干旱区生态环境知识资源中心

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.