The Sahara conceals large quantities of water stored at depth and inherited from ancient times. A recent study by the IRD and its partners has just shown that this groundwater is not entirely fossil, but resupplied every year. Using a method based on data obtained by satellite, scientists estimated the variations in the volume of water lying under the northern Sahara desert: the current rate of recharge is on average 1.4 km3 per year, for the period 2003-2010. This represents 40% of withdrawals, mainly for irrigation to support the oasis economy. The inputs therefore do not compensate for the withdrawals, but their existence means that these transboundary aquifers, the main water resource of semi-arid regions in Algeria and Tunisia, could be managed sustainably.
Non negligible recharge
Until recently, groundwater in the northern Sahara aquifer system was considered as "fossil," i.e. non-renewable, similarly to coal or oil. Precipitation in the region seemed too low and evapotranspiration too high to recharge deep aquifers. But scientists have shown that, in reality, groundwater in the northern Sahara aquifer system, to give it its exact name, is still being fed today. Indeed, the recharging exists and has been quantified, as revealed in a study published in Geophysical Research Letters. Rainwater and runoff bring an average of 1.4 km3 to the system per year, or around 2 mm per year on the aquifer recharge surface. From the period 2003 to 2010, annual recharge even reached 4.4 km3 in some years, or 6.5 mm per year.
A new satellite approach
The research team highlighted this recharge using a new satellite measuring method. The scientists analysed the data provided by the GRACE satellite mission (Gravity Recovery and Climate Experiment) by NASA and the German aerospace centre. In orbit since 2002, GRACE measures variations in Earth's field of gravity, enabling variations in the water mass contained in the surface envelopes to be deduced. These data were used by the scientists to estimate the change in the volume of water stored and to deduce aquifer recharge, one withdrawals made on the aquifers were taken into account. Among other things, this global approach means that the uncertainties in the hydrogeological models can be discounted, which are based on local piezometric measurements, i.e. the level of water noted in wells and boreholes.
Withdrawals not compensated
The average recharge of 1.4 km3 per year corresponds to 40% of the 2.75 km3 in total withdrawn every year in the region, according to data from the Sahara and Sahel Observatory (OSS). As a result, 60% of annual withdrawals are not compensated. Despite significant recharge, the Northern Sahara Aquifer System remains therefore overexploited.
Since the 1960's, withdrawals have continued to increase, to satisfy the growing need of various social-economic sectors: industry, agriculture, tourism, household use. Wells and boreholes have multiplied and annual withdrawals have risen from 0.5 km3 in 1960 to 2.75 km3 in 2010, leading to a general drop in water levels, in some places reaching 25 to 50 m. Numerous artesian wells and natural springs, around which oases have developed, have already run out.
The reduction in artesianism, i.e. the water pressure within groundwater, risks affecting the viability of the oasis economy. By quantifying current recharge, this work will enable the development of tools for to manage the resource responsibly, while more economical systems of irrigation are put in place. The challenge is considerable: these groundwater resources will have to meet the growing needs of a population which should reach 8 million inhabitants by 2030 according to the OSS.
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