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 2 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.”
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.
The new studies do not come as a surprise to hydrologists like Jerad Bales, chief scientist for water at the U.S. Geological Survey. But for him and other experts, an open question is whether the governments and individuals who control groundwater can or will work to gain more knowledge about the extent of the resource and how much use is sustainable.
Another question is whether those with responsibility for managing the aquifers will act to limit groundwater use, particularly if groundwater is essential to their livelihoods.
“We still have a ways to go in terms of learning how, and having the willpower, to manage our groundwater systems,” Bales said. “We need to think about it more. Water – people all over the world think, ‘If it’s under my property, it’s my resource.’ But it affects everybody.”
Pradeep Aggarwal, who leads the isotope hydrology division of the International Atomic Energy Agency in Vienna, said there was growing recognition of the extent of groundwater depletion but that the problem remains “an orphan.”
“Unless the government has an alternative to provide for their livelihoods, who is going to stop it?” A farmer will figure that “my livelihood depends on pumping that water – if I stop pumping it, my neighbor keeps pumping it.” The problem of groundwater depletion, Aggarwal said, cannot be solved by individuals. “This requires action on a larger scale.”
The stress on the most-used groundwater, measured over broad geographies by a NASA satellite that has provided 13 years of data, is a matter of real concern because, as the study said, “groundwater is currently the primary source of freshwater for approximately 2 billion people.”
Marc Bierkens, who holds a chair in earth surface hydrology at Utrecht University in the Netherlands, estimated that about 20 percent of the world’s population depended on crops irrigated by groundwater. “Humans are overexploiting groundwater in many large aquifers that are critical to agriculture, especially in Asia and North America,” a Bierkens study said.
Details about individual aquifers are hard to come by. The data from NASA’s twin Gravity Recovery and Climate Experiment (GRACE) satellites cannot show a level of detail below 150,000 square kilometers.
Famiglietti said that for the managers who have some control over the use of aquifers, the data from Grace is “too coarse” to provide useful data for local decisions. “They are waiting for us to do the research – we call it downscaling it to a resolution they can use, that makes it actionable for them,” he said.
The volume of water in 11 of the 37 aquifers studied has declined over more than a decade, according to the study, published in the journal Water Resources Research.
“Quantifying our understanding of how we use water in the world is very important, especially when the resource becomes limited,” Famiglietti said. “It’s important to understand where the big users are because that is key to affecting management in the future.”