Bottom line: the Earth is running out of water.

Two new NASA studies led by researchers from the University of California Irvine and published Tuesday show that the depletion of global groundwater resources, due to the dueling impacts of global warming and growing human demand, has caused the world's water supply to drop to dangerous levels.

The first report compares statistical analysis of water withdrawal to GRACE satellite analysis, which measures variations in gravity on the Earth's surface, between January 2003 and December 2013. The study compares the difference between the use and availability of these resources to determine the amount of overall renewable groundwater stress, or RGS.

According to the findings, at 21 of the 37 largest aquifers, water is being drained at a greater rate than it is being naturally replenished, 13 of which fell into the most troubled category.

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In the United States, the Central Valley aquifer in California—a region known as much for its heavy agriculture as for its ongoing record drought—falls into this group.

"The water table is dropping all over the world," said Jay Famiglietti, senior water scientist at NASA’s Jet Propulsion Laboratory, who took part in the research. "There’s not an infinite supply of water."

The second study examines total groundwater storage capacity and found that many estimates are outdated and may even be smaller than previously thought.

Whereas previous definitions of water stress do not account for groundwater as a water supply source, the researchers explain that groundwater is now "increasingly relied upon during times of drought as a resilient water supply source." Further, they state, "Groundwater is currently the primary source of freshwater for approximately two billion people."

The researchers warn that as water resources are strapped to meet future demands "due to population growth and climate change"—both of which, they note, may alter the distribution of available freshwater— "the global population without access to potable water will likely increase."

"We need to get our heads together on how we manage groundwater," Famiglietti added, "because we’re running out of it."