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New data from NASA's GRACE satellites show that many of the world's biggest aquifers are being sucked dry at a rate far greater than they are being replenished. Although scientists don't know how much water is left, they hope their findings will serve as a "red flag" for regions that may be overusing water.

Globally, scientists estimate that roughly 2 billion people rely on water supplied from underground aquifers as their main source of freshwater. Groundwater is also used for farming, especially during times of drought.

GRACE measures water under the ground by measuring its gravitational tug, and it watched 37 aquifers around the world over the course of 10 years to see what was happening to the water they held.

The results are not entirely surprising.

Groundwater is being rapidly depleted in the Middle East, North Africa and Central Asia. The aquifer under California's drought-stricken Central Valley is also being overused. Other places, such as the sparsely populated and very wet Amazon Basin and the U.S. Great Plains, where crops are grown using mainly rainwater, fared better.

Researchers at the University of California, Irvine, which led the study, say that, while the data provide reliable estimates of use, they can't say for sure how much water is left in the aquifers. Nevertheless, they fear that depleting the underground water supply could increase the risk of conflict around the world.

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