Assessment of Signal Leakage on GRACE-Derived Seasonal Variations in Dakhla Subbasin, Egypt

Abstract:

The Nubian Sandstone Aquifer System in northeast Africa is formed of three subbasins, the Dakhla, Kufra, and Northern Sudan Platform subbasins. The Dakhla subasin (DSB) receives negligible precipitation, yet displays significant seasonal variations in GRACE_TWS (up to 77 mm/yr) across the entire subbasin. The origin of these variations could be related to one or more of the following: (1) leakage in from surroundings (Kufra basin to the west, the Northern Sudan Platform from the south, and the Mediterranean sea to the north, and the Red Sea to the east, (2) leakage out from Lake Nasser, and (3) recharge and rapid groundwater flow from Lake Nasser and the northern Sudan Platform. Using a three-fold exercise, we cite evidences in support of option 3 and rule out options 1 and 2. First, the simulated TWS was derived from the widely applied Land Surface Model (LSM), GLDAS (Global Land Data Simulation System) to estimate the leakage in signal over the DSB due to finite spherical harmonic expansion. Second, we applied forward modeling technique to test whether the observed seasonal variation can be accounted for by leakage alone from Lake Nasser to its surroundings. Finally, the iterative forward modeling procedure was applied to reconstruct the true mass variations of GRACE_TWS over the Dakhla subbasin. Findings from our simulations suggest the following: (1) The leakage in signal over the DSB cannot account for the observed seasonal GRACE_TWS patterns and neither can the leakage out from Lake Nasser, (2) the leakage in (up to 10 mm) from the DSB surroundings is restricted to the northern peripheries of the DSB (~200 km deep) and is probably caused by northerly precipitation along the Mediterranean coast and negligible (< 4 mm) over the rest of the subbasin, (3) 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), (4) the observed seasonal variations in GRACE_TWS is related to an increase in groundwater storage related to seasonal recharge from Lake Nasser, and (5) Lake Nasser is the main source of modern recharge for the DSB, and a network of faults, fractures, and karst topography rapidly channels groundwater flow across the entire DSB.