Groundwater supplies around the world are scanter than previously thought and are depleting fast in many places, according to a set of two studies published yesterday online in Water Resources Research.

Groundwater is the primary water source for about 2 billion people worldwide. But estimates of supplies are based on rough estimates of withdrawals and deposits, and as such, are all over the map.

“It is absolutely insane that we do not know how much water we have in the world’s major aquifers, and that the range of estimates is so great that the numbers are effectively meaningless,” said study co-author Jay Famiglietti, a senior water scientist at NASA’s Jet Propulsion Laboratory and a professor at the University of California, Irvine.

Famiglietti and his fellow researchers used NASA’s Gravity Recovery and Climate Experiment satellites, which orbit the Earth at a distance of 400 kilometers, to measure the gravitational pull of masses of water. The data spans from January 2003 to December 2013.

By looking at total storage in groundwater basins, rather than just estimating the amount that leaves and re-enters them each year, satellite measurements can add a significant dimension to current understanding of water storage.

Yesterday’s studies found that of the world’s 37 largest aquifers, 13 are being depleted, with little to no water re-entering them. The most-stressed aquifer is the Arabian Aquifer System, followed by the Indus Basin in northwestern India and Pakistan and the Murzuk-Djado Basin in northern Africa. Fourth is California’s Central Valley, where drought has been increasingly driving farmers to tap groundwater to supplant meager river flows (ClimateWire, Dec. 17, 2014).

“The insufficient knowledge of total groundwater supplies will continue to limit effective governance of groundwater systems until a significant effort is made to improve groundwater storage estimates,” one of the studies says. “We have shown that transparent knowledge on groundwater stocks is lacking in the majority of the world’s large aquifer systems.”

‘We can no longer tolerate this level of uncertainty’
Using self-reported statistics from groundwater users isn’t as accurate as satellite models, the studies found. Calculations are often based on national per-capita water usage and therefore don’t even reflect national groundwater usage accurately, let alone effects on individual groundwater basins.

The estimates can be large orders of magnitude off-base: The methods used in a 1969 study of global groundwater storage, applied in the Central Valley, would give an estimate of 16,000 cubic kilometers, but the new studies found a huge degree of uncertainty—too much to make a concrete estimate. They also found previous estimates were likely far too optimistic because they assumed aquifers were much deeper and uniformly porous than they really are.

“We don’t actually know how much is stored in each of these aquifers. Estimates of remaining storage might vary from decades to millennia,” said lead author Alexandra Richey, a doctoral student at the University of California, Irvine, at the time of the studies. “In a water-scarce society, we can no longer tolerate this level of uncertainty, especially since groundwater is disappearing so rapidly.”

The Ganges Basin, in India, had the biggest discrepancy between self-reported and satellite-based measurements, with the satellite finding a net depletion of 19.6 millimeters per year compared to previous calculations of 63.1 millimeters per year. The basin still ranks as the most-used aquifer worldwide, though.

The studies actually found that five basins—including the Central Valley—are experiencing less withdrawal than previously thought.

The Central Valley is still in bad shape, though, the study finds. The new measurements put the basin into the “highly stressed” category, rather than “extremely stressed.”

“The current depletion rate shows that the [Central Valley] aquifer is unable to balance the combined impact of groundwater use and drought, either through capture or active management, and is therefore not a resilient system,” researchers found.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500