Groundwater modelling and assessment under uncertain hydrological conditions for Egyptian sahara

Abstract

Lack of hydrogeological data is the main reason for the difficulty of groundwater management, especially in arid zones. Egypt’s Sahara Desert is located in Western Egypt and is lacking hydrogeological data. Recent development of the Egyptian Sahara is mainly due to the Nubian Sandstone Aquifer System (NSAS) as a unique source of water there. NSAS covers a great part of Egypt, Sudan, Chad and Libya and is considered as a main source of freshwater. During the last two decades, excess pumping of groundwater at the Egyptian Sahara brought about a significant drawdown of the groundwater table. This chapter will discuss a new technique that was developed to overcome the uncertainty from data gaps to facilitate the implementation of numerical models to improve strategies for optimal groundwater management. The core of this developed method is to understand the temporal and spatial variation of groundwater table. In the Egyptian Sahara, the hydrogeological data needed for groundwater simulation are lacking, thereby introducing a problem for numerical models calibration and validation. A newly developed model named the modified grey model (MGM) was proposed to analyse groundwater flow. At its core it is a finite element method (FEM) with a new developed modified genetic algorithm (MGA) to obtain the goodness of fit with observations. The MGM is an attempt to determine a selection process of the best input models’ trends with the appropriate values of input parameters for achieving acceptable fitting to the measured data. Kharga Oasis was selected as a case study for application of the developed MGM in groundwater flow analysis. The MGM simulation results clearly show that the future groundwater table will face a severe drawdown in the northeastern part of the study area compared with that in the southwestern part. On the other hand, by 2060, the hydraulic head difference between these two parts will reach 140 m. Considering the uncertainty and lack of available data, the MGM produced more realistic results compared with those obtained from only FEM. Three development scenarios of groundwater withdrawal were proposed. These scenarios include either expanding the current extraction rate or redistributing the groundwater withdrawal over the recent working production wells (RWPWs). The results concluded that, for the northern part of the oasis, the groundwater table could be temporally recovered to an economical piezometric level; however, the table in the southern part is severely decreased. Conclusively, the MGM could be applied to other cases with similar data limitations.