For the past 14 years, drought has afflicted the Colorado River Basin, and one of the most visible signs has been the white bathtub rings around the red rocks of Lake Mead and Lake Powell, the two biggest dammed lakes on the river. But there is also an invisible bathtub being emptied, belowground. A new study shows that ground water in the basin is being depleted six times faster than surface water. The groundwater losses, which take thousands of years to be recharged naturally, point to the unsustainability of exploding population centers and water-intensive agriculture in the basin, which includes most of Arizona and parts of Colorado, California, Nevada, Utah, New Mexico, and Wyoming.

The study is the first to identify groundwater depletion across the entire Colorado River Basin, and it brings attention to a neglected issue, says Leonard Konikow, a hydrogeologist emeritus at the U.S. Geological Survey in Reston, Virginia, who was not involved with the work. Because ground water feeds many of the streams and rivers in the area, Konikow predicts that more of them will run dry. He says water pumping costs will rise as farmers—who are the biggest users of ground water—have to drill deeper and deeper into aquifers. “It’s disconcerting,” Konikow says. “Boy, water managers gotta do something about this, because this can’t go on forever.”

To document the groundwater depletion, James Famiglietti, a hydrologist at the University of California, Irvine, and his colleagues relied on a pair of NASA satellites called the Gravity Recovery and Climate Experiment (GRACE). The instruments are sensitive to tiny variations in Earth’s gravity. They can be used to observe groundwater extraction, because when the mass of that water disappears, gravity in that area also drops.

In the 9 years from December 2004 to November 2013, ground water was lost at a rate of 5.6 cubic kilometers a year, the team reports online today in Geophysical Research Letters. That’s compared with a decline of 0.9 cubic kilometers per year from Lake Powell and Lake Mead, which contain 85% of the surface water in the basin.

Famiglietti says it makes sense that cities and farmers turn from surface water to ground water during drought. But he is surprised by the magnitude of the loss. The groundwater depletion rate is twice that in California’s Central Valley, another place famous for heavy groundwater use.

Regulation and monitoring of groundwater extraction are rare. The basin’s surface water is apportioned precisely under the Colorado River Compact, a 1922 agreement among seven states. In contrast, groundwater extraction is often the local right of the landowner. “If you own the property, you can drill a well and pump as much as you want,” Famiglietti says. “That’s just the way it is.”

A few states in the western United States are changing their approach. In 1980, Arizona passed the Groundwater Management Act, which created five tightly regulated basins and limits groundwater pumping. The law was progressive for its time, says Rita Maguire, a lawyer in Phoenix specializing in water law and the former director of the Arizona Department of Water Resources. In California, a state with little oversight of groundwater use, change is also afoot. Governor Jerry Brown has called for a crackdown on excessive withdrawals, and legislators have proposed bills that would give the state more authority to monitor and regulate groundwater withdrawal. But with so many livelihoods depending on unfettered access to ground water, change will come slowly, Maguire says. “It’s like turning the Queen Mary,” she says. “It’s a big deal and it takes a long time.”