More than a half-dozen Western states that depend on the Colorado River Basin for the bulk of their water are drawing much more heavily from groundwater supplies than previously thought, according to a new study, which is the latest indication that the historic drought gripping much of the region is threatening their future access to water.
According to The Washington Post, over the past nine years the basin — covering Wyoming, Utah, California, Colorado, New Mexico, Arizona and Nevada — has lost some 65 cubic kilometers of fresh water, which is nearly double the volume of the country’s largest reservoir, Lake Mead. It was a figure that truly surprised the study’s authors, who used data from a NASA weather satellite to examine groundwater supplies.
“We don’t know exactly how much groundwater we have left, so we don’t know when we’re going to run out,” said Stephanie Castle, a water resources specialist at the University of California, Irvine, and the study’s lead author. “This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking.”
About two-thirds of water lost over the past near-decade came from underground supplies, instead of surface water, the Post reported, citing the study’s findings.
“We were shocked to see how much water was actually depleted underground,” Castle told the paper.
Although surface water in the Colorado River Basin is regulated closely by the U.S. Bureau of Reclamation, states regulate groundwater on their own. And some of them, such as California, do not have groundwater management regulations, though others, such as Arizona, have gone as far as transferring surface water from the Colorado River to underground aquifers for later use.
‘It can take hundreds of years to refill groundwater basins’
As noted by the Post:
The Bureau of Reclamation allocates water in strict proportions to each of the seven states within the basin, where 40 million people rely on the Colorado River.
Those allocations have gotten smaller as drought has swept the West over the past 14 years. Lake Mead is at its lowest level since it was created, after construction of the Hoover Dam in the 1930s, leaving a “bathtub ring” around the lake. Most years, every drop of water is pumped out of the Colorado River before it empties into the Gulf of California.
Yet, what surprised researchers in particular was how much groundwater had been accounting for the difference. Indeed, in excess of three-quarters of water lost over the past nine years came from underground. And groundwater does not replenish itself as quickly as surface water, which collects from rain and snow. The unexpected heavy use is straining resources that were already limited.
“You get a wet year, you get some precipitation, and those reservoirs can fill right back up,” Castle said. “It can take years, or hundreds of years, to refill groundwater basins.”
‘We really don’t know how much water is down there’
“The Colorado River Basin is the water lifeline of the western United States,” said senior author Jay Famiglietti, senior water cycle scientist at NASA’s Jet Propulsion Laboratory on leave from UC Irvine, where he is an Earth system science professor.
“With Lake Mead at its lowest level ever, we wanted to explore whether the basin, like most other regions around the world, was relying on groundwater to make up for the limited surface-water supply. We found a surprisingly high and long-term reliance on groundwater to bridge the gap between supply and demand,” he said.
The study’s authors concluded that federal officials allocated 30 percent more water from the Colorado River than was available; the gaps were then made up from groundwater.
Across the Western states, farmers and urban centers that rely on groundwater are already seeing their water tables decline, which indicates that supplies are running low.
“We really don’t know how much water is down there. We’ve already depleted a lot of it. There could be more, but when we have to start to dig deeper to access it, that’s a bad sign,” Castle said. “If [ground water basins] continue to be depleted, they don’t come back up.”