Water is a vital natural resource in the eastern Sahara, and the social and economic well-being of the people there depends on managing it wisely. Economic development plans for this region now include the expansion of agriculture into desert areas to utilize the vast water resources of the Nubian aquifers. But before agricultural expansion can occur, a thorough evaluation of these resources is needed.
Since radar waves penetrate sand to reveal the courses of ancient rivers below today's surface, synthetic aperture radar (SAR) data offers a useful starting point for such evaluation. Until a recent study in southwestern Egypt,1 the practical role of SAR data in groundwater resource-management decisions had not been explored. Instead, research focused on confirming the ability of radar waves to penetrate desert sand as deep as 2m,2 as well as on understanding the implications of a water-rich past in this region.3 Ultimately, we hope that advanced evaluation schemes will decrease regional drawdown on the aquifers by informing strategic groundwater-pumping decisions.
As a precursor to the southwestern Egypt study, SAR images were georeferenced and stored in a geographic-information-systems database. Digital enhancements were applied to produce high-contrast images suitable for producing drainage and fault distribution maps.4 (For quantitative analysis, phase information can be extracted using multi-polarization datasets.5) Fracture-density maps were generated and overlain on the drainage maps to identify which wadi systems acted as the underlying aquifer's potential recharge zones and which fractures could serve as storage zones, collectors, or transmitters of groundwater. Topographic data from the NASA Space Shuttle radar topography mission (that also reflects palaeotopography6) was used to check regional drainage directions and establish slope: areas with optimal slope conditions (for surface water infiltration) and high drainage-fracture intersections are most favorable to groundwater accumulation.
Figure 1. Coverage of the Radarsat-1 images overlain on a Landsat ETM+ color composite. The ETM+ image shows the nature of the surface sand fields in the study area. ETM+: Enhanced Thematic Mapper Plus.
Multispectral images of southwestern Egypt show a sand-covered desert surface (see Figure 1). However, new fluvial and structural interpretations from SAR data (see Figure 2) reveal that the landscape was produced by fluvial action, including newly-mapped alluvial fans. In central locations, braided channels are spatially aligned with a northeast structural trend, suggesting preferential water-flow paths that are consistent with the direction of groundwater flow in this area today. The alluvial fans and structurally enclosed channels coincide with gentle slopes and optimal recharge conditions, indicating an increased likelihood of groundwater in these areas.
SAR interpretations were correlated with anomalies observed in groundwater data from 383 wells. Results suggest a relationship between the spatial organization of fluvial features and occurrence of low-salinity groundwater, which exists adjacent to the alluvial fans and in southwestern reaches of the structurally enclosed channels. Further, wells near these structures contain low-salinity water, emphasizing that knowledge of structural features is essential to understanding groundwater flow paths. A distributed groundwater flow and transport model was constructed to consider the role of structures as preferential flow and transport paths by choosing appropriate hydraulic and transport parameters. Fluvial features were considered as recharge sources.
Figure 2. Radarsat-1 mosaic, with fluvial and structural interpretations overlain, reveals the fluvial action that produced the desert landscape.
Thus, SAR images combined with topographic and groundwater data have improved understanding of the heterogeneity of local aquifers in southwestern Egypt. This new approach is cost-effective, noninvasive, and applicable throughout the eastern Sahara, where assessment of water availability is critical to the expansion of food production to alleviate population pressures.
Future work should include the analysis of multispectral data for complete surface feature identification; further field documentation to confirm and supplement SAR image interpretations and to perform structural analysis aimed at determining the hydrological potential of the faults; and groundwater modeling. Water samples should be collected using packers to isolate different water zones to comprehend vertical as well as horizontal variations.
SPCS, Dept. Earth and Enviromental Sciences, Northeastern University
Center for Remote Sensing, Boston University
Cordula A. Robinson is an academic specialist in the School of Professional and Continuing Studies at Northeastern University's Department of Earth and Environmental Sciences in Boston, Massachusetts, and a research associate with Boston University's Center for Remote Sensing.