Photo Courtesy of California Dept. of Water Resources

Scientists from California's Dept. of Water Resources measure subsidence in the Central Valley.

Two new studies show that some of the largest groundwater basins in the world are being drained by human consumption but that there is little information about how much water remains in them.

Published in Water Resources Research on June 16 and conducted by a team of researchers from the University of California at Irvine, the studies draw upon data supplied by NASA Gravity Recovery and Climate Experiment satellites. The study evaluates ground- water levels in basins around the world.

The satellites measure gravity levels in different parts of the world, which are affected by the weight of water.

Lead investigator Jay Famiglietti, also senior water scientist at NASA's Jet Propulsion Laboratory, said in a statement, "Given how quickly we are consuming the world's groundwater reserves, we need a coordinated global effort to determine how much is left."

In the first paper, researchers examined the planet's 37 largest aquifers between 2003 and 2013. They found eight of the aquifers to be "overstressed," with no natural replenishment to offset usage. Another five were found to be "stressed"—still threatened, but with some water flowing back to them.

According to the research team, the most distressed—which include the Arabian aquifer system and, in northwestern India and Pakistan, the Indus Basin aquifer—are located in the world's driest areas, which draw heavily on underground water. Climate change and population growth are expected to intensify the problem, the researchers concluded.

The second paper concludes that the total volume of the world's remaining usable groundwater is poorly known. William Alley, former head of the U.S. Geological Survey office of groundwater and now director of science and technology for the National Groundwater Association, says the studies "point to the need for groundwater worldwide" and the need for more and better measurement of groundwater resources.

Bad News for California

The UC Irvine team found that water in California's Central Valley, which is widely used by the agricultural industry, is being rapidly depleted.

In recent years, overuse of ground- water resources in the Central Valley have led to subsidence, in which the land sinks due to overextraction of groundwater.

Last year, the California Legislature passed and the governor signed the Sustainable Groundwater Management Act to preserve and protect groundwater sources. Part of the law requires some replenishment or recharging of groundwater sources.

Alley says the UC Irvine papers also point to the need to "restore our water underground and to look a lot harder at [Sustainable Groundwater] Management Act recharge."

The research team included co- authors from NASA, the National Center for Atmospheric Research, the National Taiwan University and University of California Santa Barbara.