Detection and assessment of the waterlogging in the dryland drainage basins using remote sensing and GIS techniques
Mohammed El Bastawesy, Rafat Ali, Daniel Deocampo, Mohammed El Barodi
M. El Bastawesy is with the Geological Application Division, National Authority
of Remote Sensing and Space Sciences, Egypt, and also with the Geography
Department, Umm Al-Qura University, Makkah, Saudi Arabia (corresponding
author, e-mail:
m.elbastawesy@narss.sci.eg).
R. R. Ali is with the Soils and Water Use Department, National Research
Centre, Egypt.
D. M. Deocampo is with the Department of Geosciences, Georgia State University,
Atlanta, Georgia.
M. S. Al Baroudi is with the Geography Department, Umm Al-Qura University,
Makkah, Saudi Arabia.
IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING, VOL. 5, NO. 5, OCTOBER 2012 . PP 1564 - 1571:
Abstract
Waterlogging and elevated soil salinity commonly develop in the cultivated and arable areas of the Sahara, particularly within closed drainage basins. Multi-temporal remote sensing data of the Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) for the Farafra Oasis in the Western Desert of Egypt were collected and processed to detect land cover changes, cultivations, and the extent of water ponding and seepage channels. The Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) has been processed to delineate catchment morphometric parameters and to examine the spatial distribution of cultivated fields and their relation to the extracted drainage networks. The soil of these closed drainage basins is mainly shallow and lithic with high calcium carbonate content. Therefore the downward percolation of excess irrigation water is limited by the development of subsurface hardpan, which also saturates the upper layer of soil with water. The subsurface seepage from the newly cultivated areas in the Farafra Oasis has revealed the pattern of buried alluvial channels, which are waterlogged and outlined by the growth of diagnostic saline shrubs. Furthermore, the courses of these waterlogged channels are coinciding with their counterparts of the SRTM DEM, and the recent satellite images show that the surface playas in the downstream of these channels are partially occupied by water ponds. The geomorphology of closed drainage basins has to be considered when planning for new cultivation in dry land catchments to better control waterlogging hazard. Accordingly, several management strategies can be adopted to combat land degradation by water logging including; cultivation of certain areas and localities within each drainage basins, and conveying the drainage and seepage water through the inactive alluvial channels into abandoned playas, which are reserved for evaporation. Moreover, farm management and balancing irrigation water, salt leaching, and evapo-transpiration are also critical to lessen the development of waterlogging.
Index Terms—DEM, dry land, geomorphology, GIS, remote sensing, soils, waterlogging.
I. INTRODUCTION
SOIL and water resources of drylands are limited. To meet the growing demands for water and food production, large scale reclamation projects have been implemented in arable areas, supported by groundwater extraction or withdrawals from nearby rivers. For example, 5 billion cubic meters of water will annually be conveyed from Lake Nasser into the Tushka region by a powerful pumping station and sets of artificial channels, in order to cultivate 500,000 acres in the southwestern desert of Egypt (i.e., The South Valley Development Project) [11]. Originally, the agricultural areas of the Sahara were typically isolated, occuring in small areas fed by groundwater or ephemeral runoff from the surrounding mountains. Significant problems have arisen from the disturbance of the hydrological balance of these drainage basins. In particular, irrigation of poorly drained soil has led to high perched water tables, waterlogging, and salinisation [5]. Over time, if there is inadequate drainage system or considerable soil depth; a shallow perched water depth is likely to develop and, consequently, some irrigable lands are eventually abandoned [48]. In the presence of a shallow saline water table, crop production can suffer when salts accumulate in the soil surface through capillary action and/or directly as a result of waterlogging [21], [27], [16]. Conventionally, waterlogging (i.e., excess soil moisture content) and soil salinity can be controlled through maintaining a net flux of salt away from the root zone and controlling the water table through drainage systems [30]. This can be achieved by the construction of artificial drainage including both open surface drains and subsurface drainage pipes, in order to maintain a balance between soil recharge and discharge by draining the excess soil water table [1]. Practically, these engineering solutions could be limited by several factors including; cost, disposal of the drainage water, and the suitability of local hydrological conditions.
Therefore, the understanding of dryland geomorphological and hydrological setting is required to fully comprehend how waterlogging and soil salinisation develop and expand in newly cultivated areas. The importance of fluvial environments in dry land catchments is often overlooked as the drainage channels are usually abandoned and inactive for long periods. However, the modern hydrology of many areas was shaped by riverine environments in the geological past. This region has hosted perhaps dozens of lakes and other freshwater features during the previous wet climatic period known as “The Pluvial” [42]. Under dramatically different climate conditions, freshwater environments led to the development of fluvial channels and lacustrine deposits in local depressions. More significantly, groundwater aquifers were replenished during these times of net recharge [34]. Such freshwater environments likely existed sporadically at different times during the Pleistocene, depending on climatic and hydrologic changes in the environment. The remnants of some of these freshwater paleoenvironments can be seen where fresh groundwater is still found today, such as in the oases of Kharga and Dakhla in the Western Desert of Egypt. Most of the evidences of preceding wet phases and the alluvial channels have widely been obliterated by the erosion and obscured by aeolian deposits [33], [19], [35]. Therefore, most of the Sahara landforms remain of unknown soil profiles. These unique environmental conditions and their paleoenvironmental contexts have created profound implications for water and soil resources [26].
On the other hand, the surface hydrological features of the dryland and the buried- drainage networks are the only parameters that can be regionally inferred using remote sensing techniques, digital elevation models (DEM) analyses, radar, and geophysical investigations (e.g., [4], [36]). The traditional drawing of catchments and drainage networks from available topographic maps, aerial photos, and multi-spectral satellite images, is fraught with a great deal of uncertainty. Hydrologic analysis based on these data sources can potentially yield misleading results. Thus, the automated extraction of drainage networks and catchments from digital elevation models (DEM) has widely been considered more objective in hydrological analysis than manual extraction [44]. The importance of the automatic delineation of drainage networks is obvious in the Saharan areas, where the surface expression of drainage is relatively poor and the quality of available topographic maps is questioned. However, the surface drainage channels and catchment outlets can be mapped with reasonable accuracy using space-born DEMs such as the Shuttle Radar Topography Mission (SRTM) DEM (e.g., [17], [13]).
The accuracy and reliability of the hydrographic parameters simulation using DEMs are known to be influenced by source, resolution, and processing algorithms [50], [49]. However, different sources and resolution of high quality DEM are expected to give highly correlated results [10]. The different existing processing algorithms such as the D-8 [31] and multiple flow direction algorithm (MFD) [38] have also significant influence and result in relatively different networks densities and distributions and catchments areas when applied on the same DEM sets [7]
The concept of integrated catchment management in dry land areas are often neglected when expanding agricultural development. The productivity of soils is increasingly affected by unfavourable drainage conditions and improper farming and irrigation practices, leading to widespread waterlogging and salinisation of soils [28]. As a result, the lack of understanding of soil, irrigation and drainage management can result in rapid land degradation. The current study aims to investigate the landcover changes of cultivated areas in typical dryland catchments, in order to assess the development and extent of waterlogging. The DEM-derived hydrological parameters are integrated with the multi-temporal remote sensing images in order to elaborate the relation of waterlogging pattern to the drainage networks, geomorphology and soil properties of the catchments. This is to determine the most suitable remedial action, which should be uniquely compatible with the inherited hydrogeological setting and characteristics of each catchment.
Fig. 1. Location map of the study area.
II. STUDY AREA
A typical dryland closed drainage basin in the Western Desert of Egypt (i.e., the depression of Farafra Oasis), was selected to assess the interaction of different landforms, the hydrogeological setting and the agricultural management on the land degradation of cultivated areas.
The Farafra Oasis is located in the middle part of the Western Desert of Egypt, oriented NE-SW, and covering over 10,000 km2 (Fig. 1). The floor of the northern part of Farafra depression is mainly underlain by chalk of the Upper Cretaceous Khoman Formation and the Paleocene Tarawan Formation, whereas the shale of the Upper Cretaceous-Lower Paleocene Dakhla Formation is mainly exposed in the southern areas [3]. Limestones of the Lower Eocene El-Naqb and Farafra Formations cap prominent escarpments surrounding the semi-closed depression [9]. Several anticlines and synclines trending NE-SW (i.e., the Syrian Arc System) affect the Baharia depression, which is located to the northeast of the Farafra depression, and several major faults have affected the rock units within the Farafra Oasis. Also the western escarpments of Al Quss Abu Said plateau as well as the northern escarpments are outlined by faults.
The study area is in a hyper-arid climate, receiving less than 1 mm of rainfall per annum; many years could pass without any rainfall. Aeolian processes therefore dominate, and large areas are mantled by different forms and patterns of wind-blown sand. The Farafra depression is covered by extensive fields of sand dunes, which extend from the northern areas to the southeast for 200 km . 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 [14]. Sand sheets also cover large areas of the region, obscuring most of the fluvial channels. In a number of areas, calcareous bedrock developed small karstic depressions during wet paleo-climatic periods; some of these depressions have been partially or fully filled by playas [18].
Fig. 2. The Landsat TM and ETM+ images for the Farafra Oasis, which show the extent of agriculture areas (green) in 1984 (left), 2003(middle) and 2011 (right). Note the development of seepage channels and water ponds (black ) during the period of 2003 to 2011.
The large depressions are composed of separate, yet interconnected local basins, that vary in size from several tens of meters up to hundreds of square kilometers in area. For example, the Al Gunnah playa covers an area of more than 100 km2 at the northeast foot of the slope of the Al Quss Abu Said plateau, representing one of the oldest agricultural and settlement areas in the Farafra depression. Recently, the Farafra depression and other oases in the Western Desert have undergone large scale development including reclamation by land leveling, and drilling hundreds of deep wells to tap the Nubian Sandstone aquifer for irrigation and other purposes. The Nubian Sandstone aquifer is one of the largest groundwater aquifer in the Sahara, and it extends for over 630,000 km2 in the Western Desert of Egypt, representing the only source of water in the oases [6]. But there are growing concerns on the safe yield production of the aquifer given the negligible 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 [43], [23]. These new cultivation projects are being developed on large tracts of soil mainly available in extensive playas, plains and outwash which are developed at the foot slope of bounding scarps and on the floor of the depression. However, large areas of the depressions have already been irrigated despite the drainage problems, in the hope that the natural drainage capacity of the deep sandy soil profiles is sufficient to control rising soil water tables and salt accumulation. Unfortunately, these hypotheses were not true and the cultivated areas have developed widespread waterlogging problems that are clearly visible in the field as well as on satellite images of moderate resolution such as the Landsat TM and ETM+.
III. REMOTE SENSING DATA
The identification of waterlogging and soil salinisation on satellite images can be achieved using either visual interpretation or digital analyses of bare and cultivated soil using change detection techniques, ratios and vegetation index [47]. Equally these different methods require ground truth and field verification; which are time-consuming and costly. Waterlogging and ponding can easily be detected on the satellite images as soil reflectance significantly decreases (in the visible part of the spectrum) in poorly drained and waterlogged areas compared to well-drained soil. However, selecting optimum satellite images is very crucial, as reflectance of soils and crops vary with different stages of vegetation growth and irrigation status. Therefore, the visual interpretation is more reliable than digital analyses and classifications when multi-temporal satellite images covering the cultivated areas are available only for a few dates. Several Landsat TM and ETM+ satellite images were collected for the Farafra depression (1984, 1989,1999, 2003 and 2011); these images were acquired during the autumn season (i.e., late October and early November) in order to minimise the uncertainties of vegetation and cultivation reflectance during a single season. False colour composite images with 30 m spatial resolution were produced by stacking the spectral band 7 (2.08–2.35 um , shortwave infrared), band 4 (0.76–0.90 um near infrared), and band 2 (0.52–0.60 visible green). These bands were displayed respectively in red, green, and blue, as it provided the best overall discrimination of the surficial materials for mapping purposes. These bands were also selected to minimise inter-band correlation, thereby maximizing information content of the resulting composite image. Then the images were processed, enhanced and visually interpreted to detect the land cover changes and to map the waterlogged areas (Fig. 2).
Fig. 3. Field photos show the land degradation and waterlogging within the Farafra Oasis.
The Landsat TM satellite image of 1984 (path 178/ row 41) shows that agricultural fields are of small areas and mainly concentrated on the floor of Al Gunnah playa (i.e., the village of Qasr Al Farafra), which is located at the north-eastern foot slope of Al Quss Abu Said plateau. The satellite images of 1989, 2003 and 2011 show that considerable area to the south-eastern area of Al Gunnah playa in the Farafra Oasis has been cultivated; the fields are mainly arranged in arrays of square and rectangular parcels. Field investigation showed that several irrigation and drainage channels have been developed through these parcels and the main irrigation method is bordering; the field is subdivided into smaller levelled parcels being separated by low raised levees. Extensive tracts of uncultivated soils are waterlogged and have supported the growth of diagnostic species of saline soils (Fig. 3).
A. The Automatic Detection of Drainage Networks
Extraction of the drainage networks from the DEM was accomplished using digitised data from the satellite images as well as using the standard processing techniques within a GIS platform as shown in the flow chart (Fig. 4). The DEM of SRTM (Fig. 5) was processed to automatically extract the drainage networks and sub-catchment boundary for the studied areas in order to investigate the spatial relationship of agriculture fields and the catchment-drainage networks and aerial extent. The hydrological analyses of DEM was carried out using the widely used D-8 algorithm embedded in ArcInfo software with some modifications in the filling of the DEM step [12]. 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 [25]. Therefore it is important to adjust the flow directions to cope with the fact that the filling step of the DEM does not distinguish between naturally occurring sinks (i.e., playas), which is the case in the study area and the artifacts resulting from the generation technique of DEMs. The main playas dotting the area were delineated by visual interpretation of satellite images and their relative low elevations to surrounding were assessed using the available DEM. The playas were considered as terminals for surface flow and seepage from a fixed catchment area, thus they were masked from the processing steps of DEM to locally entrap the surface flow in separate and terminal locations. Therefore, the contributing drainage networks and sub-catchments of the different terminal playas have been determined following the routine application of the D-8 algorithm routine in ArcInfo as follow:
1) The terminal-masked DEMs were filled;
2) The flow direction of each cell into the lowest elevation cell of the surrounding eight cells was determined for the study areas;
3) Once the route of flow is determined for each cell, it is possible to accumulate the number of upslope flow contributing cells (i.e., areas) and the flow pathways;
4) Selecting a threshold of the minimum flow accumulation number is required to extract the fingertips of delineated channels and different catchments within the area.
B. The Soil Landforms and the Crop Water Requirements
The soil map of the Farafra Oasis was extracted from the available soil map of Egypt [2], the original nomenclature of soil order, suborders and great groups have been updated using the latest American Soil Taxonomy of USDA [45]. The ETM+ satellite images were pre-processed, and enhanced using ENVI 4.7 software [24] to delineate the extent of agricultural developments, seepage and water ponds. The different landforms were initially determined from the satellite images and DEM following the methodology developed by Dobos et al. [8] (Fig. 6). The digitized soil units were correlated and combined with the delineated landforms to define the dominant soil sets, which indicate the vulnerability to waterlogging and salinisation hazards. Assessment of the crop water requirements is also important to determine the impact of adopted agricultural management on waterlogging. Indeed, disturbance of the balance be-tween water input (i.e., the total depths of irrigation, leaching of the salts) and output (i.e., evapotranspiration and infiltration into the deep soil layers) must also considered defining the control on waterlogging. Roughly, the evoptranspiration and crop water requirement were estimated for the various crops using the CROPWAT software developed by FAO [15] depending on climatic, soil and crop types data.
Fig. 4. The flow chart of the utilized data, processing and analyses.
Fig. 5. The SRTM DEM and the automatically extracted drainage networks for the cultivated catchments within the study area.
Fig. 6. Map shows the main landforms delineated from the satellite images and DEM for the Farafra Oasis. These units were checked for accuracy in the field.
Although there is no quantitative data or measurements available for the utilized water for leaching or the infiltration to the deep layers of soil, it is apparent that these high evapotranspiration rates should have limited the development of waterlogging. The excessive use of water for leaching and the irrigation of crops via flooding of the land parcels may also accelerate the saturation of the shallow soil. Therefore, other irrigation methods such as sprinkling or dripping could be used, but the financial issues may be an obstacle.
Fig. 7. The relevance of extent of cultivation and drainage networks on the pattern of waterlogging in the Farafra Oasis
IV. RESULTS
The interpretation of satellite images and field observations showed that the cultivation of large areas in closed drainage basins has developed extensive tracts of waterlogging and water ponds on the low playa surfaces. Most of the waterlogged areas are distributed in a unique pattern resembling channels of drainage basins. Most of the Farafra drainage networks (formed during the Quaternary wet pluvial) are of poor surface expression due to the prevailing aridity, and are now buried by sand sheets. The automatically extracted drainage networks from the DEM are in coincidence with the extent of seepage and waterlogging within the new cultivated fields in the Farfara Oasis. The inactive alluvial channels are being saturated with excess irrigation water, which seeps through the subsurface and gradually migrates downstream (Fig. 7). This high correlation clearly demonstrates the control of geomorphology and landforms on the dynamics of waterlogging. The buried shallow channels focus the seepage along their natural courses downstream, leading to the development of surface ponds in distant cultivated low areas. In 1989, the total cultivated area in the Farafra depression was approximately 143 km2 , and it increased to 190 km2 in 2011. During the same period, waterlogged and ponded areas increased from approximately 22.7 km2 to 36.1 km2 . Moreover, the 2011 TM satellite image clearly shows that these seepage channels have partially filled the local playas with ponds, and are only at short distances from the old agricultural fields.
The current study clearly shows that development has not accounted for the regional physiographic setting of soil, hydrological setting, and geomorphology of the cultivated areas. The agricultural fields in the Saharan oases are mainly developed on the floors of depressions, mudflats, and outwash plains. Most of the soils in these landforms are usually shallow (less than 0.5 m deep). These shallow soils are: Lithic Torrifluvents, Lithic Torriorthents, and Lithic Torripsamments. On the other hand, deep soils (from 1.5 to 1.7 m) are mainly represented by Vertic Torrifluvents and Typic Torrifluvents soils and occupied by the ’old’ cultivation. The calcium carbonate content is very high, particularly in the Typic Haplocalcids soils. This implies that calcic horizons in these soils can develop subsurface hardpan, which encourages the rapid development of waterlogging hazards. The widespread development of waterlogging in the new cultivated areas of the Farafra depression has initiated a mitigation strategy to combat waterlogging. The construction of spaced, parallel open drainage channels to combat waterlogging is a widely used technique in the Nile Delta and Valley, but the adoption of this technique in the Farafra depression seems to be ineffective. The designed open-drainage channels locally intersect the waterlogged buried channels, but considerable areas still affected by waterlogging remain, particularly those associated with the neglected buried channels. Alternatively, the main natural drainage channels themselves must be considered and used to collect the excess water, which usually moves laterally toward channels as through flow
V. DISCUSSION AND CONCLUSION
The hydrology of dryland catchments in different parts of the globe has been disturbed by the large scale and unprecedented agricultural development in these areas. This is mainly driven by the stress of population increase, economic growth, and development. The huge surplus irrigation water whether conveyed from nearby river catchments, or pumped from underlying groundwater aquifer has slowly engaged the fluvial processes of these catchments. Planning for the new Saharan projects seems not preceded by sufficient multi-disciplinary integrated research on soil physiography, catchment hydrology, evaporatively-driven geochemistry, and their roles on adopting certain irrigation and discharge strategies (e.g., [46]). It is of utmost importance to consider the context of catchment hydrological processes in planning for new cultivation projects in the Saharan soil. Although the fluvial and hydrological processes of these catchments have been inactive for prolonged period under prevailing aridity, irrigation has provoked low rate, yet signifi- cant, hydrologic activity within these defunct channels.
The mapping of soil units and their relation to the fluvial channels is also necessary to better manage the waterlogging problems, particularly in closed drainage basins. For example, the lithic soils are much more susceptible to waterlogging in low lying landforms (e.g., playas) than those developed on higher elevation, such as on foot slopes. Within each catchment only certain areas should be cultivated, the fluvial channels should be utilised for agriculture drainage, which must be conveyed into local non-developed playas. The system of reserving certain areas within the agricultural fields for seepage and collection of drainage water is known as the ’dry-drainage’ concept [30]. This system is suitable to be implemented in the Saharan areas. The proportion of cultivated lands within each catchment, the irrigation water requirements and methods, and evaporation from drainage ponds should be balanced, to prevent waterlogging and salinisation hazards. This system proved significance in controlling waterlogging in Indus basin in Pakistan, and in rice-growing areas of West Africa [30]. Recently, the ’bio-drainage’ approach has also widely been used to combat soil salinisation and waterlogging, particularly in dry land areas. Certain plant species are capable of lowering the rising ground water tables and can be adopted as an alternative and cheaper strategy to control salinity [51]. The increasing expansion of agricultural areas and the accompanying widespread waterlogging and salinisation require time-effective and reliable remote sensing monitoring and observation, to record changes and to anticipate further degradation. Remote sensing data have been used as a rapid and efficient tool to monitor, assess and evaluate the progress of different land use and land cover changes (e.g., [37], [40]). Thus, proper and timely decisions can be made to modify the management practices or undertake remedial actions that are most appropriate [32]. The use of advanced techniques for irrigation (i.e., dripping and sprinkling) also can maximize the benefit of saving limited water resources and reduce the development of harmful waterlogging.
In conclusion waterlogging and soil salinisation is the major threat facing the development of the Saharan areas. Extensive waterlogging hazards have occurred as the geomorphologic setting was not sufficiently considered when developing new agricultural areas. The playas and buried channels of closed drainage basins are the most vulnerable areas for waterlogging, particularly when the soil of higher surrounding areas is cultivated. It is recommended that the management strategies for waterlogging should consider the natural existing drainage system when planning for the distribution of farmlands to minimise the need for artificial drainage to control subsequent waterlogging. The value of remote sensing, and DEMs of different sources and resolutions is highly appreciated in the dryland environments; as the essential topographic data, field observations and in situ collected data are very limited. The integration of these multiple sources of data and their interpretation can enable us to better understand the complex processes of land degradation, thus improving the management of limited water and soil resources within the fragile dryland.
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Mohammed El Bastawesy, photograph and biography not available at the time of publication.
Rafat Ramadan Ali, photograph and biography not available at the time of publication.
Daniel M. Deocampo, photograph and biography not available at the time of publication.
Mohammed Saed Al Baroudi, photograph and biography not available at the time of publication.
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