Why pumping groundwater isn’t a long-term solution to drought

SIGNS OF DROUGHT proliferate across the American West. California is rationing water for farmers in the state’s Central Valley. Salmon are dying en masse in the Pacific Northwest as river temperatures climb. Lake Mead, on the border of Nevada and Arizona, is drying up. The country’s largest reservoir is so depleted that the Bureau of Reclamation, an agency within the Interior Department, declared the first-ever water shortage for the Colorado River on August 16th. Facing cuts to their supplies of surface water, some farmers in the region are pumping more groundwater. Is pumping a sustainable way to weather the drought?

Groundwater is stored in aquifers (bodies of porous rock) that can be tapped by wells and used for drinking water or irrigated agriculture. Groundwater is the source of drinking water for half of Americans, and nearly all of the country’s rural communities. Worldwide, about 70% of the groundwater pumped is used for agriculture. But groundwater has become dangerously depleted in places where pumping has exceeded the rate at which aquifers are naturally replenished.

An analysis from the United States Geological Survey in 2013 found that between 1900 and 2008, groundwater was depleted by 1,000 cubic kilometres nationwide, or about twice the volume of Lake Erie, one of North America’s Great Lakes. A quarter of that was lost after 2000. The regions where depletion was most severe were the high plains, the south-west and the Gulf coast. America isn’t alone. More than half of the world’s population lives in countries where aquifers are overpumped for crop irrigation. Groundwater depletion can also cause the earth to slowly sink, a phenomenon known as land subsidence. As the ground settles, aquifers lose some of their storage capacity. In coastal regions subsidence can also contribute to rising sea levels.

In America, states eventually began to recognise overpumping and legislate against it. Arizona passed a law regulating groundwater use in 1980. To its chagrin, California’s state government didn’t do so until 2014. In many cases, farmers could replace their groundwater use with water from rivers. A massive, costly aqueduct carrying Colorado River water to cities and farms in thirsty central Arizona was completed in 1993. But rather than encouraging judicious use of water, says Newsha Ajami, a hydrologist at Stanford University, the added supplies helped farmers in California increase their acreage. “Eventually it became, oh, we have groundwater. And on top of that, we have this surface water that’s coming. So let’s just make the best out of these two resources”, she says.

Now the tables have turned. Reservoirs across the region are drying up. Farmers in Arizona will be among the first to see cuts to their share of Colorado River water in January. Some may return to pumping to keep yields high. Many rural areas in the Grand Canyon State aren’t covered by the law from 1980, which focused on cities. Policymakers are exploring ways to more quickly replenish groundwater, known as “artificial recharge”. But that takes time. Meanwhile, the West keeps on getting drier.