From the Arabian Peninsula to northern India to California’s Central Valley, nearly a third of the world’s 37 largest aquifers are being drained faster than they are being replenished, according to a recent study led by scientists at the University of California, Irvine. The aquifers are concentrated in food-producing regions that support up to two billion people.

A companion study indicates that the total amount of water in the aquifers, and how long it will last at current depletion rates, is still uncertain. “In most cases, we do not know how much groundwater exists in storage” to cover unsustainable pumping, the study said. Historical estimates, it argues, probably have unrealistically overstated total groundwater volume.

“We’re depleting one third or more of the world’s major aquifers at a pretty rapid clip,” said Jay S. Famiglietti, a professor of earth system science at the University of California, Irvine, and a leading researcher for the two studies. “And there’s not as much water there as we think.”

Dr. Famiglietti and his colleagues found that eight to 11 of 37 major world aquifers are overstressed, meaning they are losing much more water than man or nature returns to them.