PALEODRAINAGE NETWORKS OF THE NUBIAN AQUIFER SYSTEM REVEALED FROM SIR-C AND SRTM DATA

The Nubian Aquifer system of northeast Africa is one of the world's largest potable groundwater reserves. Because it occurs in one of the world's driest climates, understanding its history and behavior has both scientific and practical importance. The sparse settlement of the huge (2x10⁶ km²) area underlain by this aquifer, and the limited resources of the nations in which it occurs, result in a situation where knowledge of its origin and history is incomplete.

We defined the paleodrainage channels in Southwestern Egypt and northern Sudan using recently released 30 arc second Shuttle Radar Topography Mission data (SRTM) and ARC/INFO watershed delineation routines. The SRTM-based streams, which are now partially covered by sand sheets and dunes, were validated by comparison to stream distribution inferred from co-registered Space-borne Imaging Radar-C/Synthetic Aperture Radar (SIR-C) data. A good correspondence between the SRTM-derived channels and the SIR-C derived channels is evident.

Results indicate that the paleodrainage pattern extends from the highlands at the intersection of the political boundaries between Egypt, Sudan, and Libya to the northeast towards the Kharga oasis. Within the catchment area there are two major closed drainage basins (Dakhla, Kharga). In these areas there are thick playa deposits that probably mark the locations of paleolakes that developed in these depressions. We postulate that extensive recharge of the underlying Nubian aquifer must have occurred beneath the paleodrainage network during previous wet climatic periods since the Nubian sandstone crops out across the entire area covered by the delineated watershed. The autochtonous recharge is supported by the presence of surrounding highlands that are largely formed of basement uplifts suggesting that this sub-basin is probably largely disconnected from the Nubian aquifer to the south. This suggestion is further corroborated by the progressive increase in ages of Nubian Aquifer groundwater along the hydraulic gradient from the southwest to the northeast. For example, the youngest ³⁶Cl age (~50 kyr) was obtained from East Uweinat and the oldest ⁸¹Kr age (~1million yr) was obtained from Bauti-1 (Baharyia).