Quantifying the modern recharge of the “fossil” Sahara aquifers
Résumé
The North-Western Sahara Aquifer System (NWSAS),
one of the world’s largest groundwater systems, shows
an overall piezometric decline associated with increasing
withdrawals. Estimating the recharge rate in such a
semiarid system is challenging but crucial for sustainable
water development. In this paper, the recharge of the
NWSAS is estimated using a regional water budget based
on GRACE terrestrial water storage monthly records, soil
moisture from the GLDAS (a land data system that
assimilates hydrological information), and groundwater
pumping rates. A cumulated natural recharge rate of
1.40 ˙ 0.90 km3 yr–1 is estimated for the two main aquifers.
Our results suggest a renewal rate of about 40% which partly
contradicts the premise that recharge in this area should
be very low or even null. Aquifer depletion inferred from
our analysis is consistent with observed piezometric head
decline in the two main aquifers in the region. Annual
recharge variations were also estimated and vary between
0 and 4.40 km3 yr–1 for the period 2003–2010. These
values correspond to a recharge between 0 and 6.75 mm yr–1
on the 650,000 km2 of outcropping areas of the aquifers,
which is consistent with the expected weak and sporadic
recharge in this semiarid environment. These variations are
also in line with annual rainfall variation with a lag time of
about 1 year.
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