The nation's largest water reservoir is the euphemistically named Lake Mead. More than a decade of drought has pushed its water level down to a height not seen since shortly after the Hoover Dam was built in the 1930s to hold the reservoir in place. Mineral-scorched rings more than 100 feet tall now encircle the rocks above the water's surface, highlighting the difference between water levels today and those of wetter times.
But to truly grasp the depth of the water crisis that's draining the Colorado River Basin, which provides water to more than 30 million people, you would need to peer even deeper than those rings—into the aquifers that are buried beneath parts of the snowpack-fed river.
Peering that deep into the Earth might sound like an impossible thing to do. But thanks to data produced by a NASA gravity-sensing satellite system, named GRACE, scientists have been able to do just that.
It's not known how much groundwater remains in the aquifers at the bottom of the basin. But wells that are being dug deeper than ever could eventually run dry.
By tracking changes in measurements of gravity beneath the two satellites, the researchers calculated that about about 65 cubic kilometers (km3) of water and moisture were lost from 2004 to 2013 from the basin, which begins in the Rocky Mountains and covers seven states plus parts of Mexico. They subtracted the amount of water that was skimmed from Lake Mead and the smaller Lake Powell during the same period. And they concluded that 50km3 were pumped out of the system's groundwater during the study period.
Those are all entirely abstract numbers. But 50km3 is equivalent to more than the water capacity of Lake Mead and Lake Powell combined.
Spikes in groundwater withdrawal closely followed the imposition of new water restrictions designed to preserve dwindling reservoir supplies. A brief period of recovery was detected from early 2009 to early 2010 amid moderately wetter conditions.
Stephanie Castle, a University of California-Irvine researcher who worked on the study, described the results, which were published online Thursday in Geophysical Research Letters, as alarming.
"We didn’t think it was going to be that bad," Castle says. "The reservoirs are being micromanaged right now because their levels are so low. Yet the groundwater is not being managed like the reservoirs."
It's not known how much groundwater remains in the aquifers at the bottom of the basin. But wells that are being dug deeper than ever could eventually run dry. That day might be looming ever closer as climate change continues to parch the American West and threatens to permanently reduce the snowpacks that feed the rivers that recharge the aquifers. And it could occur at the same time that pressure for that groundwater is swelling to make up for reservoir shortfalls.
"If the snowpack continues to decline, it's not going to just be our reservoirs that go dry," Castle says. "Should we manage it differently? Can we be more sustainable in times of drought? These are all questions for water managers."