970 PbgAlong Ware--Oborso valley

gs_4MadaWalabu

Oborso Oborso 542548 678970Qs and Jsst

Along Ware--Oborso valley

gs_5 Berbere Jibri Jibri 657330 736468 Jlst2

gs_6 Berbere Sirima Eladhoke 636346 736962 Tapb

gs_7 BerbereHaroDumalto Cheketa

636624 750821 Jlst2

gs_8 Goro Melyu Doyo Abayi 643530 765093 Qs

gs_9 Berbere HambelaHambela river valley

622880 730404 AlluviumJust east of Hambela village

gs_10Harenna--Buluk

HawoHawo before crossing Wel-mel River

569645 726580 Tapb

gs_11 Haruluk HawoHawo after crossing Wel-mel River

565000 732515 Tapb

gs_12Harenna--Buluk

KumbiNear Kumbi dug well

563264 712457Pgt and Tapb

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groundwater level at 10 m below the surface. Each of these wells can provide 86,400 l/d and can supply a small town or group of villages with about 4,320 inhabitants considering a daily consumption of 20 l/c.d.

The first step in groundwater development should be to provide a safe water supply to people living within the area. In this respect it is recommended to drill wells for the water supply of particular towns. Drilling sites should be checked by detailed hydrogeological investigation and by geophysical measurements. Considering these in addition to the geophysical survey previously conducted by the Bale Zonal Water Bureau, about twelve sites were proposed in the study area for vertical electrical sounding (VES profiling). Details of the sites are shown in Tab. 6.3. and the positions are shown on the hydrogeological map.

The Bale Zone Water Bureau carried out a geophysical investigation at five sites to assess the groundwater potential. Results of the site investigation are discussed in the following text.

Wereda: Delo Mena, Kebele Berak; Locality: Bururi – site 1X UTM 608823, Y UTM 665960 at an altitude of 1,294 m a.s.l.

Geophysical investigation revealed 6 (BVES-1) and 7 (BVES-2) geoelectric layers. The fifth layer with low resistivity should be the probable water bearing layer and drilling should be done up to 212 m to penetrate the full aquifer thickness. This proposal can be checked by a geologist supervising the well drilling. The groundwater level is expected at a depth of approx. 100 m but it could be deeper.

Layer Resistivity [.m] Thickness [m] Depth [m] Possible lithology

1 350.9 0.9 0.9 Top clay soil

2 233.9 1.7 2.6 Clay soil

3 18.8 8.6 11.2Highly weathered and fracturedlimestone

4 115.8 39.4 50.6Slightly weathered/fracturedlimestone

5 22.1 51.6 102.2Highly weathered and fracturedlimestone

6 6,924.6 Fresh limestone

Tab. 6.4 Results of vertical electrical sounding - Bururi - BVES-1


1 83.6 1.1 1.1 Clay soil

2 268.8 4.0 5.1Non-weathered/fractured limestone

3 41.5 13.4 18.5 Weathered/fractured limestone

4 229.1 32.3 50.8Non-weathered/fractured limestone

Tab. 6.5 Results of vertical electrical sounding - Bururi - BVES-2 (Part 1)

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Wereda: Gura Demole, Kebele Yedi; Locality: Yedi – site 2X UTM 652013, Y UTM 0737979 at an altitude of 1,081 m a.s.l.

This site is geologically covered by limestone (Jlst2) and the Oromia Pastoralist Development Commission drilled 144 m deep well in 2006 G.C. near this site. The well was later abandoned. The well log is as follows.

The geophysical investigation consisted of a single YVES-1 located upstream of the abandoned well in a northerly direction. The maximum current penetration depth was 309 m and this could be the minimum safe drilling depth. Measurement identified 6 geoelectrical layers from which the 4th and 5th layers possess a low resistivity value and this layer may be the water bearing aquifer. The results also show depth of groundwater level about 100 m, but could be deeper.


5 30.9 85.3 136.1Highly weathered/fracturedlimestone, may be water bearing

6 199.7 75.9 212.0Slightly weathered/fractured limestone

7 1,319.5Fresh (non-weathered/fractured limestone)

Tab. 6.5 Results of vertical electrical sounding - Bururi - BVES-2 (Part 2)

Depth [m] Possible lithology

0–5 Black cotton clay soil

5–10 Gravel and boulder embedded clay

10–35 Fractured basalt

35–93 Marl

93–124 Marly limestone

124–144 Massive limestone

Tab. 6.6 Well log of Yedi abandoned well


1 28.4 0.9 0.9 Clay soil

2 15.4 2.0 2.9 Clay soil

3 83.4 9.1 12.3 Slightly weathered basalt

4 12.3 60.5 72.5 Fractured/weathered limestone

5 50.4 236.5 309.0Weathered limestone, may be wa-ter bearing

6 21.5Highly fractured/weathered limes-tone, may be water bearing

Tab. 6.7 Results of vertical electrical sounding - Yedi - YVES-1

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Wereda: Harenna Buluk, Kebele Mekana Gobelle; Locality: Mekana Gobelle – site 3X UTM 560384, Y UTM 684161 at an altitude of 1,252 m a.s.l.

The geophysical measurements have identified eight geoelectric layers in both MGVES-1 and MGVES-2 with a maximum detected depth of 166 m. Data from the two VES showed the groundwater could be found at depths from 8 to 19 m. The minimum safe drilling depth should be up to 54 m.

Wereda: Harenna Buluk, Kebele-Gerbigallo; Locality: Gerbigallo – site 4X UTM 559744, Y UTM 694046N at an altitude of 1,545 m a.s.l.


1 396.7 1.0 1.0 Top sandy clay soil

2 313.7 1.6 2.6 Sandy clay soil

3 48.1 5.3 7.9Highly weathered and foliated ba-sement rocks (Pbg), possibly water bearing

4 257.7 22.6 30.5 Weathered basement rocks

5 356.2 23.8 54.3 Slightly weathered basement rocks

6 3,109.6 65.3 119.6 Massive basement rocks


8 6,134.9 Massive basement rocks

Tab. 6.8 Results of vertical electrical sounding - Mekana Gobelle - MGVES-1


1 109.7 1.1 1.1 Top sandy clay soil

2 80.3 2.9 4.0 Sandy clay soil

3 176.3 2.8 6.8Slightly fractured basement rocks (Pbg)

4 11.2 11.2 18.8Highly weathered and/or fractu-red basement rocks(Pqfg), possi-bly water bearing

5 331.4 10.3 29.1Slightly weathered basement rocks, may be water bearing

6 308.4 5.9 35.0Slightly weathered basement rocks, may be water bearing

7 888.9 14.4 49.4Very slightly fractured basement rocks

8 14,414.3 Massive basement rocks

Tab.6.9 Results of vertical electrical sounding - Mekana Gobelle - MGVES-2

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In this area three geophysical measurements GGVES-1, GGVES-2 and GGVES-3 were conducted in a NE-SW oriented local fracture and the cold Oda-dima valley near Csp-17. Two of them align across the fracture while the third one was along the fracture. On the basis of the measurements the maximum penetration depth was 98.8 m and this was also the recommended minimum safe drilling depth during groundwater exploitation in the Gerbigallo area.


1 43.9 1.0 1.0 Clay soil (dry)

2 14.6 2.4 3.4 Clay soil (wet)

3 127.2 7.8 11.3 Fractured basement rocks (Pbg)

4 461.3 10.6 21.9 Slightly fractured basement rocks

5 1,831.4 22.3 44.2 Fresh basement rocks

6 25,601.1 Fresh basement rocks

Tab. 6.10 Results of vertical electrical sounding - Gerbigallo - GGVES-1


1 58.9 1.1 1.1 Clay soil (dry)

2 12.7 2.6 3.6 Clay soil (wet)

3 1,775.8 26.6 30.3 Fresh basement rocks (Pbg)

4 456.6 23.4 53.6Weathered/fractured basement rocks, may be water bearing (Pbg)

5 718.1 32.4 86.1Slightly weathered/fractured base-ment rocks, may be water bearing

6 6,096.9 Massive fresh basement rocks



1 1,885.4 0.8 0.8 Top soil (undifferentiated)

2 119.7 1.0 1.8 Clay soil (dry)

3 17.7 3.2 4.9Wet clay soil, may bearing some water

4 520.6 40.0 44.9Slightly weathered/fractured base-ment rocks (Pbg)

5 360.5 54.0 98.8Weathered/fractured basement rocks (Pbg), may be water bearing

6 19,315.5Massive fresh basement rocks (Pbg)


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Wereda: Meda Welabu, Kebele Oborso; Locality: Chefa– site 5X UTM 543837, Y UTM 677833 at an altitude of 1,581 m a.s.l.

The geophysical measurement OVES-1 was conducted in a unit of Pbg around Oborso and the maximum current penetration depth was 241 m. From the seven geoelectric layers determined the 5th one seems to be a water bearing layer. The 6th layer also has a relatively low resistivity value and could also be a water-bearing layer and layer three may also develop a perched or localized aquifer. From general observation of VES data the minimum recommended drilling depth was 241 m as the rocks are weathered and fractured up to this depth.

The proposed depth of boreholes in limestone is designed based on the optimum cost and yield of individual wells additional to geophysical measurements. During the final siting of each well it is necessary to consider that the final depth of the proposed wells is governed by the level of groundwater which is given by the drainage level e.g. nearby spring altitude, altitude of surface water level of the nearby river and surface level of the site selected for well drilling.

The most difficult question will be supply to rural areas with a widely spread population. This should be done from local centers where water wells will be drilled and connected to places of water use with relatively long distribution pipes. Effectiveness and cost of water supply systems for the rural population should be studied as a site specific problem in the future.

Most rural schemes, especially gravity schemes, do not have water levies. The tariff rates of schemes with water charges range from 0.10 Birr/family/month to 6 Birr/m3 of water. Schemes with a motorized borehole source have higher rates ranging from 3 to 6 Birr/m3 of water.

Potential groundwater resources developed in the area surpass the current needs of people living in the area. It even surpasses the potential demand of water when agriculture, living standards and industry will be developed in the future in the area. Groundwater is generally of good quality without harmful substances and can be used for drinking purposes after the supply system is secured by chlorination. There is a chance to use the groundwater of the highlands for water supply of the adjacent lowlands–the transfer of water to the arid areas where there is a problem with water scarcity and quality (salinity). The groundwater of the highlands can be easily developed at the foot of the Harenna escarpment. This action represents the second step when the groundwater resources of the area will be used for development of human resources and agriculture resources not only within the area, but also in other areas with high demand.


1 245.0 0.8 0.8 Dry sandy clay soil

2 23.9 1.3 2.1 Wet sandy clay soil

3 6.3 5.3 17.4Totally weathered basement rocks, possibly water bearing


5 70.1 100.5 150.9Highly weathered basement rocks, possibly water bearing

6 99.8 50.1 241.0 Slightly weathered basement rocks

7 487.2 Massive basement rocks

Tab. 6.13 Results of vertical electrical sounding - Chefa - OVES-1

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Some of the existing water points do not represent safe water supplies as they show an increasing content of nitrates in shallow water supply systems. Deeper wells currently represent a safe type of water supply; however, they have to be protected against pollution from local sources like human and animal waste (sources of pathogens and nitrates) as well as from potential industry (tanneries, textile industry, flower plantations, etc.). The minimum required distance of water supply wells and potential pollution sources should be maintained during water resources development in towns and villages. The same level of interest should also be applied to the development and protection of groundwater resources for rural communities. It should be necessary to start with relatively concentrated communities where the feasibility and impact of developed schemes will be most significant. This problem is accelerated by the fact that the main aquifers of the area are highly vulnerable karstified aquifers developed in limestone.

In addition to priority in development of groundwater for safe drinking water supply it should be possible to select the most fertile soil to be developed by irrigation based on groundwater to increase the stability of food supply in prolonged periods of drought. The problem was discussed by Tsur and Issar (1998) who stated that if, as it commonly found in reality, the supply of surface water is uncertain then groundwater plays a role in addition to that of increased water supply: the role of a buffer that mitigates the undesired effects of uncertainty in supply of surface water.

Development and protection of the water resources of the area and the environment as a whole have a principal importance for the development of the infrastructure with subsequent impacts upon the eradication of poverty (development of irrigated agriculture, maintaining livestock during drought). Access to drinking water changes the life of women, when a shorter distance for fetching water provides more time for family care and improves the health level of the population (statistics show that 40 % of child death rates is related to water born diseases). About 15 % of the rural population has access to safe drinking water in the area and about 70 % of infections are related to contaminated water resources. This is a serious problem for the creation of strong farm communities capable of full time engagement in agricultural activity. It is therefore important to provide safe drinking water to rural communities. Protection of the environment, particularly prevention of soil erosion and degradation leading to food and water scarcity, is an important development aspect for rural communities within the area. This aspect is based on the importance of water retention which is of primary importance with regard to the increase in population numbers, bringing with it an increase in demands on soil use.

Another important task for the future development of knowledge about the groundwater resources of the area is the monitoring of fluctuations in groundwater levels and quality. It would be necessary to drill several monitoring wells within the aquifers for this purpose. It is recommended to drill these wells as additional monitoring equipment for climatic stations and conduct groundwater monitoring together with measurements of climate characteristics (Dodola, Delo Mena, etc.). Selection of monitoring points for observation of groundwater level (quantity) and quality fluctuations in aquifers should be discussed with the Wereda Water Offices.

Results of water resources assessment show that the area is rich in both surface water and groundwater providing a good potential for future development. From the point of view of food security it is highly recommended to make the use of surface water for irrigation and subsequent increase in agriculture production a priority. Several rivers can be used for small hydropower schemes. Considering the surface and groundwater potential of the Dodola area:

1. Surface water is sufficient for irrigation of 16 % of the Dodola map sheet area (Yadot Welmel, Sawe, Deyu, Dumal rivers) considering the use of 10,000 m3/ha annually.

2. In the case of groundwater consumption of 20 l/d by the recent population the demand will represents 1.6 % of the assessed groundwater resources with the potential to supply people with relatively good quality drinking water (more than 80 % of inventoried water points meet requirements of drinking water standards).

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The potential of the area provides feasible and environmentally sound water management.

6.3 Human and Land Use Resources and DevelopmentThere is a large human resource potential within the area. The total assessed population is 0.8

million and average urban and rural population growth in the Oromia as well as in SNNP regions is 2.9 %. Taking this into account the population of the area will double in the next 20– 25 years. This represents a large potential of manpower for agricultural production as well as for developing industry using the area s natural resources. Agricultural irrigation should be practiced on the arable land and part of the area cover classified as pasture should also be used for arable land, and livestock husbandry should use more effective methods of livestock breeding.

Improvement of the health status of inhabitants using safe water supply systems and utilization of the remaining water resources for agricultural irrigation and the possibly for small hydropower schemes and industrial development (using other natural resources of the area) will improve the standard of life and help to eradicate poverty within this part of Ethiopia.

6.4 Wind and Solar Energy DevelopmentThe area has a good potential for the development of solar and wind energy. It should be feasible

to use the produced energy for local supply e.g. running pumps for groundwater development or for distribution of irrigation water. It could also be feasible to use this electricity for running local small businesses as grain mills, food processing and conservation industry etc.

6.5 Environmental Problems and their Control / ManagementAttention is paid to the eradication of poverty, protection of the environment and natural resources

as well as the increase in education in this field. The explanatory notes provide information for planning in sustainable economical development, other sectorial planning, management in the use of natural and human resources and protection against natural hazards. The study concentrates on the identification and protection of water resources, soil (particularly protection of soil against erosion), protection against natural hazards and wastewater and solid waste management.

Protection of water resources should be concentrated on better practices in sanitation within towns, villages and rural settlements. Most of the surface and groundwater is good in quality and can be used directly for drinking, agricultural and industrial purposes (see Chapter 5). Indication of improper sanitation practices is reflected in the increase of nitrates from human and animal waste in the shallow groundwater that is used by developed springs and dug wells. Water development practices should be based on basic principles of protection as follow:

1. The source of groundwater should not be drilled directly in the center of the village/town.2. The final design of the well and distribution system should prevent direct percolation of

water from the surroundings of the well along its casing to the groundwater.3. A well should be designed upstream from the groundwater flow direction in respect to

existing and potential pollution sources.4. The required minimal protection zones should be respected by land use development in the

vicinity of wells.5. Regular monitoring of water levels and quality should be performed.6. There should be improvements in the general application of sanitation and waste

management practices.

Soil erosion and protection is one of the limiting factors of sustainable development of agriculture within the area. The Vice-Minister (ENA) of Agriculture disclosed that Ethiopia is losing 1,900 million

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tons of soil through erosion every year. In the opening of a three-day workshop on soil fertility management, the Vice-Minister Ato Getachew Tekelemedhin said the country is losing 600 million Birr per annum due to reduced agricultural production triggered by the effects of soil erosion. If the current trend continues unabated, a sizeable farming community in the country would be forced to earn their livelihood from sources other than farming. The prominent factors for soil degradation in Ethiopia, according to the Vice-Minister, were population pressure, deforestation, poor agricultural techniques, overgrazing and drought. He noted that the Soil Fertility Initiative (SFI) launched by the World Bank and the UN Food and Agriculture Organization played an important role in preventing soil degradation in sub-Sahran countries including Ethiopia. Addressing the workshop, Mr. Ismail Serageldin, the Vice-President of a World Bank special program, expressed the bank´s readiness to support Ethiopia´s soil fertility initiative.

Data about soil erosion in the area are scare. The human causes of soil erosion relate mainly to Meher ploughing and seeding and the Belg harvesting seasons and the Kiremt season with the heaviest rainfall when crop cover is limited. Another human factor which contributes to soil erosion is the short fallow period (one to four years). Soil burning which destroys the organic matter content of the soil is another adverse factor.

Traditional soil cultivation and conservation techniques use ditches for drainage. The ditches run diagonally across the slope, usually with a gradient of more than 5 %. These ditches are made by ploughing deep into the ground. The spacing of the drainage ditches in a field depends on the steepness of the slope, the steeper fields having more drainage ditches than fields on gentler slopes.

Anti-erosion measures consist of several techniques. Some of the most frequent techniques can be defined as follows:

1. The appropriate parts the highlands and the Harenna escarpment should be reforested and/or existing forests should be preserved.

2. This area as well as parts of gorges, where reforestation is not possible, can be terraced (similar to the Konso area and/or on the slopes at the northern part of the country).

3. Retention of water in the countryside – construction of small dams for irrigation can help not only for the accumulation of water for irrigation, but also to slow down runoff after heavy rains and the accumulation of suspended material (eroded soil) in small dams. The accumulated material can be subsequently excavated and used as a fertilizer for arable land.

4. Wicker fascine – is a cheap and very simple anti-erosion measure that can be practiced in all parts of the area either separating agricultural fields of individual owners or implemented inside the field when the fields are big enough and highly prone to erosion.

5. Creation of shrubs/tree rows preventing wind erosion and slowing down surface runoff.6. Covering artificial cuts (along roads and other constructions) by nets or geo-textile.7. Other technical measures and agricultural practices.

A focus on soil conservation is one of the most important factors for environmentally sound land use. Soil conservation contributes significantly to food security in the area.

Natural hazard and protection against the consequences of earthquakes, land slides, rock falls and other hazards is important for the preservation of human lives, property and arable land.

Susceptibility to exogenous risks differs both in quantity and quality between the valley and plain engineering geological provinces. The following natural hazard potentials have been identified:• Slopes of the deep erosion valleys and mountain slope with repeated rockslides of all sizes and

small to medium sized rockfalls. • Repeated rock falls along the upper rims of the deeply cut valley sides. • River flood plains have been included into risk susceptible units because of the possibility of

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floods which can be very severe in arid areas. The observed lithological-structural changes in cuts of alluvial soils indicate the occurrence of catastrophic floods carrying substantially increased volumes of coarse materials in sub-historical times.

• Generally, the clay rich soils covering sedimentary rocks are prone to high plasticity and swelling when wet. That makes them rather problematic not only for building but also as material for earth roads especially during the rainy season.

• Soil erosion and protection has been address above so we can say that areas especially susceptible to erosion are medium energy relief in residual and colluvial soil units. An intensive deforestation in these areas will result in a further increase in the erosion susceptibility.

Susceptibility to endogenous risks has to be taken seriously also. Earthquakes are common in Ethiopia, but there is not enough information to assess the hazard potential of the Dodola area.

Waste water and solid waste management is important for environmentally sound development of the area. Appropriate management in this field protects not only the environment and soil and water resources but also human health against exposure to harmful pathogens and chemicals.

Recent practice is to release wastewater from households as well as from small scale industrial production directly to the environment. Wastewater is discharged directly to rivers without appropriate treatment. Wastewater is mixed with surface water and is used for irrigation as well as for drinking by people living downstream from wastewater discharge. People use this polluted water from the river without any knowledge about the potential harm to their health. There is little chance to educate a large number of people about the possible adverse health impact of using polluted water and that is why the waste water producers have the responsibility to treat the water to remove substances harmful for human health.

Infiltration of polluted water to groundwater threatens the groundwater resources of the area. It is very well documented by the increasing content of nitrates in groundwater.

Solid waste management is not practiced in any of the sites visited within the area. Increasing environmental care and protection of natural resources will contribute to better living standards of the people living within the area and also to an increase in their working output leading to an increase in food security.

6.6 Touristic Potential of the AreaThe area has a high touristic potential because of its beautiful landscapes and places of high

naturalist, especially geo-touristic, interest can be found here. An important aspect for the development of the touristic potential of the area is its accessibility. The area is accessible by asphalt roads (under construction during the field work).

The main tourist attraction of the area is the Bale Mountains National Park. The park was created in 1970 and covers about 2,200 km2 of the Bale Mountains to the west and southwest of Goba town. Within its boundaries are some of the highest points in Ethiopia, which include Tullu Deemtu (Mount Batu) – the highest point in Southern Ethiopia 4,377 m a.s.l. (Fig. 6.1). The Bale Mountains contain three distinct eco-regions: the northern plains, bush and woods; the central Sanetti Plateau with an average elevation of over 4,000 m a.s.l.; and the Southern Harenna Forest known for its mammals, amphibians and birds including many endemic species. The Central Sanetti Plateau is home to the largest population of the rare and endangered Ethiopian wolves. The endangered Painted Hunting Dog, Lycaon Pictus once existed in the park (with relict packs reported in the 1990s), but may now be extinct due to the human population pressures in this region.

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It is recommended to develop the touristic potential of the forest area at the foot of the Harenna escarpment. There are several forest areas i.e. Hawo, Kumbi, Godu, and Herele Forest that can provide trips to visit places with managed as well as wild coffee, bird watching, forest biotope study, etc.

Geology is the most influential factor controlling the natural scenery and landforms. Geology and erosion resulting in the formation of deep gorges and escarpment has also influenced Ethiopian history which is imprinted in the rocks in many parts of the country. It seams that little attention is given by the tourist industry to geological features like the Harenna erosional escarpment and

Fig. 6.1 Mount Tullu Deemtu – Bale Mountains National Park

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waterfalls along escarpment additional to trachyte plugs of Sanetti Plateau. Geotourism in Ethiopia is a new product which was introduced by Asfawossen et al. (2008) as a result of the project “Contribution of Geology to the Growth of the Tourism Industry in Ethiopia”.

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Over the past 40 years natural disasters on the Ethiopian territory have increased both in frequency and intensity and have led to severe social impacts, particularly in the southeastern part of the country. Evidence has long suggested that disaster risk reduction has a high cost-benefit ratio. Disasters also divert a substantial amount of national resources from development to relief, recovery and reconstruction, depriving the poor of the resources needed to escape poverty. Disasters cannot be avoided but there are ways to reduce risks and to limit their impacts. The action comprises preparedness, mitigation and prevention. It aims to enhance resilience to disasters and is underpinned by knowledge on how to manage risk, build capacity, and make use of information and communication technology as well as earth observation tools. Ethiopia is prone to natural risks like landslides, rock falls, flooding and particularly drought as reflected in geological, historical as well as recent records. Two or three subsequent periods of intense drought can cause severe crop losses, famine and population displacement in the country. The country also faces an increased risk due to climate change and more extreme weather which can be accelerated particularly in the vulnerable semi-arid part of the country. The insufficient quality of drinking water, the natural risks and the overall degradation of the environment are all fundamental problems and contribute to an increase in the rate of migration to urban areas.

These explanatory notes to the hydrogeological and hydrochemical map of the Dodola area provide the results of the joint Czech Ethiopian projects. The mapping activity was carried out by field groups of hydrogeologists of the GSE in framework of the project “Groundwater Resources Assessment of the Southeastern Highlands and Associated Lowlands of Ethiopia” in 2010. The mapped area covers 18,356 km2 and is inhabited by 0.8 million people.

Groundwater accumulates in porous aquifers of alluvial and elluvial origin and in fissured and karst aquifers hosted in sedimentary (particularly limestone), volcanic and basement rocks. The peak forming trachytes in the Bale Mountains represent minor fissured aquifers (aquitards) of the area.

There is a relatively good potential for development of surface water for small-scale irrigation and electricity generation in the area because the Genale, Welmel and Dumal rivers and several intermittent rivers drain groundwater of volcanic, limestone and basement aquifers. It is necessary to consider that the groundwater level in the aquifers will fall to greater depths during periods with inadequate precipitation and river flow fed by groundwater will disappear during periods of drought in most of rivers of the area.

Groundwater is relatively of good quality and the groundwater resources can be directly used for drinking, industrial as well as agricultural purposes. Groundwater should be primarily used for drinking water supply; it should be also used for irrigation should there be clear evidence that pumping for irrigation does not lead to over pumping of the aquifer, undermining of groundwater

ConclusionsConclusions

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resources and causing degradation of the aquifer. Should the aquifer be used for irrigation, monitoring wells are recommended to be drilled together with the production wells for systematic observation of changes in groundwater levels, quality of pumped water and optimization of the pumping system.

Local pollution of groundwater by nitrates is common in rural as well as in urban areas. In the case of developed springs their surroundings should be protected against pollution because most of the springs have shallow groundwater circulation and human as well as animal waste (problem of watering animals directly from the spring) can easily and quickly penetrate the groundwater resources. This is also a problem in karst aquifers which are highly vulnerable to pollution because of their high permeability. The spring should be developed by a solid concrete box and it is preferable that the water will flow from the spring by a tube and distributed to people 10–20 m from the spring (lower position of water distribution point). The area of the protection box should be protected against the entry of people and animals; in particular animals should be completely prevented entry.

It is advisable to use geophysical investigation to select locations where the regolith is thick and volcanic, sedimentary and basement rocks are deeply fractured, weathered and soft for siting wells. Groundwater can be totally missing when the regional groundwater table is not reached in cases where the drilled part of the basalt or basement is massive without joints and fissures. It is also true for aquifers in limestone where groundwater is deep and its level is controlled by the level of surface water or the level of principal springs representing the regional drainage of the area.

The water distribution well should preferably be equipped with a system minimizing discharge of water when it is filled into containers. In the case that water is used for animal watering it should be transported by a tube and distributed to the animals about 20–30 m from the well (lower position of water distribution point – cattle bin). The area of the well head should be protected against accumulation of surface water by drainage ditches and the entrance of animals to the well’s surroundings should be completely eliminated.

The proposed development should take into consideration the protection and conservation of the natural resources of the area. Particular interest should be paid to soil conservation and groundwater protection using the appropriate agricultural methods to decrease soil erosion and to the implementation of water resource protection to protect groundwater against pollution and over pumping, particularly in rural and urban settlements where pollution by nitrates is increasing. Monitoring of environmental components, particularly surface water flow and sediment load, in gauging stations in the lower reaches of the river should be enhanced. Recent inappropriate wastewater and waste management has to be considerably improved.

The area has high touristic potential because of its beautiful landscapes and places of high naturalist, especially geo-touristic, interest can be found in the Bale Mountains National Park.

Despite some local and regional environmental problems the Dodola area provides the potential for feasible and environmentally sound natural and human resource management.

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Agrotec-C.R.G.-S.E.D.E.S. Ass. (1974): Report on The Sothern Rangelands Livestock Development Project.

Asfawossen, Asrat; Metasebia, Demissie; Aberra, Megessie (2008): Geoturism in Ethiopia, Shama Booke, Addis Ababa, Ethiopia 185 pp.

BEICIP (1985): Petroleum potential of Ethiopia. Confidential report of Ministry of Mines and Energy, Addis Ababa.

Belay (1978): Preliminary geological map of Dodola sheet in scale 1:250,000. MS GSE, Addis Ababa.

Bogena, H.; Kunkel, R.; Schrobel, T.; Schrey, H.P.; Wendland, F. (2005): Distributed modeling of groundwater recharge at the macroscale, Ecological Modeling 187, 2005, 15-26.

Bosellini, A. (1989): The continental margins of Somalia: their structural evolution and sequence stratigraphy. Memorie di Scienze Geologische, Padova 41: 373–458.

Brooks, Nick (draft, 2005): Cultural responses to aridity in the Middle Holocene and increased social complexity, Quaternary International, April 2005.

Central Statistics Authority (2007): Statistical abstract.

Daniel, Gamatchu (1977): Aspects and Water budget in Ethiopia, AA University Press, Addis Ababa.

Edet et al. (1998): Application of remote sensing data groundwater exploration: A case study of the Cross River State, Southeastern Nigeria.Hydrogeology J. 6:394-404.

FAO-UNESCO (1978): FAO-UNESCO, Soil map of the World, 1:5,000,000 (download for ArcGis) - http://www.lib.berkeley.edu/EART/fao.html

Gani, N.DS.; Abdelsealm, M.; Gera, S.; Gani, M.R. (2008). Stratigraphic and structural evolution of Blue Nile Basin, North Western Ethiopia. Geological Journal.

Gilboy, C.F. (1970): The geology of the Gariboro region, southern Ethiopia. Unpub. Ph.D. Thesis, Univ. Leeds, Leeds.

Gozálbez, Javier; Cebrián, Dulce (2006): Touching Ethiopia 2nd English edition, 2006, Shama Publisher.

Hambisa et al. (1997): Geology of the Dodola area, GSE, Addis Ababa (unpublished).

HSDP (2008): Ministry of Health (MOH) [Ethiopia]. 2008. The Health Sector Strategic Plan (HSDP). Addis Ababa, Ethiopia: Ministry of Health.

http://www.moh.gov.et

Chater, A.M. (1971): The geology of the Megado area of southern Ethiopia. Unpub. Ph.D. Thesis, Univ. Leeds, Leeds.

Jelenc, D. (1966): Mineral occurrences of Ethiopia. Ministry of Mines, Addis Ababa.

Kazmin, V. (1972): Geological Map of Ethiopia. Geological Survey of Ethiopia: Addis Ababa, Ethiopia.

Kazmin, V.; Seifemichael, Berhe (1978): Geological map of Nazreth Sheet, Scale 1:250,000, Geological Survey of Ethiopia, Addis Ababa.

ReferencesReferences

134

Koeppen in NMSA (1989): Climatic and Agroclimatic Resources of Ethiopia, NMSA, AA, August 1989.

Lahmeyer International (2005): Genale-Dawa river basin integrated resource development master plan study. Ministry of Water Resources.

Linsley, R.K.; Kohler, M.A.; Paulhus, J.L.H.; Wallace, J.S. (1958): Hydrology for engineers. McGraw Hill, New York.

Mohr, Paul (1962): The Geology of Ethiopia. Addis Ababa University Press, Addis Ababa, Ethiopia: 268.

MoWR (2002): Negarit Gazeta No. 12/1990 and The Guidelines of Ministry of Water Resources.

Nehmia, Solomon; Raghuvanshi, Tarun K. (2005): Assessment of Small Hydropower Potential in Muger, Jemma and Weleka Sub-basins of Abay Basin (Central Ethiopia) 9th The Symposium on Sustainable Water resources Development, Arba Minch University, Ethiopia.

NMA (2006): Annual Climatic Bulletin 2006, Federal Democratic Republic of Ethiopia, Ministry of Water Resources, National Meteorological Agency (NMA) Climatolocical Service Team.

NMSA (1996): Assessment of Drought in Ethiopia, Meteorological Research Reports Series No. 2, Addis Ababa, Ethiopia.

Reddy (2001): A Text Book of Hydrology. Laxmi publication, New Delhi.

RRC (1985): The Challenges of Drought, Ethiopia’s decade of struggle in relief and rehabilitation, RRC, Addis Ababa, 280p.

Setegn, S.G. (2010): Modeling Hydrological and Hy-drodynamic Processes in Lake Tana Basin, Ethiopia, KTH. TRITA-LWR PhD Thesis 1057.

Shaban, A.; Khawlie, M.; Abdallah, C. (2006): Use of remote sensing and GIS to determine recharge potential zone: the case of Occidental Lebanon. Hydrogeol J 14:433–443.

Sima, Jiri et al. (2009): Water Resources Management and Environmental Protection Studies of the Jemma River Basin for Improved Food Security. ISBN 978-80-254-5021-5. AQUATEST a.s., Prague, Czech Rep.

sine (1981): Atlas of Ethiopia. Mapping Agency, Addis Ababa.

Tesfay, Tafese (2001): Nile question: Hydropolitics, Legal Wrangling, Modus Vivendi and Perspectives.

Tesfaye, Chernet (1993): Hydrogeology of Ethiopia and Water Resources Development – MS EIGS, Ministry of Mines and energy, Addis Ababa (Library 880-051-17).

Tsur; Issar (1998): Custodio E, Gurgui A. Groundwater economics Developments in Water Science 39, Elsevier p. 373–379.

WHO (2006): World Health Statistics, http://www.who.int/countries/eth/eth/en/

Worku, H. (1996): Geodynamic development of the Adola Belt (southern Ethiopia) in the Neoproterozoic and its control on gold mineralization. Pub. Ph.D. thesis,Tech. Univ. Berlin. Germany.

WWDST (2003): Wabe Shebelle River Basin Integrated Development Master Plan Study, MWR, Addis Ababa.

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