About one third of Earth’s large groundwater basins are being rapidly depleted by human consumption even though we have scarce and inaccurate data about how much water remains in them, according to two new studies published in June 2015 in Water Resources Research. This means significant segments of Earth’s population are consuming groundwater without knowing when it might run out.

“Groundwater is currently the primary source of freshwater for approximately two billion people,” the researchers wrote. “Despite its importance, knowledge on the state of large groundwater systems is limited as compared to surface water, largely because the cost and complexity of monitoring large aquifer systems is often prohibitive.”

The map above shows the annual change in groundwater storage from 2003 to 2013 in the 37 largest aquifer systems in the world. Basins shown in shades of brown have had more water extracted in the study years than could be naturally replenished; basins in blue saw increases in underground water storage, perhaps due to changes in precipitation, ice or permafrost melting, or changes in surface water.

The study was partly based on data from the Gravity Recovery and Climate Experiment (GRACE), a pair of satellites that flies in close formation and measures small changes in mass and gravity near Earth’s surface. Water has mass and exerts a gravitational tug; GRACE observes those signals in ways that allow scientists to follow the movement of water—including groundwater—around the planet. The studies also included data from ground-based sources (such as national statistics on water extraction) and models of groundwater extraction and storage. The analysis was conducted by scientists from the University of California, Irvine, NASA’s Jet Propulsion Laboratory, the National Center for Atmospheric Research, National Taiwan University, and the University of California, Santa Barbara.

The multidisciplinary research team found that 13 of Earth’s 37 largest aquifers are being depleted while receiving little to no recharge. Eight were classified as “overstressed,” with almost no natural replenishment to offset usage, while the other five were found to be highly stressed, with that rate of extraction far exceeding the little bit of natural replenishment. Climate change and population growth are expected to intensify the problem.

“What happens when a highly stressed aquifer is located in a region with socioeconomic or political tensions that can't supplement declining water supplies fast enough?” asked Alexandra (Sasha) Richey, the lead author on both studies, who conducted the research while she was a doctoral student at UC Irvine. “We are trying to raise red flags now to pinpoint where active management today could protect future lives and livelihoods.”

The most overburdened aquifers are found in the world’s driest areas, where populations draw heavily on underground water. The Arabian Aquifer System, an important water source for more than 60 million people, is the most overstressed in the world. The Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, and the Murzuk-Djado Basin in northern Africa is third. Previous research has shown that California’s Central Valley, used heavily for agriculture, has been suffering rapid depletion.

“We don't actually know how much is stored in each of these aquifers. Our current estimates mostly date back to very crude methods developed in the 1960s and 1970s. Few aquifers have improved their storage estimates, and those that have still carry a huge range and uncertainty,” said Richey, who is now a post-doctoral fellow at Washington State University. In the Northwest Sahara Aquifer, for example, “time to depletion” estimates vary from 10 years to 21,000 years. “In a water-scarce society, we can no longer tolerate this level of uncertainty, especially since groundwater is disappearing so rapidly.”

Groundwater aquifers are typically located in soils or deeper rock layers beneath Earth's surface. The depth and thickness of many large aquifers can make it tough and costly to drill to bedrock and understand where the moisture bottoms out. But it has to be done, the authors say.

“Available physical and chemical measurements are simply insufficient,” said UCI professor and principal investigator Jay Famiglietti, who is also the senior water scientist at NASA’s Jet Propulsion Laboratory. “Given how quickly we are consuming the world's groundwater reserves, we need a coordinated global effort to determine how much is left.”

NASA Earth Observatory images by Joshua Stevens using GRACE global groundwater data courtesy of Jay Famiglietti NASA JPL/University of California Irvine and Richey et al. (2015). Caption assembled by Mike Carlowicz, based on stories by Alan Buis (JPL) and Janet Wilson (UCI).