Groundwater Disappearing Much Faster Than Lake Mead in Colorado River Basin

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The steepest rates of water loss occurred in 2013, which followed one of the driest years on record.

The total decline in the Basin’s water reserves was 64.8 cubic kilometers (52.5 million acre-feet), with 50.1 cubic kilometers (40.6 million acre-feet) from groundwater. The capacity of Lake Mead, for comparison, is 32 cubic kilometers (26 million acre-feet). At least 70 percent of the Basin’s water use is for irrigated agriculture.

Chuck Collum, Colorado River manager at the Central Arizona Project, which delivers more than half of Arizona’s Colorado River allocation, said the study was helpful.

“The study is useful in using GRACE to verify at a coarse scale what water managers in the Basin know,” Cullom told Circle of Blue.

To check their conclusions, the researchers compared the satellite data against measurements taken from individual wells. Measurements from nine U.S. Geological Survey wells and 65 Arizona Department of Water Resources wells showed similar trends at the smaller scale.

“That gives us confidence in what GRACE is seeing,” Castle said.

Still, GRACE data has limits. For one, the satellites cannot tell what share of the groundwater decline comes from an increase in pumping versus a decrease in the recharge rate, the amount that flows into an aquifer. In a drought, pumping would be expected to go up and recharge down, Famiglietti said.

GRACE also has spatial constraints. The satellites work best for land areas larger than 200,000 square kilometers (77,220 square miles), roughly the size of Nebraska. The satellites cannot pick out hotspots of groundwater depletion within the states.

“It’s difficult to break GRACE data down farther than we have,” Famiglietti told Circle of Blue. “We need additional data on the ground to tell what is happening.”

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