The study offers a bird's-eye view of groundwater in the Colorado River Basin, one of the most heavily used and closely watched water resources in the West. More than 40 million people rely on Colorado River water in the United States alone (water is also allocated to Mexico), the U.S. Geologic Survey estimates.
"We thought the picture could be pretty bad, but this is shocking," lead study author Stephanie Castle, a water resource specialist at the University of California, Irvine, said in a statement.
The Colorado River Basin stretches across seven states: from Wyoming across Colorado, Utah, Arizona, New Mexico, Nevada and California. Its groundwater is stored in underground aquifers and is sucked from the ground by wells. If water is removed from an aquifer faster than it can be replaced, eventually the wells will go dry.
Castle and her co-authors tracked groundwater loss in the basin with NASA's twin GRACE satellites (for Gravity Recovery and Climate Experiment). The satellites circle the Earth, monitoring the slight changes in Earth's gravity from increases or decreases in water. More water means more mass, which strengthens the pull of gravity on the satellites. (The satellites also track ice - their original mission was to monitor the planet's polar ice sheets and melting glaciers.)