WATER RESOURCES ASSESSMENT IN SIWA OASIS

Abstract

Siwa is one of the five main Oases in the Western Desert in Egypt. The area is bounded by the zero elevation Qontour line. The main aquifer system supplying the oases is the regional Nubian Sandstone, and it is overlain by the fractured limestone aquifer. The rainfall is practically nil. The only source of water in Siwa Oasis is the groundwater. The hydrogeological conditions are controlled by geological structure. The upper and the lower aquifers are hydraulically connected. Because of the excessive groundwater uses, the drainage lakes became inefficient to accept the water lost from the wells. The Research Institute for Groundwater (RIGW) studied this problem. RIGW blocked most of the uncontrolled wells and replaced them by new ones. A groundwater flow model was applied for Siwa Oasis by RIGW, (Sakr et al., 1999). Water management is a dynamic process, so this model is developed in the present study. The updating data collected from deep wells and oil wells and various management strategies are applied. From the present study of piezometeric head of Siwa depression, it is noticed that the depression is fed from all directions around the oasis and the source of water is the slow drainage around the oasis. The result of the present study shows that the feeding rate to the Nubian aquifer is 90.4Mll 3/year, water uses is 92.5Mm3/year the total extraction from the

aquifer system is 140Mm3/year.

Numerical groundwater flow model (MODFLOW) is also applied. The calibrated model is used for predicting of the aquifer system behavior for three strategic scenarios. As a result the maximum drawdown reaches 4.0 m after 80 years in the area with high extraction for the present extraction. Considering the response of Nubian sandstone aquifer due to future development with a total extraction of 800,000m3/day, the model simulation showed that

the minimum piezometric head reaches zero after one year in the western part.

To achieve better management of the aquifer system, both shallow and deep wells should be utilized in such a way, to keep the shallow wells flowing with extraction from deep aquifer. The model is used to allocate the new deep wells in the areas where shallow aquifer has low potential.