One of the world's most important sources of fresh water is being depleted at a faster rate than can naturally be replenished. Researchers from the University of California, Irvine, warn there is little to no data on how much water is left in these massive aquifers.

In the two studies published in the Water Resources Research journal, researchers collected satellite data from NASA to examine the 37 largest aquifers in the world. Researchers tracked the dips and bumps in Earth's gravitational pull, which is affected by the weight of the water, between 2003 and 2013.

They found that the Arabian Aquifer System was the most overstressed in the world, which is worrying as it’s an important water source for more than 60 million people. The Indus Basin aquifer of northwestern India and Pakistan came in second place and the Murzuk-Djado Basin in northern Africa is third. Researchers suggest that 21 of the world's 37 largest groundwater sources have already passed their sustainability tipping points.  

“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 Richey, the lead author on both studies, in a statement.

The Arabian Aquifer System, for example, is used by many people in Iraq, Saudi Arabia and Yemen, where large swaths are currently undergoing armed conflict.

“We’re trying to raise red flags now to pinpoint where active management today could protect future lives and livelihoods,” said Richey.

This growing problem is best illustrated in California, which is currently experiencing a brutal drought. The study suggested the farm-rich Central Valley, utilized heavily for agriculture, was highly stressed. While underground aquifers supply humans with 35 percent of water used worldwide, explains Todd Frankel in The Washington Post, this demand increases during droughts, putting greater pressure on underground aquifers. In California, Frankel reports, the dependency on aquifers has risen from 40% to 60%, and some experts warn that the number is set to rise.

“As we’re seeing in California right now, we rely much more heavily on groundwater during drought,” said UCI professor and principal investigator Jay Famiglietti. “When examining the sustainability of a region’s water resources, we absolutely must account for that dependence.”

Researchers warn the dearth of groundwater has led to ecological damage, including depleted rivers, declining water quality and subsiding land, which will only be exacerbated by climate change and population growth.

“Given how quickly we are consuming the world’s groundwater reserves, we need a coordinated global effort to determine how much is left,” Famiglietti added.