HOW DO AQUIFERS RESPOND TO WET AND DRY PERIODS? A CASE STUDY FROM THE NUBIAN SANDSTONE AQUIFER SYSTEM (NSAS)

An integrated research approach (remote sensing, field, geophysics) was conducted to investigate the response of large aquifers to dry and wet periods, using the less studied Nubian Sandstone Aquifer System (NSAS) of NE Africa as a test site. The aquifer extends over 2.2 x 10⁶ km² in Egypt, Libya, Chad, and Sudan and consists of thick (> 3 km), water-bearing, Paleozoic and Mesozoic sandstone with intercalations of Tertiary shale and clay. Using satellite-based precipitation (Tropical Rainfall Mission Measuring Mission [TRMM] and Global Precipitation Measurement [GPM] data, two periods were identified: an early dry period (2002 to 2012) where the Average Annual Precipitation (AAP) over recharge areas was modest (109 mm) followed by a wet period (2013-2016; AAP: 178 mm), whereas the AAP remained negligible (<5mm) over the northern areas (i.e., Dakhla Aquifer System [DAS] and northern Kufra) during both, the dry and wet periods. Using GRACE RL05 monthly mascon solutions from Center for Space Research (CSR), the following were extracted: (1) trends for Terrestrial Water Storage (TWS) for dry and wet periods over DAS (dry: -4 mm/yr; wet: +11.4 mm/yr), northern Kufra (dry: -1.3 mm/yr; wet: +7.2 mm/yr), and NSAS (dry: +1.3 mm/yr; wet: +15.9 mm/yr), and (2) mass movement directions (from phase and difference images). A conceptual model was developed to account for GRACE observations: (1) the NSAS witnessed near-steady conditions in dry periods, except for DAS (depleted by – 4.4 km³/yr) and an increase in groundwater storage (+6.5 km³/yr) during the wet period, (2) groundwater extraction, not climatic changes over the DAS, is responsible for the observed TWS depletions in dry years, (3) TWS depletion over DAS is related to the presence of an E-W trending uplift that impedes replenishment by groundwater flow from the south, (4) TWS in DAS is controlled by groundwater flow from the south, from east (Lake Nasser), and from the west (Kufra basin), (5) only in wet periods does DAS receive substantial replenishment by groundwater flow from the south, and (6) the fast response (groundwater flow: meters/day) of the aquifer in wet periods can be explained by preferential flow in fractured media. Groundwater discharge (and soil moisture) is increasing in lowlands during wet periods and vice versa in dry periods.