Vast 'Fossil' Aquifer Beneath Sahara Desert is Slowly Refilling

A vast water supply stored deep beneath the Sahara Desert thought to be a relic of ancient times is actually being resupplied each year, according to a new study published in the the journal Geophysical Research Letters. But the rate of resupply does not meet or exceed water demand, which poses long-term water challenges for the future.

The northern Sahara aquifer system extends across an area nearly double that of mainland France and is thought to hold more than 30,000 cubic kilometers of water accumulated during wet periods that occurred over the last 1 million years.

The aquifer system supports urban and agricultural development in semi-arid regions such as Tunisia, Libya and Algeria.

Using estimates based on satellite data, scientists at the French Institute for Research and Development (IRD) estimated the variations in the volume of water in the aquifer. They suspect the rate of recharge is an average of 1.4 cubic kilometers per year for the years 2003 - 2010. The estimated recharge represents 40 percent of the withdrawals from the water bank made 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," the researchers wrote, adding that, "Precipitation in the region seemed too low and evapotranspiration too high to recharge deep aquifers."

According to the IRD, water from the northern Sahara aquifer system was thought to be "fossil," as in a non-renewable resource like coal or oil. But the recent research shows that the aquifer is recharging, and now researches have empirical data to account for how much refilling is going on.

Each year the system receives an average of 1.4 cubic kilometers or aquifer recharge in the form of rainfall, about 2 millimeter rise across the surface. From the period 2003 to 2010, annual recharge even reached 4.4 km3 in some years, or 6.5 millimeters per year.

Using data from NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission and data from the German aerospace center, the researchers were able to calculate the volume of water going into the aquifer. The GRACE system has been orbiting Earth since 2002 and can measure variations in Earth's field of gravity, which enabled researchers to deduce the water mass contained in pockets beneath the surface. Scientists used the data to estimate the change in volume of water in the aquifer and determine the amount of aquifer recharge once withdrawals from the surface were taken into account.

The data show a stark reality, however. The researchers found that while the aquifer system is recharging, the withdrawal from it still exceeds deposits, and 60 percent of the annual take is not being replenished. That percentage is likely to increase as demand for more water continues to increase and population in the region grows and the economy expands, generating a further demand for water.