The world's aquifers are draining. Groundwater worldwide is being pumped away faster than it can be replenished -- at rates that have more than doubled since the 1960s, according to a new global study of groundwater.
It is mostly agricultural irrigation that is driving the increase, because it accounts for 70 to 80 percent of global groundwater usage.
What's more, the vast majority of the pumped groundwater is ending up in the ocean, where it is a major contributor to the world's rising sea level.
Groundwater usage since the 1960s has more than doubled, the new study says. Already by the 1960s, people were pumping groundwater faster than it could be replenished by water soaking back into the ground.
Since then, the degree to which pumping is outpacing groundwater recharge has also more than doubled, according to the findings.
A team led by Marc Bierkens of the Utrecht University and the International Groundwater Resources Assessment Center at Deltares in the Netherlands used a model of water flow across the landscape to estimate how much water soaks into, runs off of or evaporates from parcels of land across the globe.
They combined this with the best available information on groundwater usage to calculate the net amount of water leaving or returning to aquifers.
Certain areas emerged as hotspots for groundwater depletion: northeastern China, northwestern India, Iran, northeastern Pakistan, southeastern Spain, the central United States, California's Central Valley and Yemen, findings that are consistent with local studies of these regions, the authors said.
The team published their findings in Geophysical Research Letters.
If the water being pumped is not returning to the aquifers, where is it going? More than 95 percent ends up in the ocean, the authors say, where the quantity of groundwater reaching the sea is making an important contribution to sea level rise. The team concluded that groundwater has contributed 25 percent of the sea level rise observed since 2000.
While over a third of the world's population suffers from water stress, the most important consequences of overusing groundwater will be in agriculture, where most groundwater is used.
"A lot of this is abstracted by small farmers with small wells," Bierkens said. "It doesn't mean that the water is depleted immediately, but it could mean that the small farms wouldn't be able to reach it anymore."
"If you run out of water you cannot grow crops anymore," he added. "You are basically extending your crop production on borrowed water."
Declines in food production could affect survival directly in developing areas. In developed areas, they would likely affect economic growth.
It's not just the hotspots that are affected, since people everywhere buy food from areas of water overuse. "The food that I eat is connected through global trade to groundwater depletion in another part of the world," Bierkens said. "It's not only a local problem, it's a global problem."
Slowing groundwater use will require using more sophisticated irrigation techniques that use less water, developing crop types that can survive on lower quantities of water, and redirecting water on the landscape so that a higher proportion soaks back in to replenish the groundwater.
"We've known that groundwater depletion is becoming more and more of an issue," said Matthew Rodell of NASA Goddard Space Flight Center in Greenbelt, Md., who has used satellite data to come to similar conclusions about groundwater usage. "This paper does a nice job of trying to quantify that on a global scale."
"People need to take a long-term perspective and not use more groundwater than is recharged," Bodell said.