The earth’s biggest groundwater basins are being depleted far more quickly than previously believed, according to two new studies by the University of California, Irvine (UCI).
The studies used data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites gathered between 2003 and 2013 to examine the 37 largest aquifers on the planet. Twenty-one of those aquifers have exceeded their sustainability “tipping points,” the analysis revealed, meaning that more water is being removed than replaced from these vital underground reservoirs. What’s more, 13 of 37 aquifers fell at rates that put them into the most troubled category, meaning there is “nearly no natural replenishment” to restore water used by humans, according to a statement from UCI.
“The situation is quite critical,” lead investigator Dr. Jay Famiglietti, senior water scientist at NASA’s Jet Propulsion Laboratory, told the Washington Post.
Aquifers supply 35 percent of the freshwater used by humans worldwide, serving as the primary water source for more than 2 billion people. The water held underground in layers of rock and soil is an essential emergency supply to make up for surface water loss during droughts, when rivers and streams shrivel, as is the case today in California. These groundwater reserves take thousands of years to accumulate and only slowly replenish with water from snowmelt and rains. Deeper aquifers, however, contain ancient water locked in the earth by geological changes millions of years ago and thus typically cannot recharge — so once the water is gone, it’s gone for good.
As drilling for water has taken off across the globe — fueled by population growth, agriculture, and industries like mining and hydraulic fracturing (“fracking”) — reliance on these largely non-renewable water sources has soared, leading to what scientists describe as a growing “global water security crisis.”
“The water table is dropping all over the world,” Familglietti said. “There’s not an infinite supply of water.”
The world’s most overstressed groundwater source, according to the researchers, is the Arabian Aquifer System, which supplies water for more than 60 million people. The Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, and the Murzuk-Djado Basin in northern Africa is third.
In the United States, California’s Central Valley Aquifer was in the most trouble, with rapid depletion linked to agriculture and climate change. Now in its fourth year of exceptional drought, California is currently tapping aquifers for 60 percent of its water use, up from the usual 40 percent. Many communities rely on groundwater from aquifers exclusively for their water needs, according to the California Department of Water Resources, and scientists expect groundwater will account for virtually all of the state’s fresh water supply by the end of the year.
The findings are the first to characterize groundwater losses via data from space, using readings from NASA’s two GRACE satellites, which measure subtle variations in the earth’s gravitational pull. When an area either gains or loses a large volume of water, the change in mass allows the satellites pick up the difference in the gravitational pull. This lets researchers determine the rate at which large aquifers are being depleted.
Scientists warn that the situation would only worsen with climate change and population growth. The most overburdened aquifers are in the world’s driest places, where there is little natural replenishment.
Startlingly, although scientists are able to tell how quickly the water is being depleted, they do not know exactly how much is left. However, scientists say that some of these aquifers may be much smaller than previously thought. Only a few of the aquifers have been mapped in detail and most commonly accepted estimates of groundwater storage have little relationship to actual conditions, the researchers said. Estimates developed in 1969 and 1974, which are still cited today, assumed that each aquifer had the same characteristics, that the depth to water and the water-holding capacity of the soil were the same. They are not.
By altering the assumptions of those original studies, based on findings from regional groundwater studies — which are smaller in scope but have more detailed information on local conditions — the research team found a much emptier balance sheet, with “a lot less groundwater than we thought.” The regional estimates were between 10 and 1,000 times lower than the rough estimates from 40 years ago, an astonishing difference.
“We don’t actually know how much is stored in each of these aquifers. Estimates of remaining storage might vary from decades to millennia,” said co-author Alexandra Richey. “In a water-scarce society, we can no longer tolerate this level of uncertainty, especially since groundwater is disappearing so rapidly.”
Famiglietti compared the problem to someone writing checks on an account while not knowing how much money is in the bank. It would be possible to determine the present groundwater supply by drilling into the aquifers, Famiglietti said, but scientists currently lack the funding to do so. However, similar investigations have been undertaken to quantify the oil supply, he said, so it’s certainly possible — we just need to start placing the value of water on par with the earth’s wealth of minerals and fossil fuels.
“Given how quickly we are consuming the world’s groundwater reserves, we need a coordinated global effort to determine how much is left,” Famiglietti said in the UCI statement. He said he hoped the new findings would spark renewed interest in exploring what’s left of our underground water supplies.
“We need to get our heads together on how we manage groundwater,” he warned, “because we’re running out of it.”