Many of Earth’s groundwater basins are drying out

Around the world, large reservoirs of fresh water lie hidden underground. These groundwater basins are like banks. Water can be deposited, stored or withdrawn. 
Now, changes in climate and human water usage are emptying those reservoirs, a new study finds. And it's happening at an alarming rate. Of Earth’s 37 largest groundwater basins, 21 lose more water each year than they gain. Details appear in a paper to be published in Water Resources Research.

The conclusion is troubling. That’s according to study coauthor Sasha Richey. She is a hydrologist at Washington State University in Pullman. (Hydrology is the study of Earth’s water.) Groundwater quenches the thirst of about 2 billion people. It also irrigates crops.

“People need to think about groundwater as an important resource,” Richey says. “We’re not managing that resource adequately, or even at all, in most of the world.”

People extract groundwater by drilling into underground reservoirs called aquifers. Aquifers are refilled when water seeps down through the soil.

Scientists can monitor groundwater using wells. Water levels drop as an aquifer is drained. This method fails to provide a global picture of changes in water levels, though.

Richey and colleagues instead used data collected by the GRACE mission. These twin satellites measure small changes in Earth’s gravity. Variations in the density of Earth's surface cause those small variations in gravity. (Density is a measure of how much mass is contained in a given volume.)

The emptying and refilling of groundwater basins is one way that the density of Earth’s surface changes. The GRACE satellites pass over these buried reservoirs regularly. As they do, the satellites “weigh” the mass of the water stored inside.

The researchers examined gravity changes over Earth’s largest aquifers from 2003 through 2013. Eight of the studied aquifers lost significant volumes of water over that decade. The researchers classified these aquifers as “overstressed.” That means almost no water naturally trickled in to replace water being pumped out. The regions of greatest concern were in the Middle East, northern Africa and northwestern India and Pakistan.

The most dried-up aquifers were in areas near large cities, in heavily agricultural areas or in arid climates. All three characteristics probably contributed to the extreme stress affecting the basins below central California, Richey says. California has both people and farms. It’s also in a multi-year drought. As a result, the state’s pumping of groundwater recently has skyrocketed.

The GRACE mission provides valuable information about how global groundwater has changed. It can’t measure exactly how much water is left in the aquifers, however, says Gordon Grant. He is a hydrologist with the U.S. Forest Service in Corvallis, Ore. Still, the new work allows scientists to "better understand, like an accountant would, withdrawals and deposits of groundwater around the world."

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