ROLE OF GROUNDWATER EVAPORATION LOSSES ON THE AVAILABLE WATER RESOURCES, WESTERN DESERT OF EGYPT

The Nubian Sandstone forms an aquifer underlying some two million squarekilometers of Egypt, Libya, Sudan and to a lesser extent of Chad. Aridity dominates the area and the limited amount of existing cultivated land is subject to various conditions that induce desertification. The knowledge of the terms of water balance of the vast groundwater reservoir is rather scarce and based on indirect estimation. A proper groundwater extraction plan should be based on long term forecasts of the evolution of all water balance terms. A problem of calculating natural groundwater losses is complicated due to the large variability of the hydrogeological situation and the extensiveness of the areas where evaporation occurs. Direct experimental evidence of larger evaporation losses with Bowen-ratio measurements at the sites of field experiments in the Western Desert has been obtained. The natural evaporation losses obtained are rather different from ·the former groundwater losses studies. This has an overall effect on the regional groundwater balance. Since groundwater development schemes are based on simulation models, the impact of future extractions has to be considered. Lowering. of the hydraulic head by artificial extraction decreases the natural losses, resulting in a compensation of water available for·pumping. Proper long term groundwater development plans has to take into account extraction and natural losses versus the hydraulic head and horizontal recharge rates.