Construction of a hydrologic model for estimating Wadi runoff and groundwater recharge in the Eastern Desert, Egypt

Abstract

We constructed a hydrologic model to estimate the groundwater recharge rate for alluvial aquifers of the Eastern Desert from sporadic precipitation over the Red Sea hills. To estimate initial losses over sub-basins, transmission losses through channel routing, and downstream runoff, we developed an integrated model combining spatial rainfall distribution, an appropriate basin unit hydrograph, and appropriate infiltration parameters. Watersheds and stream networks identified from digital terrain elevation data were verified by comparison with co-registered Landsat thematic mapper scenes and geologic maps. Records of a November 1994 storm event acquired from rain gauges along the Nile River and the Red Sea shore were used to generate a spatial precipitation distribution for the study area. A 2 hour design hyetograph was adopted from rain gauge data for the 1994 flood event. The model was tested against records from the November 1994 flood event at the outlets of the Tarfa and Hammamat watersheds.Groundwater recharge rates were estimated for the alluvial aquifers within the major watersheds of the north Eastern Desert. We estimated that during the 1994 flood event, the ground water recharge through transmission losses ranged from 21 to 31% (Tarfa: 15.8×106m3; Asyuti: 20×106m3, Qena: 49×106m3, Hammamat: 59×106m3) of the precipitated volume. The initial losses ranged from 65 to 77%. Only 3–7% of the precipitation reached the watershed outlets. Archival data show that rainfall events of the size of the November 1994 storm or larger occur every 40 months; thus, the annual recharge rates for the Tarfa, Asyuti, Qena, and Hammamat alluvial aquifers are estimated at 4.7×106m3, 6×106m3, 14.7×106m3, and 17.7×106m3, respectively. Implications for the use of these renewable ground waters and similar water resources in other arid areas of Egypt and in neighboring countries are clear.