Quantifying the modern recharge of the “fossil” Sahara aquifers

The North-Western Sahara Aquifer System (NWSAS),one of the world’s largest groundwater systems, showsan overall piezometric decline associated with increasingwithdrawals. Estimating the recharge rate in such asemiarid system is challenging but crucial for sustainablewater development. In this paper, the recharge of theNWSAS is estimated using a regional water budget basedon GRACE terrestrial water storage monthly records, soilmoisture from the GLDAS (a land data system thatassimilates hydrological information), and groundwaterpumping rates. A cumulated natural recharge rate of1.40 ˙ 0.90 km3 yr–1 is estimated for the two main aquifers.Our results suggest a renewal rate of about 40% which partlycontradicts the premise that recharge in this area shouldbe very low or even null. Aquifer depletion inferred fromour analysis is consistent with observed piezometric headdecline in the two main aquifers in the region. Annualrecharge variations were also estimated and vary between0 and 4.40 km3 yr–1 for the period 2003–2010. Thesevalues correspond to a recharge between 0 and 6.75 mm yr–1on the 650,000 km2 of outcropping areas of the aquifers,which is consistent with the expected weak and sporadicrecharge in this semiarid environment. These variations arealso in line with annual rainfall variation with a lag time ofabout 1 year.