Boaters launch their boats hundreds of yards away from designated boat ramps at Folsom Lake on Aug. 19, 2014, in Folsom, Calif. As the severe drought in California continues for a third straight year, water levels in the state's lakes and reservoirs are reaching historic lows. / Justin Sullivan, Getty Images
SAN DIEGO (AP) - About 63 trillion gallons of water have been lost to drought in the western United States, enough to blanket the region with 4 inches of water, according to a study published Thursday.
Researchers at the Scripps Institution of Oceanography at the University of California, San Diego, arrived at the conclusion by measuring the level of the earth's crust with a network of GPS stations that is normally used to predict earthquakes.
When water is lost because of a lack of rain and snow, the earth's crust rises. The sensors show that the earth's crust has risen an average of 4 millimeters in the western United States since last year and as much as 15 millimeters in the California mountains.
The earth's crust typically sags in the winter and spring, weighed down by water, and it rises during the dry season in summer and fall, said co-author Adrian Borsa. The authors removed those seasonal factors when analyzing about a decade of data from GPS stations within the National Science Foundation's Plate Boundary Observatory.
Last year, an area stretching west of the Rocky Mountains witnessed a "massive uplift," Borsa said. The rise was most striking in the Sierra Nevada mountains and California coastal regions, but it was spread over the entire region, unlike previous years when some pockets have gone up and others went down.
"It's just amazing to us that this covers the entire western United States," Borsa said.
The loss of water since last year is equivalent to the annual loss from the Greenland Ice Sheet, according to the study published in the journal Science
.The findings do not appear to raise any serious concerns about earthquake hazards, said Borsa, who hopes authorities use the measurements as a tool to measure the impact of drought. The findings cannot be compared to the severity of earlier droughts because the measurements were not used then.
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