While rivers and lakes dry up in the west, due to years of drought, their loss is easy to measure—not only is the loss visible, but aboveground reservoirs are regulated by the federal government. Sadly, this isn’t the case for groundwater. Use of water in underground aquifers often isn’t regulated and can be difficult to calculate.

The drought-stricken Colorado River Basin has experienced the driest 14-year period in the last hundred years. Given the water shortage, scientists wondered how much groundwater usage there was throughout that period. The basin supplies water to about 40 million people in seven states and Mexico, in addition to irrigating roughly four million acres of farmland.

To look underground, the team took to the skies. They used data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite mission to track changes in the mass of the basin, which changes with the amount of water on and below the surface. Monthly measurements of the change in water mass from December 2004 to November 2013 revealed the basin lost nearly 53 million acre feet (65 cubic kilometers) of freshwater. More than three-quarters of the total—about 41 million acre feet (50 cubic kilometers)—was from groundwater.

GRACE is like having a giant scale in the sky, says Jay Famiglietti, of NASA/JPL and UC Irvine. Within a given region, the change in mass due to rising or falling water reserves influences the strength of the local gravitational attraction. By periodically measuring gravity regionally, GRACE reveals how much a region’s water storage changes over time.

“The Colorado River Basin is the water lifeline of the western United States,” says Famiglietti. “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.”

“We don’t know exactly how much groundwater we have left, so we don't know when we're going to run out," says Stephanie Castle, a water resources specialist at UC Irvine. “This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking.”

Famiglietti notes that the rapid depletion rate will compound the problem of short supply by leading to further declines in streamflow in the Colorado River.

Perhaps in the short term, more regulation at the state level can help reduce groundwater depletion. According to an article in Science, in California:

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.

Castle’s and Famiglietti’s work was published last week in Geophysical Research Letters.

Image: U.S. Bureau of Reclamation

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