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.

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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.