Assessment of Temporal Hydrological Variations Due to Land Use Changes Using Remote Sensing/GIS

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Selome Mekonnen Tessema TRITA LWR Masters Thesis

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Assessment of Temporal Hydrological Variations Due to Land Use Changes Using Remote Sensing/GIS, KTH

ABSTRACT

Land use change is one of the major reasons for variations in the hydrological parameters of awatershed. The major aim of this work was to analyze the land use change in Gilgel Abay Sub-basin of the Lake Tana basin between the years 1987 and 2001. Lake Tana is located in the

Northwestern Plateau of Ethiopia and is the largest lake in the country. Apart from previouswork assessment and field survey, historic data from two satellite images, Landsat TM (1989-1992) and Landsat ETM+ (2001), were used as inputs to produce three GIS-based land covermaps of the area. The results showed that in between the referenced time period, cultivated landdecreased totally by 14% in a continual sense. Bush and shrub land decreased between 1987 and1992 by 6% but increased during the years 1992 and 2001 by 53%, much more than to theirprevious level. Open grassland gained 35% from other cover types in the first period but lostaround 49% in the second. Forest cover increased by more than two times its previous levelduring the first period but decreased about 15% in the later one. Water, with major input of thelake, increased by nearly 1% in the first period but decreased by about 117 km2 (4%) in the sec-ond period. Population change and the consequential demand for land and trees were considered

to be the major driving forces behind the changes. This study showed that in addition to the soildegradation, the land use changes affect the hydrological parameters of the area in a sensible way,both in quality and quantity. Thus, the assessment indicated the significance of the anthropo-genic changes on the lake level changes.

Key words: Land cover changes; Remote sensing; GIS; Hydrological parameters; LakeTana

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ACKNOWLEDGMENTS

First of all I would like to thank my supervisor Ulla Mrtberg for her advice and support in eve-rything I needed, which contributed for the improvement of my work. I also would like to thankKirlna Skeppstrm for her time and vital guides. My regard goes to Swedish International Devel-

opment Association (SIDA) for sponsoring my stay in Stockholm and my trip to the study area.My best regard goes to Sofia Norlander for her patience and support that has made my stay bothin Stockholm and at the study area very smooth and enjoyable.

My appreciation goes to Dr. Seifu Kebede for his continual advice and encouragements throughthe whole program. I would like to thank the stuffs in Geology and Geophysics department of

Addis Ababa University with special thanks to all postgraduates of the year 2005 in GIS andRemote Rensing particularly Henok, Addisu and Zeleke; and Yohannes from Technology Fac-ulty. I also thank the Lake Tana Research Center in Bahir Dar University particularly Ato De-salgn, Korbaga and Seyum for making it easy to access the necessary data I needed for my re-search. I thank The Ministry of Water Resources and National Meteorological Service Agency forproviding me with the whole raw data for my study.

My special thanks goes to my beloved mother, Ewuye, and all my brothers for being there withme through thick and thin by supporting me and providing their love. I thank all my geo friendsfor their moral support during my study that has made me feel happy and at home. I would liketo thank my friend Fasil Bekele for supporting me in moral and logistics and for making my stayin Stockholm very enjoyable. Last but not least my deepest gratitude goes to my best friends 40s.

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TABLE OF CONTENTS

ABSTRACT................................................................................................................iii

ACKNOWLEDGMENTS........................................................................................... v

TABLE OF CONTENTS......................................................................................... vii

1. INTRODUCTION.................................................................................................. 1

1.1 Aim and Objectives .............................................................................................................. 1

1.2 General Description of the study area ............................................................................... 11.2.1 Physiography ................................................................................................................. 31.2.2 Climate ........................................................................................................................... 31.2.3 Geology.......................................................................................................................... 31.2.4 Soil .................................................................................................................................. 41.2.5 Land Cover/Land Use................................................................................................. 51.2.6 Aquatic Species ............................................................................................................. 5

2. MATERIALS AND METHODS............................................................................6

2.1 Hydrological data analysis .................................................................................................... 6

2.2 Field survey ............................................................................................................................ 6

2.3 GIS/Remote Sensing............................................................................................................ 6

3. RESULTS................................................................................................................ 9

3.1 Lake Tana Basin .................................................................................................................... 9

3.2 Hydrology............................................................................................................................. 113.2.1 Precipitation................................................................................................................. 113.2.2 River Discharge........................................................................................................... 113.2.3 Sediment flow.............................................................................................................. 123.2.4 Lake Tana surface level.............................................................................................. 12

3.3 Land Cover Analysis........................................................................................................... 143.3.1 Dynamics in land cover types ................................................................................... 153.3.2 Accuracy Assessment................................................................................................. 20

4. DISCUSSION........................................................................................................ 224.1 Causes of land cover dynamics ......................................................................................... 22

4.2 Implications of land cover dynamics................................................................................ 244.2.1 Soil degradation........................................................................................................... 244.2.2 Hydrological processes in the catchment................................................................ 25

5. CONCLUSIONS AND RECOMMENDATIONS .............................................. 28

REFERENCES......................................................................................................... 29

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Assessment of Temporal Hydrological Variations Due to Land Use Changes Using Remote Sensing/GIS,KTH

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1. INTRODUCTION

Land use change is one of the major reasons for any change in the hydrological parametersof a watershed, next to climate change. Land degradation is becoming a serious problem

for the whole country, Ethiopia, and is induced by land use changes in different ways,including deforestation and poor management of land. According to a previous work doneby Berry (2003) land degradation, in addition to the direct damage, is a major reason forsilting of dams and riverbeds, increasing irregularity of streams and rivers and reducedgroundwater capacity.

1.1 Aim and Objectives

The major aim of this work was to analyse the land use changes in Gilgel Abay subbasin ofthe Lake Tana basin between the years 1987 and 2001. The analysis was then used to rea-son out any possible hydrological parameter changes in the study area.

The specific objectives included in the research were:

- to produce a land cover map of the Gilgel Abay subbasin for each reference time

- to analyse the land use changes that occurred within the referenced time

- to analyse the hydrological parameter changes in the area and to correlate it withthe results of the land use analysis

- to recommend possible mitigation measures in order to minimize the effects ofland use changes on hydrological parameters

1.2 General Description of the study area

Ethiopia is located at the Horn of Africa and has a total surface area of 1.13 million squarekilometres (Fig 1.1) that is characterized by three physiographic regions (Mesfin, 1972):

Northwestern plateau, Southeastern Plateau and Rift Valley. The population is 63 million(CSA, 2000) with the rate of growth estimated to be 3.1%. Agriculture has always been thebackbone of the economy that engaged 86% of the population. About half (51.8%) of thegross national product and 90% of the export earnings is supplied by agriculture, as well asit is supplying the bulk of raw materials for the agriculture-based industries (CSA, 1999).

The country became land locked in 1993 with 8,800 km2 total area of inland water, repre-senting 0.72 % of the total surface area of the country (Bonzon and Horemans, 1988) thatcomprises of 10 lakes. Among these, Lake Tana is the largest and covers nearly 50% of thetotal lake area and is located on the Northwestern Plateau of the country.

Lake Tana Basin (LTB) is situated at the top of the Blue Nile Basin, forming its uppercourse (Fig 1.2). Geographically, LTB is bounded between latitude 1057`1247`N andlongitude 3638`- 3814`E. The basins landscape is part of the western plateau of Ethiopiaand includes the escarpments of Gonder and Gojam, the lower plains surrounding the lakethat forms extensive wetlands during the rainy season, such as Dembiya, Fogera, Kunzilaplain in the north, east and southwest respectively and Bahir Dar city in the south. Thebasin is estimated to have a total area of 15,320 km2 (Tana-Beles Project Studies, StudioPietrangeli, 1988) while the lake covers an area of 3060 km2 with about 70 km length and60 km width. The lake is shallow with an average depth of 9 m it is oligotrophic and fresh,

with weak seasonal stratification (Wudneh 1998). The surface level of the water is situated1785 m asl.


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Fig 1.1 Location map of the study area

Source: The World Fact Bookwww.cia.gov/cia/publications/factbook.

Lake Tana gains water from four major rivers; Megech, Ribb, Gumara and Gilgel Abbayfrom the northern, northeastern, eastern and southern side of the lake, respectively. Inaddition, about 30 or more seasonal and perennial rivers flow into the lake, Gilgel Abbay

River is the major source that originates from a small spring located 2850 m asl (Gish-Abbay), and flows about 96 km before it drains out from the lake, together with the rest ofthe inflows, at Bahir Dar city where it is called Blue Nile River. The Blue Nile River then

goes a long way until it reaches Sudan to join the White Nile River. Together these riversform the world largest river, the Nile.

N

l50 0 50 100 Kiometers

Lake Tana Basin

Blue Nile RiverBasin

Fig 1.2. Lake Tana Basin in Relative to Blue Nile River Basin

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http://www.cia.gov/cia/publications/factbookhttp://www.cia.gov/cia/publications/factbook


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Gilgel Abbay catchment: is the largest of four catchments that form the big LTB (Fig.1.3). It covers the southern part of LTB with the south part of the lake as its northernboundary and contributes almost 60 % of the inflow.

1.2.1 PhysiographyGilgel Abbay catchment is situated at the northwest part of the country, on the Northwest-ern Plateau between latitudes 1057`1154`N and longitudes 3638`-3723`E.Topog-raphically the elevation of the land varies between 1800 and 4000 m asl. The higher eleva-tion is concentrated at the southeastern corner with the rest of the portion lying relativelyuniform and gentle under it. According to the study of the Tana Beles project (Tana-BelesProject Studies, Studio Pietrangeli, 1988) the catchments area is about 5,004 km2. Gilgel

Abbay River has one major tributary known as Koga River.

1.2.2 Climate

Based on the climatic zones classification, the catchment falls within the Cool semi-humidzone that represents altitudes of 1800-2400 m asl, with mean annual temperature of 17-20C and the Cool and humid zones that represents altitudes of 2400-3200 m asl, withmean annual temperature of 11.5-17C. The dry season occurs between November and

April while the wet season occurs mostly between May and October. Small rains also occursporadically during April and May. Mild temperatures and conductive rainfall are typical ofthe area in between 1900 and 2400 m asl, making it more suitable for most crops and live-stock production, whereas cooler temperatures and high rainfall is a character of the areasabove 2400 m asl.

N

l50 0 50 100 K i o m e t e r s

Gilgel Abbay Catchment

Lake Tana

Fig 1.3 Drainage pattern of the Lake Tana Basin

1.2.3 Geology

According to Mesfin (1972) and Kazmin (1973) the area is comprised of sedimentary,effusive and intrusive rocks. Formations occurring in the LTB include Tertiary Volcanics,Chelga Beds, Quaternary Volcanics and Recent Alluvium and Lake Sediments.


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Tertiary Volcanics: are mostly represented by alternating basalts and related pyroclasticdeposits, with almost horizontal attitude. Basalt lavas are generally fresh and have usually amassive structure. It is likely that those most closely surrounding the Lake are mostly ofMiocene age.

Chelga Beds: are constituted by tuffs, shales and sand stone, with local lignite beds, the

Chelga formation corresponds to lacustrine sediments, deposited in one or more Palaeo-tana, formed by tectonic sinking of an area larger than the present lake. These beds arepossibly of Pliocene age, contemporary with the last occurrence of Tertiary volcanicity.

Quaternary Volcanics: are mostly represented by olivine alkali basalts, often interbeddedwith clayey palaeosoils. Near Bahir Dar, rhyolite lava is also present. The basalts are partlyvesicular with various weathering degree. Pyroclastic deposits are also observed. The lastoccurrences of the quaternary volcanicity, mostly explosive, have left perfectly preservedcrateric edifices, as a sign of a very recent age. Other edifices, mostly lavic, rise over theplain covered by basalt flows, extending from Bahir Dar to beyond the Gilgel Abbay. Alsothe small Dek Island, rising close to the larger and flat Nargadaga lavic island, is one ofsuch edifices.

Recent Alluvium and Lake Sediments: cover the banks of the lower reaches of the mainLake Tana emissaries. They are composed of fine sediments, partly of marsh environment,

which usually are represented by clays. Very fine sediments often line the Lake bottom. Inthe deepest part of the lake they cover consolidated sediments.

1.2.4 Soil

There are seven types of soil groups observed in this area, Alisols, Fluvisols, Leptosols,Luvisols, Nitisols, Regosols, Vertisols (BCEOM, 1998) in combination with four diagnostichorizon modifiers: chromic, eutric, haplic, and lithic.

According to the work of BCEOM (1998), from these the Gilgel Abay catchment is mostlycovered by Haplic Luvisols with an areal coverage of around 2583 km2. The soil units with

their areal coverage are summarized in Table 1.1.The FAO (1991) description states thatthe Luvisols are, in general, fertile soils because of their mixed mineralogy, relatively highnutrient content and the presence of weatherable minerals. The cultivated areas mostly lieon this type of soil throughout the study area.

Table 1.1 Gilgel Abay Catchments major soils

Major Soil Area (Km2)

Eutric Regosols 45

Eutric Fluvisols 133

Haplic Nitisols 193

Eutric Leptosols 213Lithic Leptosols 457

Eutric Vertisols 491

Haplic Alisols 860

Chromic Luvisols 1034

Haplic Luvisols 2583

(Source: BCEOM 1998, Soil Map of Lake Tana Basin)

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1.2.5 Land Cover/Land Use

Agriculture, predominantly rain-fed, is the main stay of the whole Tana basin economy.The surrounding flood plains known as Fogera, Dembia, Alefa and Achefer, are intensivelycultivated areas for more than centuries. Irrigated land and perennial crops, mainly coffee,also hold a great amount of the area. Yields are very low because of the shortage of skilledmanpower, backward technology, poor infrastructure in rural Ethiopia, recurrent drought,and land degradation. Land degradation is perhaps the most significant factor. For exam-ple, a case study in Gojjam, which includes the study area, indicated that the amount ofnutrients lost from an unprotected cultivated land by sediment and runoff is estimated at830 kg to 2,068 kg ha-1 (Kejela, 1992).

One of the major natural forests covers a small part of the Gilgel Abay catchment at thenorth, which is known as Zege, the peninsula of the lake. Some marshes and wetlandsoccur at the floodplains, especially near the lake, with a dominant flora type of Papyrus(Cyperus papyrus). Bushes, shrubs and grassland including grazing land cover the rest of

the basin.Many small urban areas are distributed through out the whole basin. The biggest of these isBahir Dar city, which is located on the south corner of the lake where the Blue Nile Riveroutlet starts. In addition to the urbanization there are two hydroelectric power plants thatuse the outflow of Lake Tana to start their turbines with the help of seven gates of a weir,known as Chara Chara. Tis AbayI & II hydroelectric plants are located on the Abay River,some 32 km downstream of Lake Tana, at a site where the riverbed suddenly drops byapproximately45meters; thus creating the well-known Tis Issat Water Falls. Apart fromthese, seven irrigation schemes have been proposed with an overall water demand greaterthan 600 million m3 per year within the watershed of the lake (USBR, 1964).

Koga Irrigation Project is one of the seven proposed irrigation schemes that lie in the

Gilgel Abay catchment area. The Koga River is a major tributary of the Gilgel Abay, whichflows into Lake Tana. The Koga catchment covers 27,850 ha and about half of this area isintensively cultivated in the rain season (Acres International Limited and Shawel ConsultInternational, 1995). By the time of the field survey the project was on a preliminary con-struction level.

1.2.6 Aquatic Species

According to the work of Wudneh (1998) the major fish categories of Lake Tana are Barbusspp., Clarias gariepinus, and Oreochromis niloticus, which contribute to the important fisheryactivity of the area.O. niloticusis most abundant in the shallow littoral zone, while C. garie-

pinusand the larger piscivorous Barbusspecies are found mainly in the deeper open water

area of the lake. The breeding activity of all major species is associated with the rainy pe-riod and increase in the water level of the lake.

In addition, there are endemic aquatic species localized in this area. Among these Barbusintermedius, species of large barbs (LakeNet, 2004) is one of those, which is the only intactcyprinid species flock in the world. Makedia tanensis(Manconi et al., 1999) is an endemicfreshwater sponge that resides in Lake Tana. Many wetland and migratory water birds arealso dependent on the lake for feeding and resting grounds.


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2. MATERIALS ANDMETHODS

The analysis started with a literature review followed by a field survey, in which samples ofground truth were taken. During the field survey efforts were made to observe locations inthe study area mainly based on accessibility. Then, using GIS and remote sensing, thechanges in land cover was analysed by interpretation of temporal satellite image data.

2.1 Hydrological data analysis

The existing data of rainfall, discharge, lake water level and sediment load in Lake Tanabasin were collected and analysed. Raw data was collected and prepared by the governmentoffices. National Meteorological Service Agency measured daily rainfall in different gaugingstations located in Lake Tana Basin. The Ministry of Water Resources collects hydrometricdata at eight gauging stations, including the outflow data of the Lake at Bahir Dar and thesuspended solid load of the five major emissaries of Lake Tana.

The work of JICA (1977) and Kebede et al., (2005) were analyzed and compiled. JICA(1977) used the data recorded from five gauged stations during 1964-1975 and calculated

the water balance based on a simple principle of water budget method expressed as:I + P O E = S

Where, I inflow from basin to lake

P direct precipitation into lake

O outflow from the lake

E evaporation and infiltration from lake

S storage in lake

Kebede et al., (2005) used data recorded during 1960-1992 and the inflow from the un-gauged catchments that was estimated from runoff coefficient. Efforts were made to com-pare the above two works in order to see any temporal changes that may have occurred

during the years.The water level of Lake Tana has a series of measured data at a station near Bahir Dar city.The measurements took place in the years 1964-2003 on a daily basis from a datum level of1783.5 m asl. The data was analysed using Excel software and compared with the relativelake height variations computed from satellites altimetry of TOPEX/POSEIDON (T/P)and Jason-1 by NASA (2005).

2.2 Field survey

Samples of land cover were taken during the field survey in about 10 locations, and most ofthem were near the lake and on the islands. The record shows that the area is highly mixed

with each type of land cover and therefore it was very difficult to differentiate into specificclasses. The cultivated land and the trees were well mixed and had to be represented aspercentages in the map. Global Positioning System (GPS) was used to locate the samplesof ground truth data and use the data during the satellite image classification. Informalinterviews were also made with local people that included farmers, fisheries, boatmen andresearchers.

2.3 GIS/Remote Sensing

The materials used to assess the land use changes for this study were historic data from twosatellite images. The first was an orthorecitified Landsat TM image dated 1989-1992 that

included three bands (3, 4 & 5) and the second one was Landsat ETM+

image dated Febru-ary 2001. Topographic maps of the area with a scale of 1:50,000, produced by the Ethio-

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pian Mapping Agency (1987), were used as a background for both the GPS locations andfor geo-referencing of the satellite images. Different types of maps that included the Landcover map of the Tana Basin produced by BCEOM (1998) in the Abbay River Basin Inte-grated Development Master Plan project were used as a reference and background for the

image analysis. Different scenes from different years (1986-1989) were used to produce theresult maps, in which the study area was covered by the 1987 image. The part covered bythe 1987 image particularly was used for the final analysis to compare the changes that hadtaken place during that period of time. The software used to prepare and analyze the im-ages were Idrisi32 (Eastman, 2001), ArcView GIS 3.2 (ESRI, 1999), MapInfo Professional7.8 (MapInfo, 2001), ENVI 3.5 (RSI, 2001) and CartaLinx (CartaLinx, 2001).

The initial step taken to prepare the images was the selection of Ground Control Points(GCP) from the 1:50,000 topographic maps for computations of a deformation model.

Then geometric correction to the Universal Transverse Mercator (UTM) cartographicprojection, zone 37 north, was made. Since the main study area, the Gilgel Abay catchment,lied completely within one scene, radiometric balancing and digital mosaicking of contigu-ous scenes were unnecessary. Bands 3, 4 and 5 were combined together as blue, green andred respectively to provide the false color composite image (RGB image) that was used as abackground to perform a supervised classification on both the 1992 and 2001 images.

The next step was delimiting and cutting out the study watershed by tracing it from1:50,000 topographic maps and digitizing it in ArcView 3.2. The traced boundary was thenused in Idrisi 32 by superimposing it on the images during the analysis. The references usedto select the training sites for the supervised image classification were the samples takenduring the fieldwork and the land cover map produced by BCEOM (1998). The trainingsites were selected distributively, in order to cover the whole image with a minimum of 70pixels for each class. During the selection of the pixels it was noticed that each class had

more than one or two different pixel values to represent it. This is due to the heterogene-ous pattern of the land use types of the area, which makes it very difficult to perform adistinguished value for each. In order to avoid errors that can occur as details increase, theclassification scheme was kept simple. For the purpose of comparison, many land covertypes produced from the multispectral Landsat image were synchronized to fit the numberof classes picked from the previous work (BCEOM, 1998) and the current ground truthdata. In so reducing the number of classes, proper care was taken to minimize errors due togeneralization. Thus, six land cover classes were identified: cultivated, grassland, grass-bush-shrub land, forest, water and urban with different dominancy values.


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Fig 2.1 A highly complicated land use pattern with cultivated land (maize) and Junipers,

near Bahir Dar city

After the signature was achieved for each class and after trying different methods of imageclassification, the selected method for this particular case was the Maximum LikelihoodClassification (MLC) on account of the mixed pixel values. Then the on-screen digitizing ofthe 6 different land cover classes was made using MapInfo 7.8 on the basis of the MLCimage. From these land cover types the forest and urban classes were delineated by handdue to the heterogeneity of the pixels around these areas (Fig 2.1), that in the classification

would give a totally different output on the final images. In order to minimize the error,signatures for these two classes were not used during the MLC step. Finally two land covermaps were produced corresponding to the two reference years, 1992 and 2001, and tempo-

ral changes in land cover were determined.

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3. RESULTS

3.1 Lake Tana Basin

Previous works indicate that the area is susceptible to land degradation and sediment run-

off from the surface that goes directly into the lake. The population of the area showsincrement with a total average growth rate of 2.6% per year (CSA, 1994). The populationdensity in the Tana Basin is 50-200 persons/km2 and in Gilgel Abay catchment it rises to100-150 persons/km2.

Among the previous work done on the area, Ministry of Water Resources in associationwith BCEOM (Bureau Central d'Etudes pour les Equipments d'Outre-Mer) EngineeringConsultants did a major study on Abbay River Basin Integrated Development Master PlanProject (1998). The output of the study included basic information about land cover, hy-drology, geology, soil and socio economic activities in much detail, which included mapsand reports. The statistics from the Land Cover map of Tana Basin mapped during 1986-1989 showed that the majority of the area is covered by cultivated land (BCEOM, 1998).

According to the map, the total area of cultivated land is around 8,194 km2, which covershalf of the basins total area. The eucalyptus trees are growing everywhere in the fields andthe farmers use it as a source of income and firewood. A very disturbed forest is mappedaround the lake, covering around 5 km2 and at the eastern corner of the basin with a cover-age of 90 km2 there exists afro-alpine forest. Shrubs, woods, bushes and grassland, with theexception of the water bodies that include the lake and the streams that flows into it, coverthe remaining part of the area.

The work of Kebede (2001) stated the fact that the nearby urbanization of the city of BahirDar is one of the causes for the pollution of the lake through the uncontrolled nutrientinflow. A study by Tsehale et al., (2002) stated that the catchment area of the Lake Tana

experiences a soil loss rate ranging from less than 5 to more than 250 tons per hectare peryear (Fig 3.1). The increasing turbidity following the rainy season resulted in a decline ofplankton production and in the blooming of weeds.


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Fig 3.1 Gully erosion near Merawi town: land degradation and widening up of gullies were

observed during the stay, the smaller and seasonal streams pushed the process of

land degradation by increasing the slopes.

According to the study made by Merid (2005), the unavailability of any form of waste col-lection and treatment facilities in the major towns like Bahir-Dar and Gonder and villagessituated in the Lake Tana sub-basin, makes it obvious that the waste is directly dischargedto the lake. Hence, the population pressure and industrialization trend of the area will causepollution problems in Lake Tana in the near future. The high sediment load in the rivers isa combined result of the high topographic slope, the strong rain-fall pattern and of defores-tation as a result of the population pressure.

Kebede et al., (2005) calculated the water balance of Lake Tana and its sensitivity to fluc-tuations in rainfall. They concluded that variation in climate plays a far greater control onLake Tana hydrology than human impact or local forces such as deforestation and accom-panied changes in runoff, and diversion for irrigation, during the last century. The dataused for this research was a series of meteorological and hydrological measurements of1960-1992.

According to the interviews the deforestation is increasing. The two major reasons they putforward were for getting an income by selling the wood and for agricultural land due to theincrement of the population. One boatman said that the level of the lake fluctuates highlyto the degree of hindering the bigger boats movements, making the transportation processineffective.

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3.2 Hydrology

The catchment area of the whole Lake Tana Basin differs in different works: 15,017 km2(JICA, 1977); 15,320 km2 (Tana-Beles Project Studies, Studio Pietrangeli, 1988); and 16,500km2 (LakeNet, 2004). Among the four major rivers feeding Lake Tana, the Gilgel AbbayRiver is the largest, covering around 5,004 km2 catchment area (JICA 1977).

3.2.1 Precipitation

National Meteorological Service Agency measured a daily rainfall data in different gaugingstations located in Lake Tana Basin. The recording periods differed in each station, butgenerally lie in between 1964-2004. Fig 3.2 shows the mean annual rainfall calculated fromthe available ten stations. As shown in the graph, within the forty year period the maximum

value was in 1973 (1772.83 mm) and a minimum value was in 1991 (691.75 mm).

Mean Annual Rainfall in Lake Tana Basin

0.0

250.0

500.0

750.0

1 000.0

1 250.0

1 500.0

1 750.0

2 000.0

1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004

Recording Period (year)

MeanAnnualRF(mm)

Fig 3.2 Mean Annual Rainfall in Lake Tana Basin from ten gauging stations with 10 to 40

years of recording periods.

3.2.2 River Discharge

The Ministry of Water Resources collects hydrometric data at eight gauging stations, in-

cluding the outflow data of the Lake at Bahir Dar. The gauging stations in the basin includethe rivers Megech (642 km2), Gumaro (174 km2), Garno (94 km2) in the north sub basin:Ribb (1592 km2) and Gumara (1394 km2) in the eastern sub basins and Koga (244 km2) &Gilgel Abbay (1664 km2) in the south sub basin. All together the gauged watershed is esti-mated to be 5624 km2, which accounts 35% of the total catchment area of the lake.

According to JICA (1977) the mean annual inflow, after the compilation of the data for thefive major gauged rivers with recording periods of 1964-1975 is 104.02 m3/s and the meanannual discharge of Abbay River (outflow) is 113.31 m3/s. The calculated mean annual

water balance of Lake Tana is tabulated in Table 3.1.


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Table 3.1 Mean annual water balance of Lake Tana (JICA, 1977)

Components Mean annual (mm)

I + P 3896

O 1133

E 2741S 22

According to Kebede et al. (2005) total mean annual surface water inflow is estimated to116.2 m3/s, which was obtained from the measured river discharge data (1921-1926, 1928-1933 and 1959-1992) and the inflow from the ungauged catchments that was estimatedfrom runoff coefficient. The calculated annual average water balance from the recordeddata of 1960-1992 indicates the total inflow is more than the total outflow from the lake

with an error of22 mm/yr (Table 3.2).

Table 3.2 Mean annual water balance of Lake Tana compiled from Kebede et al., (2005)

Components Mean annual (mm)

I 1162

P 1451

O 1113

E 1478

S 22

As we can see from the above two tables, the storage of the lake shows similar value

while the rest of the components show a decline through time.

3.2.3 Sediment flow

The Ministry of Water Resources measured the suspended solid load of the five majoremissaries of Lake Tana from 1960 to 1968. According to JICA (1977), the result obtainedby correlation of the solid loads with the recorded monthly runoff was 8.75 million m3 peryear of suspended sediment entering Lake Tana from the Gilgel Abbay, Koga, Megech, Riband Gumara rivers. This figure could be increased to about 10 million m 3 if the minoremissaries are considered.

3.2.4 Lake Tana surface level

The water level of Lake Tana has a series of measured data at a station near Bahir Dar cityby the Ministry of Water Resources. The measurement took place in the years 1964-2003on a daily basis from a datum level of 1783.5 m asl. During the analysis, estimations wereused to fill unmeasured gaps of the collected data. Figure 3.3 shows fluctuations throughthe years in the past. Between the years 1964-1995 the fluctuation seemed to have beenrandom with a maximum value of 2.79 m and a minimum value of 2.24 m. In the years1996-2003 the level raised abruptly to a new value of 3.08 m. The maximum value duringthis time was 3.20 m while the minimum value was 1.08 m. The most probable reason forthe big difference is the starting of the Chara-Chara weir in 1996, which was constructed to

regulate the outflow from the lake to the Tis AbayI hydroelectric power plant.

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In Fig 3.4 the relative lake height variations are shown, which are computed from satellitesaltimetry of TOPEX/POSEIDON (T/P) and Jason-1, with respect to a 10 year mean levelderived from T/P altimeter observations. Satellite position is known relative to the centerof the Earth and so it measures lake height relative to the Earth's center. T/P has 2 altime-

ters that measure the distance from the satellite to the ocean (NASA, 2005). These altime-ters send radar signals straight down to "bounce off" the ocean surface where they arebounced back to the satellite. The time it takes for the radar signal to return to the satellitetells how far the satellite is from the ocean's surface.

1785.01785.21785.41785.61785.81786.01786.2

1786.41786.61786.81787.0

1964 1968 1972 1976 1980 1984 1988 1992 1996 2000

Record Period (Years)

WaterSurfaceElevation(masl)

Mean Annual

Fig 3.3 Measuredmean annual water surface elevation of Lake Tana near Bahir Dar

According to the description of the Figure from NASA (2005) the top panel displays theraw height variations while the height variation time series in the lower panel has beensmoothed with a median type filter to eliminate outliers and reduce high frequency noise.

The graph indicates similar trend of variation as in Fig 3.3 with an extended small amountof increment of the lake level from the year 2003 to 2006.


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Fig 3.4 Lake Tana mean water level fluctuation calculated from observed data of satellite

altimeters. (Source:http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir/)

3.3 Land Cover Analysis

The land cover analysis was based on the results found from satellite image classificationand from interviews made in the field. Generally the total area coverage that constitute theboundary of the catchments plus the Lake Tana boundary showed a decrement of onepercent in the second period, between 1992 and 2001, with almost zero percent incrementin the first period, between 1987 and 1992. The variation involved about 120 km2 areadecrement of the lake in the 2001 map. From this, 99 km2was dried up land outside theGilgel Abay catchments and the rest, around 21 km2, was inside the study area. The reasonfor this is most probably fluctuation of the lake level (Fig 3.5) during this period of time

due to several factors: rainfall fluctuation, drought seasons, and a weir constructed for theregulation of the outflow from the lake for the two hydroelectric power plants.

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http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir/gr_regional_chart.cfm?regionid=eafrica&reservoir_name=Tanahttp://www.pecad.fas.usda.gov/cropexplorer/global_reservoir/gr_regional_chart.cfm?regionid=eafrica&reservoir_name=Tana


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Fig 3.5 The fluctuation of the Lake Tana level near Bahir Dar City: the color of the rock

shows stratification that comes from weathering effect.

3.3.1 Dynamics in land cover types

The land cover maps of the study area for the two reference years and the one taken fromthe BCEOM (1998) that shows the 1987 map is illustrated in Figure 3.6. Statistical summa-ries of the different land cover types are given in Table 3.3. There was a variation in allclasses, both increasing and decreasing. In the data from the 1987 map some generaliza-tions have been made to fit the land cover types of the other two maps. Since the work wasso detailed it was needed to compress the land cover classes, combining shrub, bush and

woodlands in one class and cultivated and settlements in another class.Cultivated land: This category included cultivated land (Fig 3.7) and rural settlements.Due to the difficulty encountered in identifying the dispersed rural settlements as a separateclass it was combined with the cultivated land. Since the land cover type was classified witha dominancy factor this category is found in all land cover classes with different percentage.

The calculations made on account of each parameter showed that this category lost around73 km2 (2% decrease) and around 352 km2 (12% decrease) of the area between 1987-1992and 1992-2001, respectively. This decrease is attributable to increased tree planting at thehousehold level, especially during the second period, which has led to the formation ofclustered bushes and shrubs around homesteads. These were classified asgrass_bush_shrubland because that is how they appeared in the images.


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(a) 1987:- Cultivated - 40.76%, Bush-shrub - 15.41%, Grass - 4.18%, Forest - 0.07%,

Water - 39.59%

(b)1992:- Cultivated - 39.76%, Bush-shrub - 14.49%, Grass - 5.65%, Forest - 0.17%,

Water - 39.93%

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(c) 2001:- Cultivated 35.64%, Bush-shrub - 22.39%, Grass - 2.91%, Forest - 0.14%,

Water - 38.91%

Fig 3.6 Land cover types in Gilgel Abay Catchments in 1987(a), 1992(b) and 2001(c)including the Lake Tana: cultivated land decreased through time, bush-shrub land

decreased slightly and then increased, grass land increased slightly and then

decreased and forest coverage increased significantly and then decreased.

Bush and shrub lands: this category included bushes, shrubs and grassland all togetherdue to the mixed land use on a very small scale (Fig 3.8), which makes it difficult to sepa-rate them using the images at this scale. In addition, some of the tree species, like Eucalyp-tus, were also included here for the same reason. The analysis showed that the change that

occurred in the first time period was 69 km2 (6 % decrease) loss of area. The second timeperiod showed an increment of around 585 km2, which was 53% of the 1992 area coverage.

This shows that the culture of tree planting was successful at this time period, which hadthe consequence that the cultivated land decreased. This contributed to the present aerialcoverage by bushes and shrubs. Most of the tree species planted at that time were varietiesof Juniper and Eucalyptus. The Eucalyptus trees would though decrease the water level inthe soil, which could be an explanation of the exchange of the classes from cultivated tobush land on an individual site.


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Table 3.3 Land cover changes in Gilgel Abay Catchment, including Lake Tana, between

1987 and 2001.

Land CoverType

Area in

1987(km2)

Area in

1992(km2)

Area in

2001(km2)

Change be-tween 1987 and1992 (%)

Change be-tween 1992 and2001 (%)

Cultivated land 3129 3056 2704 -2 -12

Bush-Shrub land 1183 1114 1699 -6 53

Grassland 321 434 221 35 -49

Forest 5 13 11 160 -15

Water 3039 3069 2952 1 -4

Total 7677 7686 7587 0 -1

Fig 3.7 Cultivated land near Koga River

Grassland: this category contained the open grassland that is used for grazing, and ex-posed land that have been degraded through time. The analysis shows an increment of 113km2 (35 % increase) between 1987 and 1992. Then there was a decline of this class by about213 km2, nearly half (49% decrease), between 1992 and 2001. The first increment of thisclass may be due to the decline of the shrub land for fuel and construction use in addition

to the decline of the cultivated area. According to the interviews of older inhabitants, themajor reason for the decrease in the aerial extent of grassland and degraded land was popu-

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lation pressure, which caused much of the open grazing land to be transformed into shrubland due to the consequence demand of fuel wood (Fig 3.9).

Fig 3.8Shrub lands near Merawi town with a mix of crop and open grass.

Fig 3.9 Grassland and shrubs near Abay River.


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Forest: this category covered a very small part of the study area known as Zege, which is apeninsula to Lake Tana (Fig 3.10), located at the south of the lake. According to the localinhabitants, the forest remains this long due to a local rule abide by the followers of theOrthodox religion that forbids any cultivation activity in that particular area. The peoples

income is mainly based on coffee, lemon and papaya. In the first time interval, between1987 and 1992, the change was about 160 % of the area, 8 km2 increase. A probable reasonfor this is the growing up of new trees that the inhabitants use as an income. According tothe analysis, the change was around 2 km2 loss of area (15% decrease) on the second timeperiod. The reason for this is mainly the use of the trees for fuel and construction pur-poses.

Water: this category included mainly the Lake Tana and some very small ponds. Accordingto the image classification there was a decline of the area of this class by about 117 km 2,

which is 4% of the 1992 area coverage, during the second time period. This might indicatethe fluctuation of the lake level and the drying up of the water around its boundaries, witha probable reason of drought and the Chara Chara weir that regulate the outflow from the

lake to the Tis AbayI hydroelectric power plant.

Fig 3.10The Peninsula of Lake Tana, Zege: the only natural forest in the studyarea.

3.3.2 Accuracy Assessment

The Maximum Likelihood procedure is the most commonly used procedure for classifica-tion in remote sensing. The foundation for this approach is Bayes' Theorem, which ex-

presses the relationship between evidence, prior knowledge, and the likelihood that a spe-

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cific hypothesis is true. In this study, the classification was made using equal likelihood forall classes due to a very limited prior knowledge.

The following Figures (3.11 & 3.12) show the frequency histogram of cell values in anIDRISI image from the MLC. It identifies the largest land cover type (x-axis) detectedduring classification, based on the frequency of pixels (y-axis) for the year 1992 and 2001.

The average uncertainty associated with each of the land cover classes in the maximumlikelihood classification was extracted by using an image from the soft classifier Bayclass.

As shown in Table 3.4, the highest average uncertainties for the 1992 image was for thecultivated class followed by the grass-bush-shrubland class. In the 2001 image the highestuncertainties was found in the grass-bush-shrubland class, which might strengthen thehypothesis of the land use change from cultivated to shrub land, that would create newpixel values due to the transition.

Fig 3.11 Histogram of the MLC of the year 1992 image

Fig 3.12 Histogram of the MLC of the year 2001 image


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Table 3.4 Average uncertainty values extracted from Bayclass image based on MLC

image.

Category Legend 1992 2001

1 Cultivated 0.319653 0.223450

2 Grass-Bush-Shrub land 0.225737 0.690433

3 Grass 0.078708 0.208179

4 Water 0.000285 0.004104

4. DISCUSSION

The maximum likelihood classification showed a change between the years 1992 and 2001.One type of change is the exchange of land cover from cultivated land to grass-bush-shrub-land and from open grassland to cultivated land. Around the lake a decline of thelevel of the water could be detected, which exposed more land added to the surface area.

4.1 Causes of land cover dynamics

According to the work of Meyer and Turner (1994), land cover changes are caused by anumber of natural and human driving forces. Whereas natural effects such as climatechange are felt only over a long period of time, the effects of human activities are immedi-ate and often radical. From the human factors, population growth is the most important inEthiopia (Hurni, 1993 as cited in Tekle and Hedlund, 2000), as it generally is in developingcountries. It was not possible to obtain overall demographic data for the catchments stud-ied here because such data are compiled according to administrative structures, such aspeasant associations, districts and provinces, so this data do not correspond to watershedboundaries.

The total population of nine selected urban areas that lies in the study area in 1999 was179,434, according to the census made by the Central Statistics Authority. The projected

value, with a 3% per year average growth rate, of 2001 is 190,362. Assuming that the aver-age rate of population increase in the 9 sample villages remained constant in the wholecatchment, the population of the studied area would have been around half of its currentsize about 25 years ago. By the same reasoning it will double within less than 25 years fromnow. Thus, population growth would certainly be the most important factor causingchange in the land cover dynamics, because demand for land for cultivation and settlement

and trees for fuel and construction purposes was greater. But as mentioned above, thecultivated land is decreasing in the observed area, which might indicate the increment ofthe urbanization process.

A possible explanation could be population ageing as it is occurring much faster in devel-oping countries, most of which have inadequate resources to deal with age-related prob-lems (Food and Agriculture Organization, FAO, 2004). According to a research done byFAO many fatal diseases affect young people rather than older people, particularlyHIV/AIDS, contributing to population ageing. This process is occurring even faster inrural areas due to rural-urban migration, which sees young adults leaving to find work inurban areas and older people returning to rural areas when they retire. This might be one ofthe reasons for the cultivated land to decrease in spite of the population growth, since in

the absence of young labor force, older people have to look after crops and livestock andthis can reduce the agricultural productivity of an area.

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Fig 4.1 Eucalyptus trees near homesteads.

The change from cultivated land to bush land might also reflect infertility of the soil due todrought, which will reduce the level of production. Soil loss is another factor that comesfrom high runoff incident, which would increase soil erosion, transporting most of thenutrients to the lake. In addition, this will probably damage the lake ecosystem and fishpopulations. The afforestation program of the previous government (derg) regime, aninitiative to preserve indigenous trees or forests and planting of trees at the household levelcould be another cause. As in many places throughout the country, the community under-

took some afforestation during the dergs dictatorial rule.One of the expanding tree types is Eucalypts (Fig 4.1) that have many uses, which makethem economically important (Wikipedia, 2005). Due to their fast growth the foremost ofthese is the wood. The many species provide many desirable characteristics for use asornament, timber, firewood and pulpwood. But the roots absorb large amount of waterthat could be a disaster to the crops. The resinous of the leaves could also assist to thedepletion of soil nutrients when they fall. The oil of the leaves that would be much usefulin proper use could also be detrimental to the whole ecosystem, particularly fish, due to itstoxic behavior.

According to the local people who live in the peninsula, Zege, the only forest area, they areaware of the water consumption of the Eucalyptus tree and avoid using it in order toprotect their coffee trees from drought. The growing of coffee, lemon and papaya (Fig 4.2)is increasing which leads to the increment of bush and shrub land of the area.


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4.2 Implications of land cover dynamics

Changes in land cover by land use do not necessarily imply a degradation of the land(Meyer, 1995). Indeed, it might be presumed that any change produced by human use is animprovement, until demonstrated otherwise. But soil degradation and inadvertent impactson hydrological processes are the two major aspects that could be implied by land cover

dynamics if this went on uncontrollably in a specific area.4.2.1 Soil degradation

Human alterations of land use and cover have caused erosion rates to increase in manyareas of the world, resulting in considerable land and environmental degradation (McCauleyand Jones, 2005). The severity of erosion depends upon the quantity of soil detached andthe capacity of the wind and water force to transport it (Morgan, 1995). As both detach-ment and transport require energy the ability of soils to erode is based on erosivity, theenergy of the eroding agent (i.e., wind or water), and erodibility, the soils susceptibility toerosion. Land cover change influences both the erosivity of the eroding agents and theerodibility of the eroding subject (Morgan 1995). Among the land cover changes that

would enhance soil erosion, land clearance, agriculture (ploughing, irrigation, grazing),construction and urbanization are the major ones.

Fig 4.2 Lemon trees at Zege

From the point of view of exposure of the land to erosive storms, the land cover types inthe catchments can be classified into 2 classes: (1) land that is bare when the erosive rainsoccur, and (2) land under good vegetative cover when the rains begin, which is protected

from the threat of erosion. Cultivated fields that include urban and rural settlements andpart of the grassland and degraded land constitute the first category, whereas forest, bush

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and shrub land cover types can be included in the second category. Accordingly, the part ofthe catchments subject to possible maximum soil loss accounted for 75% and 63% of thetotal area with an exclusion of the lake in 1992 and 2001 respectively. The decrease in per-centage through time is mainly due to the increment of bush and shrub areas with an ex-

change of the cultivated areas.The whole catchment area of the Lake experiences a soil loss rate ranging from less than 5to more than 250 tons per hectare per year (Teshale et al., 2002). In some major tributaryrivers, turbidity of the water increased from 40 NTU during the dry season to 400 NTUduring rainy season. The study put population pressure, deforestation coupled with sedi-ment loading and rapid growth of the town of Bahir Dar as major threats.

Fig 4.3 Widening of gullies near Merawi town.

4.2.2 Hydrological processes in the catchment

The removal of indigenous land vegetation over the ages is likely to have affected the hy-drological cycle through biological, thermal and physical effects (Jones, 1997). As trees andshrubs have been replaced with grassland and agricultural crops, the rates of interceptionand evapotranspiration have been reduced. Potential evaporation is likely to have beenreduced further by the increase in surface albedo, and the resultant reduction in net radia-tion balances, as woodlands are cleared. Similarly, cultivation and improved land drainage

will tend to reduce soil moisture levels and so further reduce actual evaporation losses.

Rainfall tends to drain across the landscape through drainage lines, intermittent creeks andsmall streams to the major river of the catchments. Land under little vegetative cover is


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subject to high surface runoff and low water retention. The deepening and widening up ofrills and gullies as shown in Figure 4.3, and the intensification of sheet erosion is caused bythe increased runoff. If a city is expected to grow in the future by 20 %, then a certain areathat used to be grassland, agricultural area or forest will be changed into urban. The in-creased urban area is impervious and thus have a near zero storage capacity, which could

increase the direct surface runoff and decrease the soil water retention capacity. Afforestation could affect annual flow by increased interception in wet periods andtranspiration in dry periods through increased water availability to deep root systems(Maidment, 1993). As shown in Table 3.3 the bush-shrub land class increased by 53%during the second period of time, which included eucalyptus, junipers and pine types oftrees and shrubs. According to Maidment (1993), eucalypt and pine types cause an averagechange of 40 mm in annual flow for a 10% change in cover, with respect to grasslands in acorrelation of inverse proportion. This means that a 10% increase in tree cover causes adecrease of annual flow by 40 mm and vice versa.

Hydrological processes include both quantity and quality of surface and ground water. Themasses of sedimentary materials removed from the hill slopes accumulate in low-lying areas

downstream, where they could create problems of water pollution, reservoir siltation andproblematical sediment deposition on important agricultural lands. Streams can becontaminated by a range of materials from adjacent land. This can include soil particles(sediments), nutrients such as nitrogen and phosphorous, salt, plant and other materialfrom crops, chemicals and microbes. In rural regions, eroding soil and associated nutrientsare the most important and widespread causes of reduced water quality.

While phosphorous is a natural and vital nutrient in our ecosystems, changes in land useintensity and practice have radically altered the amounts of phosphorous being delivered toour water bodies, particularly river courses, reservoirs and lakes. Excessive nutrient loads inthese water bodies can cause euthrophication, a process leading to deteriorating waterquality and the increased occurrence of toxic and unsightly algal blooms such as blue-greenalgae or cyanobacteria. According to the work of Kebede (2001) Lake Tana does appear tobe in danger in the long run, specifically in the gulf area, near Bahir Dar city.

A river also transports food, in the form of nutrients, leaf litter, fine particles of organicmatter and other dissolved substances, for aquatic plants and animals. According to Te-shale (2002) the conductivity of the streams in the study area dropped during the rainyseason, which indicates that the sediment contained a high amount of fine sediment (clay)and organic matter. A decline of plankton production and blooming of weeds followed as aresult of the increment in turbidity.

Sedimentation is another hydrological problem for a lake reservoir. Of the fine particles inthe flowing water to the Lake Tana, a part will be deposited at the lake bottom and a part

will flow out to the Blue Nile (JICA, 1977). Some amount of the sediment deposited in thelake will settle near the inlets to the lake and the quantity remaining without further beingtransported will affect the reservoir capacity in the future. According to JICA (1977) thereare large proportions of fine particles, which helped to estimate that the deposition near themouths of the inlets would be half of the entire quantity at most. Therefore the estimatedamount of fine particles that will be deposited in the effective storage capacity of the lake isapproximately 5 million m3 per year.

According to Kebede et al., (2005), despite the 20 % rainfall variations in the Blue NileBasin in the years 1960-1992, the lake level remained regular. The authors stated that theanalysis of the sensitivity of level and outflow of the lake suggested that they are controlledmore by variation in rainfall than by basin-scale forcing induced by human activities.

The fact that the measured lake level is less sensitive to change in catchment characteris-tics may indicate that an extreme change in land use would be required for its effect to be

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detectable beneath the changes caused by natural variations in rainfall. (Kebede et al.,2005). The above statement could strengthen the reason for the significant variation of thelake level after 1996, the year on which the Chara-Chara weir started the operation, asshown in Figure 3.3 when compared to the rainfall variation in Figure 3.2.

As we can see from Table 3.1 and 3.2, the water balance indicated that the inflow is morethan the outflow of the whole lake basin based on the measured components. Since thecalculation method does not include soil and land use parameter, the result couldnt tell usthe effects of the soil and land use changes that took place during the recording periods.

Soil erosion is enhanced during rainy season on bared lands. According to the feasibilitystudy for the Koga Irrigation Project the construction was staged to conduct during the dryseasons. The field survey for this paper took place in July-September period, during which72 % of the annual runoff occurs. During the survey a high amount of flooding and ero-sion were observed while visiting the irrigation project area that was under active construc-tion phase (Fig 4.4). If the timing of the construction continues without consulting the

feasibility study the damage it may cause on the whole basin might increase uncontrollablyduring the proposed seven years of construction period.

Fig 4.4 Flooding of the dam site, Koga Irrigation Project, under construction phase.


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5. CONCLUSIONSANDRECOMMENDATIONS

The Lake Tana is a major source of the Blue Nile and the basin has major global environ-mental benefits. Both positive and negative impacts on the basin hold significant impactson rain regime. The Lake Tana Basin is rich in biodiversity with many endemic plant spe-cies, cattle breeds, birds and cultural and archaeological sites. It contains large areas of

wetlands and flood plains with a major input for agricultural use. The basin has also vitalnational significance as it has the potential of vast water resources for irrigation, to develophydroelectric power, for development of high value crops and livestock production andhigh potential for ecotourism.

According to the assessment made the land use change is a major input to bring both posi-tive and negative impacts on the basins hydrology. The changing of each class of the landcover has its own advantage and disadvantage to the surrounding ecosystems and needs tobe done with a caution. The assessment shows the significant indicator on the lake levelchanges as the anthropogenic changes in the area. Even though a feasibility study wasdone for each project in the area the implementation and outcome is seen to be in a differ-ent way. Sustainable land management should be applied in order to preserve and protectfrom future possible damages.

The study goes through the different parameters and attempts have been made to reasonout the relationships of the parameters and the potential of each to change the whole hy-drological system of the Lake Tana Basin. The study has limitations concerning detailedground truth data for the image classification but can give basic information for future use.

In sum the study revealed land use changes, which affect the hydrological parameters of theLake Tana in addition to the fertility and retention capacity of the soil that enhance thedegradation of the land in the study area. The methods and study itself can with furtherdevelopment contribute to knowledge of the specific changes that had occurred and quan-tify the hydrological changes.

For further study spatially distributed hydrological modeling is recommended to assess thehydrological change due to land-use changes in more specific and quantified way. Oncedeveloped the model might help greatly to differentiate the change from natural cause andhuman factors like land use change by doing calibration on an already existed digital data-base of all the necessary inputs.

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REFERENCES

Acres International Limited and Shawel Consult International, 1995. Koga Irrigation ProjectFeasibility Study, Main Report. Water Resources Development Authority. Addis

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Bonzon and Horemans, 1988.Socio-Economic Data Base on African Fisheries. FAO Fish. Circ.,(810): 109 pp. FAO, Rome as cited in Greboval, D., Bellemans, M., and Fryd, M.,1994. Fisheries characteristics of the shared lakes of the East African Rift. CIFA TechnicalPaper No. 24. Rome: FAO. Retrieved October 5, 2006 athttp://www.fao.org/documents/show_cdr.asp?url_file=/docrep/008/v3470e/v3470e00.htm

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Eastman, J.R, 2001. Idrisi 3.2, Version 2. Guide to GIS and Image Processing, Volume 1. ClarkLabs, Clark University, USA, 171 pp.

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JICA, 1997. Feasibility Report on Power Development at Lake Tana Region. Japan InternationalCooperation Agency.5/76 p.

Jones, J.A.A., 1997. Global Hydrology: Processes, resources and environmental manage-ment. Longman Singapore Publishers (Pte) Ltd. Singapore. 399 p.

Kazmin, V. 1973. Geological map of Ethiopia (1:2000000). Ethiopian Institute of GeologicalSurveys. Addis Abeba, Ethiopia.

Kebede S., 2001. Water Pollution of Lake Tana with Particular Emphasis on EutrophicationMSc.Thesis. Stockholm, Royal Institute of Technology. 70 p.

Kebede, S., Travi, Y., Alemayehu, T. and Marc, V., 2006. Water Balance of Lake Tana and itsSensitivity to Fluctuations in Rainfall, Blue Nile Basin Ethiopia. Journal of hydrology, v.316, iss. 1-4, pp 233-247. Elsevier Science.

Kejela K, 1992. Assessing Soil Degradation with Emphasis on Soil Productivity in the Anjeni Area.Natural Resources Management for Conservation and Development. Proceedingsof the Second Natural Resources Conservation Conference. 10-13 May 1990, Addis

Abeba. Ethiopia, IAR. Addis Ababa as cited in SWMnet, February 2002, Proceed-ings of the Strategic Planning Workshop held at Novotel Mount Meru Hotel,

Arusha, Tanzania, 4th 8th June 2001, Discussion Paper No. 1, pp 25-37. RetrievedDecember 11, 2006 athttp://www.asareca.org/swmnet/publication/discussion_papers/Discussion_Paper_1.doc

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Assessment of Temporal Hydrological Variations Due to Land Use Changes Using Remote Sensing/GIS

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