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Data from the Gravity Recovery and Climate Experiment (GRACE) and outputs of the CLM4.5 model were used to estimate recharge and depletion rates for large aquifers, investigate the connectivity of an aquifer's subbasins, and identify barriers and preferred pathways for groundwater flow within an aquifer system. The Nubian Sandstone Aquifer System and its subbasins (Dakhla, Northern Sudan Platform, and Kufra) in northeast Africa were used for demonstration purposes, and findings were tested and verified against geological, geophysical, remote sensing, geochronologic, and geochemical data. There are four major findings. (1) The average annual precipitation data over recharge areas in the southern Kufra section and the Northern Sudan Platform subbasin were estimated at 54.8 km 3 , and 32.8 km 3 , respectively, and knowing the annual extraction rates over these two areas (~0.40 ± 0.20 km 3 ), recharge rates were estimated at 0.78 ± 0.49 km 3 /yr and 1.44 ± 0.42 km 3 /yr, respectively. (2) GRACEderived groundwater depletion rates over the Dakhla subbasin and the Northern Kufra section were estimated at 4.44 ± 0.42 km 3 /yr and 0.48 ± 0.32 km 3 /yr, respectively. (3) The observed depletion in the southern parts of the Dakhla subbasin is apparently caused by the presence of the east-west-trending Uweinat-Aswan basement uplift, which impedes the south-to-north groundwater flow and hence reduces replenishment from recharge areas in the south. (4) A major northeast-southwest-trending shear zone (Pelusium shear system) is apparently providing a preferred groundwater flow pathway from the Kufra to the Dakhla subbasin. Our inte-grated approach provides a replicable and cost-effective model for better understanding of the hydrogeologic setting of large aquifers worldwide and for optimum management of these groundwater resources.

Flash flooding is one of the periodic geohazards in the southern Red Sea Coast. However, their freshwaters are the main source of recharging alluvial and fractured aquifers. This paper presents hydrological and geomorphologic classification of Wadi El-Gemal, Wadi Umm El-Abas, Wadi Abu Ghuson and Wadi Lahmi, along the southeastern Red Sea Coast in Egypt. The main goal is to find a relationship of flash floods and groundwater recharge potentials. Satellite imageries and topographic data were analysed via remote sensing and GIS techniques. The main four valleys' orders range from six to seven. Wadi El-Gemal was the main focus of this study; it is characterized by high stream frequency, low stream density and coarse texture, reflecting influence of highly fractured Precambrian rocks. Most of the wadis have umbrella-shaped catchment areas, due to the influence of NW-SE Najd Fault System and late E-W strike-slip faults. The main wadis were divided into 45 sub-basins. 14 of the studied sub-basins flow into Wadi El-Gemal, 7 flow into Wadi Umm El-Abas, 10 are in Abu Ghuson, and rest of the basins flow into WadiLahmi. A conceptual model was used in this study, showing that most of the sub-basins have high flash flooding and low groundwater recharge potentials. However, only two sub-basins have low potential of flooding and high potential of groundwater recharge, whereas few sub-basins have moderate potential of groundwater recharge as well as flooding. For flash floods beneficiation and mitigation, construction of multifunctional embankment dams is imminent. F. Abdalla et al.