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Two new UC Irvine studies have bad news about the world's water supply: Several of earth's underground water sources won't be able to self-replenish, and we don't know when we'll run them completely dry.

The first report, "Quantifying renewable groundwater stress with GRACE" uses information gathered by NASA's Gravity Recovery And Climate Experiment (GRACE) satellites to determine that of the world's 37 biggest aquifers, 13 are in bad shape—between 2003 and 2013, NASA writes, those aquifers "were being depleted while receiving little to no recharge."

Eight of the affected aquifers are considered to be "overstressed," or "active depletion" of groundwater. Five got the slightly better prognosis of "extremely" or "highly stressed." That means water is still flowing into the aquifier, but too slowly.

In the first paper, authors note that groundwater is not traditionally considered to be the main source of water worldwide — but that it's something we need to be cognizant of as droughts get worse (emphasis ours):

Surface water is the principal freshwater supply appropriated to meet human water demand globally, but the importance of groundwater is increasing as surface supplies become less reliable and predictable… and groundwater is increasingly relied upon during times of drought as a resilient water supply source… Groundwater is currently the primary source of freshwater for approximately two billion people.

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Of the eight overstressed water sources, the Arabian Aquifer System is in the worst shape. That system is the main source of water for 60 million people.

UC Irvine / NASA

In the second paper, researchers said that though we know the water sources are depleting, we don't know at what rate. Lead author Alexandra Richey said in a release: "We don’t actually know how much is stored in each of these aquifers. Estimates of remaining storage might vary from decades to millennia."

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Still, regardless of what is known about the shrinking aquifers, people are feeling the pressure of the diminishing water supply. Fusion's Tim Pool took a look inside the largest desalination plant in the Western Hemisphere, where researchers are trying to figure out ways to fight California's drought:

To find out more about the aquifers, scientists would have to drill through bedrock—a hard, expensive task. But sounds like it's worth the trouble.

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Danielle Wiener-Bronner is a news reporter.