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A Remote Sensing and GIS approach for the study of recent non-planned cultivation and its influence on water-logging in the Farafra Oasis (Egypt).
Mohammed El Bastawesy
Geological application division, National Authority of Remote Sensing and Space Sciences, Egypt. Email: m.elbastawesy@ narss.sci.eg Abstract
This paper presents a new approach to assess the water-logging, which has been notably developed in the Farafra Oasis following extensive and non-planned cultivations. Analysis of Landsat satellite images of 1984 shows that the main agricultural area is mainly developed on the depression floor. Here, the excess water of irrigation is drained into few local ponds, which are dotting the playas and agricultural areas. The Landsat satellite images of 1989 show a recent and extensive land reclamation project, located at 25 km to the southeast of the ‘old’ main agriculture area in Farafra Oasis. A recent satellite image of Egyptsat 1, acquired in 2009, shows that the new agricultural project area has developed seepage and vegetation-strips that confine to certain flow paths heading northwest toward the old agriculture fields. The Digital Elevation Model (DEM) of the Shuttle Radar Topography Mission (SRTM) was used to automatically extract drainage networks (i.e. wadis and dry-channels) and sub-catchments of the Oasis area, which is veneered by sand sheets. The DEM-simulated drainage networks show that The Farafra Oasis is a closed catchment, where channels are arranged in a centripedal pattern. The ‘new’ agriculture project area has been erected on the finger-tips of these centripedal alluvial channels. Furthermore, the seepage from the ‘new’ agriculture fields’ is delineated by natural vegetation strips that exactly follow the flow directions of the extracted drainage networks. It is anticipated that subsurface seepage from the new fields’ area will progress in the downstream direction to reach the old agriculture areas. The performed drainage network analysis is very useful to determine the optimal pathways of irrigation-discharge channels. Because the dry channels are partially buried by aeolian deposits, they have to be first detected and then excavated and deepened to control water-logging. It is highly recommended that newly developed seepage-channels have to be intercepted before reaching old agriculture areas. Introduction: The depression of Farafra Oasis covers an area over 10,000 sq km, and is considered one
of largest mega-depression in the Western Desert of Egypt (Fig 1). Although, hyper-arid
climate prevails the oasis and surrounding desert areas. Playa deposits of the oasis
preserve paleoclimatic and archaeological evidences, which clearly depict Quaternary
wet climatic phases that sustained early human occupations during the prehistory.
Limited archaeological remains of the eighteenth dynasty and Roman periods are also
found in the oasis (Hassan et al. 2000). In 1960, the population of the oasis was
approximately just1000 persons. Recently, unprecedented immigration from the Nile
Valley and Delta was triggered following the construction of connecting networks of road
and motivated by the staggering need for new land reclamation projects. These new
projects are being developed on large tracts of soil mainly available in extensive playas,
which are developed at the footslope of bounding scarps and on the floor of the
depression. The water resources are mainly extracted from the deep Nubian Sandstone
aquifer. Generally, the water is of reasonable quality, but there are growing concerns on
the safe yield production of the aquifer given the non to limited recharge of the aquifers
and the rapid developments of other mega-agriculture projects in other different parts of
the western desert pumping the same aquifer (Thorweihe, 1990; Idris and Nour, 1990).
Traditionally, in the realm of almost flat topography in the Nile valley and Delta (i.e. old
cultivation) the irrigation and discharge managements are largely dominated by land
leveling techniques. In the Nile valley and Delta, drainage networks and irrigation
channels are arranged in inter-leaving arrays trending from south to north as the main
Nile flow direction. The open surface drainage and subsurface drainage systems are
implemented on a large scale to sustain the irrigated agricultural and maximize crop
yields (Abdel-Dayem et al. 2007). Large areas of the newly-reclaimed land in the desert
areas have already been leveled and irrigated. But artificial drainage system have not yet
developed, as it may be thought that the natural drainage capacity of the deep sandy soil
profiles in the newly developed areas is efficient to control rising soil water tables and
salt accumulation. Unfortunately, this is not the case, remote sensing observations and
field investigation shows a large scale spread of water logging and salinization in the new
project in Farafra Oasis. The digging of traditional more or less parallel and straight open
drainage seems no effective to collect and control the widespread water logging in the
farms. It has been noticed that the seepage lines and networks developed naturally in
lower areas entailing the farms are typically identical to the catchment-drainage networks
derived from the topographic maps and Digital Elevation Models (DEM).
This paper aims to investigate the influence of original landforms and geomorphology of
catchments in the Farafra depression on productivity of newly reclaimed lands. The
water logging and soil salinity hazards are closely associated with the spatial patterns of
fluvial channels that have been defunct since the last wet climatic pluvial in the Sahara
several thousands years ago. Thus water logging hazard can be better understood and
thus mitigated by remote sensing and GIS mapping the paleohydrological parameters,
which must be considered to design the drainage of agriculture fields.
Geology and geomorphology: The Farafra depression is a semi-closed basin with an irregular shape, where it is
bounded by the escarpments from the north, east, and west. The depression floor
gradually rises southward and merges into the plateau that forms the northern escarpment
for Dakhla and Abu Minqar Oases. The depression is mainly underlain by Palaeocene-
Eocene sedimentary rocks, where the chalk of Upper Cretaceous (Khoman Formation)
and Paleocene (Tarawan Formation) cover large areas in the north and the shale (Dakhla
Formation) is mainly exposed in the south. Lower Eocene formations (El-Naqb and
Farafra limestone) cap prominent escarpments (El-Azabi and El-Araby, 2000). However,
major faults have outlined the western escarpment of El- Quss Abu Said plateau and also
along some other sectors of the northern escarpment. But the interplay between fluvial
and karastic processes under changing climatic conditions since the Miocene has
developed most of the current landforms. Chemical weathering and wind deflation have
developed several small depressions (i.e. karst) on the main floor (El Aref et al., 1987).
Some of these depressions have been partially or fully filled by playas during the
Quaternary wet climatic periods (Hassan et al. 2000). Generally, detailed mapping of
the playas in the Western Desert is yet not available. However, more than 100 playas of
different sizes were identified In Farafra depression from high resolution aerial
photographs, and in total they cover about 440 km2. Most of these playas are of small
surface areas, for example 67 playas were found to cover 20 km2 (Embabi, 2004). Few
playas are of large surface areas and they have been developed for agriculture. For
example Al Gunnah playa, which is located at the northeast footslope of Kuss Abu Said
plateau, covers alone an area of more than 100 km2 and it represents one of the oldest
agricultural and settlement areas in Farafra depression.
Under the prevailing aridity, these widespread playas are exposed to wind erosion that
scoured the former lake deposits and thus developed yardangs of various shapes and
sizes, which stand as small residual hills above the surrounding playa floor. The eastern
part of Farafra depression is covered fields of sand dunes. These dunes generally extend
southeast for about 200 km from the northeast corner of the depression to the northern
escarpment of Dakhla depression. In the northern part of this field, sand dunes mainly
exhibit whalebacks structures that are closely spaced and in some places coalesced so that
the surface is undulating. Interdune areas become wider southwards and eastwards and
therefore, the decrease in sand supply dismantle the widely separated dune ridges into
separate barchans before reaching the scarps of Dakhla depression (Embabi, 2004). Sand
sheets also cover large surface areas in different places and have consequently obscured
most of the fluvial channels developed at wetter climatic phases. Playas and sand sheets
areas were recently engaged in a large scale development project including reclamation
by land leveling, drilling tens of deep wells for irrigation and digging of parallel
irrigation and drainage channels. The rapid deterioration in land induced by spreading of
water logging and salinization are detected and assessed using the following
methodology
Data and methods:
The Shuttle Radar Topography Mission (SRTM) DEM was processed to automatically
extract the drainage networks and sub-catchment boundary of Farafra depression using
the original D-8 algorithm embedded in ArcInfo software with some modifications in the
filling of DEM step (El Bastawesy et al. 2008). This method requires, first, that all the
sinks (i.e. local depressions) of the DEM to be filled and raised in elevation to their
neighbouring cells in order to ensure the flow continuity within the catchment to an outlet
(Jenson and Dominique, 1988). The filling step of the DEM does not distinguish between
naturally occurring sinks (i.e. playas), which is the case in Farafra depression and the
artefacts resulting from the generation technique of DEMs. The main playas dotting
Farafra depression were delineated by visual interpretation of satellite images and their
relative low elevations to surrounding were assessed using the available DEM. Some of
these playas are covered in parts by sand sheets and also cultivated. However, any given
playa can be considered as terminals for surface flow and seepage from a fixed catchment
area, the individual playas can be connected by over flow channels under certain
hydrological conditions and thus their catchments are relaxed. Herein, playas were
individually delineated and masked from the processing steps of DEM to locally entrap
the surface flow in separate and terminal locations. Therefore, the contributing drainage
networks and catchment area to the different terminal playas have been determined
following the subsequent D-8 algorithm routine in ArcInfo as follow:
The terminal-masked DEM of Farafra depression was filled, and the flow direction of
each cell into the lowest elevation cell of the surrounding eight cells was determined.
Once the route of flow is determined for each in the DEM it is possible to accumulate the
number of upslope flow contributing cells (i.e. areas) and the flow paths of each cell.
Selecting a threshold of the minimum flow accumulation number to form the fingertip of
mapable channel segment is required to portray the drainage networks in the area (Fig 2).
The available Landsat TM and ETM+ satellite images for Farafra depression, covering
the period from 1984 until 2003, were obtained and processed. Additionally, a recent
satellite image of Egyptsat 1 acquired on November 2009 was also obtained and used
along with other satellite images to assess the agricultural expansion in different areas
from 1984 to date. The 1984 satellite image shows that agricultural areas are developed
around Qasr El Farafra area on the floor of Gunnah playa, which is located at the north-
eastern footslope of Kuss Abu Said plateau (Fig 3). Extensive areas of new agricultural
fields appeared on the satellite images of 1998 and onward (Fig 4). The new cultivated
areas are located on a slightly higher plain to the south east of the 'old' agriculture fields.
Interpretation of the multi-temporal satellite images and field observations show that the
new project area has developed a large scale water logging, ponds in low areas, and
entailed by seepage areas heading to the 'old' agriculture fields (Fig 5 and 6). The
automatically extracted drainage networks were overlaid ontop of the satellite images
showing seepage lines in order to investigate the correlation between the DEM-simulated
hydrographic flow patterns and the emerging seepage patterns. This is to understand the
control of geomorphology and landforms on dynamics of water logging and seepage and
how to sustainable develop new agricultural areas.
Results:
The depression floor of Farafra is occupied in many parts by playas, which usually
conforms a closed drainage catchment. El Gunnah playa, which is the key one in Farafra
depression, is also fed by the largest catchment area in the depression as inferred from
DEM analyses. The surface channels that were formed during the Quaternary wet pluvial
are often buried and veneered by sand sheets and cannot be fully traced from satellite
images and available maps. The availability of large areas of almost flat plains and soils
within the catchment of Al Gunnah playa has prompted the agriculture development of
nearly ( 25 thousand feddans ). Signs of soil deterioration and water logging have
appeared shortly after land levelling and irrigation. The digging of spaced parallel open
drainage channels to combat water logging is not effective and lots of farms were
abandoned. The designed artificial drainage channels are intersecting with the natural and
unexposed drainage networks only in some places. However, the water logging is
spatially associated with the neglected natural channel, and the detected seepage lines
from the 'new' agricultural areas is following these channels paths and slowly heading
toward the 'old' agricultural areas. The most recent satellite images of Egyptsat1 acquired
in November 2009 shows that these seepage channels are only at short distances from the
old agriculutral fields. It is therefore, required to intercept these channels and collect and
transfer the drainage water into one of the local playas that is not hydrologically
connected with the cultivated ones at the scarp footslope. The current situation clearly
indicates that the regional physiographic of soil was not comprehended and adopting the
same technique of farming in the Nile Delta and the Valley is not appropriate everywhere
in Saharan areas. The engineering structures to combat water logging are meaningless
unless supplemented by knowledge of the spatial variability of soil characteristics and the
natural drainage patterns even if inactive due to prevailing aridity. The natural drainage
channels must be considered, excavated (if buried), depends and used to collect the
excess soil water to control water logging. Water is diverted laterally toward channels as
throughflow where draw-down drainage is inhibited by less permeable soil horizons or
bedrock. Of course the water logging primarily depends on soil physical properties,
depth and gradient, and characteristics of subsoil bedrock, but the role of natural drainage
patterns is an important factor in closed and playa drainage basins in. Where the
irrigation-excess water and subsurface seepage is ultimately will accumulate under the
cultivated playa surfaces, particularly if it is underlain by an impermeably rock.
Discussion and conclusion:
The increasing demand for limited and strained resources in the Western Desert, threats
the processes of sustainable developments. Recently, the Nubian Sandstone aquifer is
over-pumped for new land reclamation projects in the Sahara. Remote sensing is a very
rapid and efficient tool to monitor, assess and evaluate the progress of different land use
and land cover changes (Palmer and Van Rooyen 1998; Shalaby and Tateishi, 2007).
Unfortunately, the new agricultural lands in Farafra oasis have developed widespread
water logging and seepage lines encroaching the old agriculture areas. The planning for
this new project seems not preceded by sufficient multi-disciplinary integrated research
on soil physiographic, catchment hydrology and their role on adopting certain irrigation
and discharge strategies. The impacts of intensive development on groundwater include
excessive decline of water levels, depletion of aquifer storage and deterioration of
groundwater quality. For example, the groundwater levels in Dakhla Oasis are gradually
being lowered by a rate of 1 to 4 meters per year as a result of over-pumping of the deep
Nubain Sandstone aquifer for irrigation. Additionally, the productivity of soils is
increasingly affected by high unfavourable drainage conditions and improper farming and
irrigation practice have caused widespread water logging and salinization of soils
(Khouri, 2003).
In conclusion water logging and soil salinization is the major threat facing inhabitation
and development in the oases. Extensive water logging hazard has occurred as the
geomorphologic setting was not considered when developing new agricultural areas. The
playas and buried channels of closed drainage basins are the most vulnerable areas for
water logging, particularly when the soil of higher surrounding areas is cultivated. It is
recommended that constructed open drainage networks should coincide with the natural
drainage system delineated using remote sensing and DEM analyses.
References:
Abdel-Dayem, S., Abdel-Gawad, S., and Fahmy, H., 2007. Drainage in Egypt: a story of
determination, continuity, and success. Irrigation and Drainage, 56, 101-111
El Aref, M.M., Abou Khadrah, M.A., and Lotfi, Z.H., 1987. Karst topography and karstification processes in the Eocene limestone plateau of El Bahariya Oasis, Western Desert, Egypt, Z. Geomorph. N.F., 31, 45-64pp. El-Azabi, M.H., and El-Araby, A., 2000. Depositional cycles: an approach to the sequence stratigraphy of the Dakhla Formation, west Dakhla-Farafra stretch, Western Desert, Egypt. Journal of African Earth Sciences, 30, 971-996.
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Fig 1:Location map of the study area.
Al Gunnah Playa (Qasr El Farafra)
Kuss Abu Said plateau
Fig 2: The processed SRTM DEM shows the playas at the footslope of Kuss Abu Said plateau. The automatic drainage networks are overlaid ontop.
Fig 3: A Landsat TM image of the study area, which acquired in October 1984 and shows the local extent of clutivations (red colours) in Qasr El Farafra area on the floor of playas.
Fig 4: An 1998 Landsat TM images shows the new agriculutral areas (compare with Fig 3), also note the developed channel patterns heading from the south areas to the playas at footslope of Kuss Abu Said plateau.
Fig 5: A field photo shows the developed seepage and water logging in one of the channels heading toward the playa at the scarp footslope.
Fig 6: A field photo shows the resulting natural vegetation cover from waterlogging and salinization in one of the recent reclaimed farms.
Fig 7: A recent Landsat image shows the close threat of seepage channels from the new irrigated lands on the old lands.