A third of the Earth's biggest groundwater basins are in trouble, because after significantly relying on this hidden resource for so long, they are being quickly tapped out, according to two new studies.

Groundwater comes from aquifers - sponge-like gravel and sand-filled underground reservoirs - that provide populations with freshwater to make up for surface water lost from drought-depleted lakes, rivers and reservoirs. With the ongoing drought in the western United States, particularly California, residents have been relying more heavily on groundwater for their water needs. And yet despite the fact that groundwater is disappearing, there is little to no accurate data about how much water remains in them - so we don't know when we'll run out.

"Available physical and chemical measurements are simply insufficient," lead author Jay Famiglietti, from the University of California, Irvine (UCI), said in a news release. "Given how quickly we are consuming the world's groundwater reserves, we need a coordinated global effort to determine how much is left."

So for the first time, two new studies, led by researchers at UCI using data from NASA Gravity Recovery and Climate Experiment satellites, aimed to find out. NASA's twin GRACE satellites measure dips and bumps in Earth's gravity, which is affected by the weight of water.

In the first study, published in the journal Water Resources Research, researchers examined the world's 37 largest aquifers between 2003 and 2013. Those worst off (eight total) were classified as "overstressed," meaning they were barely being replenished for the water taken out of them. Another five aquifers were found, in descending order, to be extremely or highly stressed, depending upon the level of replenishment in each. But even with some water flowing back into them, scientists are still concerned.

The most distressed basins are in the world's driest areas, which draw heavily on underground water. And the problem is only expected to worsen due to climate change and population growth.

"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 the lead author on both studies, Alexandra Richey. "We're trying to raise red flags now to pinpoint where active management today could protect future lives and livelihoods."

So which of the world's groundwater basins are most at risk of drying up?

The research team - which included co-authors from NASA, the National Center for Atmospheric Research, National Taiwan University and UC Santa Barbara - found that the Arabian Aquifer System, which supplies water to more than 60 million people, is the most overstressed in the planet. (Scroll to read on...)

Meanwhile, the Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, and the Murzuk-Djado Basin in northern Africa is third.

But California - which is experiencing the worst drought in over a millennium - is not far behind.

California's Central Valley, utilized heavily for agriculture and suffering rapid depletion, was slightly better off but still labeled highly stressed in the first study.

"As we're seeing in California right now, we rely much more heavily on groundwater during drought," Famiglietti said. "When examining the sustainability of a region's water resources, we absolutely must account for that dependence."

Unfortunately, according to the second study, we still don't have an accurate estimate of how much groundwater the world has left. Previous estimates were made decades ago, but researchers expect that now that number is significantly lower.

The little data that does exist, in comparison with the latest satellite-derived groundwater loss rates, shows that there are some major discrepancies. Specifically, in terms of so-called "time to depletion" - how long we have until the basins are empty.

In the overstressed Northwest Sahara Aquifer System, for example, this fluctuated between 10 and 21,000 years.

"We don't actually know how much is stored in each of these aquifers. Estimates of remaining storage might vary from decades to millennia," Richey said. "In a water-scarce society, we can no longer tolerate this level of uncertainty, especially since groundwater is disappearing so rapidly."

Not to mention that disappearing groundwater is already causing ecological damage, including depleted rivers, declining water quality and subsiding land.

Groundwater aquifers are typically located in soil or deeper rock layers beneath Earth's surface. But they are often so deep and thick that it's difficult - and costly - to drill to or otherwise reach bedrock and learn where the moisture bottoms out. And despite both studies' findings and the stress humanity is putting on these groundwater basins, researchers still say that drilling them is necessary.

"I believe we need to explore the world's aquifers as if they had the same value as oil reserves," Famiglietti concluded. "We need to drill for water the same way that we drill for other resources."

At least in the United States, groundwater can still be drilled as often as necessary because there are currently no statewide regulations limiting groundwater use. But this and other research shows that soon or later, if nothing is done, we are going to run out.

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