More than a third of the world's biggest aquifers, a vital source of fresh water for millions, are "in distress" because human activities are draining them, according to satellite observations.
Scientists from Nasa, the US space agency, and the University of California, Irvine, analysed 10 years of data from the twin Grace satellites, which measure changes in groundwater reserves by the way they affect Earth's gravitational pull.
"Twenty-one of the world's 37 biggest aquifers have passed sustainability tipping points . . . they are being depleted," said Jay Famiglietti, the study leader.
"Over a third  are so bad that they are experiencing exceptionally high levels of stress."
The problem is most serious in regions where rainfall and snowmelt cannot make up for water extracted for agriculture, industry, drinking and other human purposes.
The scientists determined aquifers' overall stress rates on the basis of their depletion over 10 years of satellite measurements, together with their potential for replenishment, taking account of regional climate and human activities.
The results, published in the Water Resources Research journal, show that the Arabian Aquifer System, an important water source for more than 60 million people, is the most "overstressed" in the world.
It is followed by the Indus Basin aquifer of India and Pakistan and the Murzuq-Djado Basin in northern Africa. California's Central Valley, currently at the center of a political battle over water rights, was classed as "highly stressed" and suffering rapid depletion — mainly for agriculture.
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Although many of the world's great aquifers are being drained rapidly, there is "little to no accurate data about how much water remains in them," the researchers added.
Professor Famiglietti said: "Available physical and chemical measurements are simply insufficient. Given how quickly we are consuming the world's groundwater reserves, we need a co-ordinated global effort to determine how much is left."
By comparing their satellite-derived groundwater loss rates to the limited data on groundwater availability, the researchers found huge discrepancies in projected times to total depletion of the aquifers.
In the Northwest Sahara Aquifer System, for example, such times fluctuated between 10 and 21,000 years. The study noted that a dearth of groundwater was leading to severe ecological damage, including rivers running dry, water quality deteriorating and land subsiding.
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Groundwater aquifers are located in soil or deeper rock layers beneath the earth's surface. Drilling down to discover the extent of an aquifer can be difficult and expensive but there is often no other option, according to the authors.
"We need to explore the world's aquifers as if they had the same value as oil reserves," Prof Famiglietti said. "We need to drill for water the same way that we drill for other resources."