GRACE-derived seasonal variations, a key to understanding aquifer sources, recharge and groundwater flow patterns

Abstract

<p>The Nubian Sandstone Aquifer System (NSAS) in northeast Africa is formed of three subbasins, the Dakhla, Kufra, and the Northern Sudan Platform subbasins. The Dakhla subbasin (DSB) receives negligible precipitation (<10 mm/yr), yet displays significant seasonal variations in GRACETWS (average: 50 mm/yr, up to 77 mm/yr) across the entire subbasin. The origin of these variations could be related to one or more of the following factors: (1) leakage out from Lake Nasser, (2) leakage in from surroundings (Kufra basin [west NSAS], Northern Sudan Platform [south NSAS], Mediterranean sea [north NSAS], and Red Sea [east NSAS], and (3) recharge and rapid groundwater flow from Lake Nasser and the northern Sudan Platform. Three approaches were used to investigate the contribution of leakage (factors 1 and 2) to the observed GRACETWS signal over the DSB subbasin: (1) forward modeling (in spherical harmonic domain) of the maximum variations in Lake Nasser levels was applied to test whether the observed seasonal variation in GRACETWS across the DSB can be accounted for by leakage from Lake Nasser alone; (2) estimate (in spherical harmonic domain) the leakage in signal using the simulated TWS from the widely applied Land Surface Model (LSM), GLDAS (Global Land Data Simulation System); and (3) apply iterative forward modeling (iterations: n=30) to reconstruct the true mass variations of GRACETWS over the DSB. Findings suggest: (1) the leakage in signal over the DSB cannot account for the observed seasonal GRACETWS patterns and neither can the leakage out from Lake Nasser; (2) the leakage out signal is centered over Lake Nasser and extends to its immediate surroundings with a maximum radius of 250 km (upper boundary of leakage error); (3) the iterative modeling indicates that the maximum leakage within the 250 km buffer zone around the lake amounted to 22.6 % of the observed GRACETWS signal; (4) minimal leakage (up to 10 mm) from northerly precipitation is observed along the northern sections (~200 km deep) of the NSAS and negligible (< 4 mm) leakage is detected over the remaining sections of the DSB; and (5) the observed seasonal variations in GRACETWS over the DSB is related to an increase in groundwater storage related to seasonal recharge from Lake Nasser and rapid groundwater flow along a network of faults, fractures, and karst topography across the entire DSB.</p>