Throughout the western United States, a network of Global Positioning System (GPS) stations has been monitoring tiny movements in the Earth’s crust, collecting data that can warn of developing earthquakes.
To their surprise, researchers have discovered that the GPS network has also been recording an entirely different phenomenon: the massive drying of the landscape caused by the drought that has intensified over much of the region since last year.
Geophysicist Adrian Borsa of the Scripps Institution of Oceanography and his colleagues report in this week’s Science that, based on the GPS measurements, the loss of water from lakes, streams, snowpack, and groundwater totals some 240 billion metric tons—equivalent, they say, to a four-inch-deep layer of water covering the entire western U.S. from the Rockies to the Pacific. (Related: “Water’s Hidden Crisis”
The principle behind the new measurements is simple. The weight of surface water and groundwater deforms Earth’s elastic crust, much as a sleeper’s body deforms a mattress. Remove the water, and the crust rebounds.
As the amount of water varies cyclically with the seasons, the crust moves up and down imperceptibly, by fractions of an inch—but GPS can measure such small shifts.
Borsa knew all this when he started to study the GPS data. He wasn’t interested in the water cycle at first, and for him the seasonal fluctuations it produced in the data were just noise: They obscured the much longer-term geological changes he wanted to study, such as the rise of mountain ranges.
When he removed that noise from some recent station data, however, he noticed what he describes as a “tremendous uplift signal”—a distinct rise in the crust—since the beginning of 2013. He showed his findings to his Scripps colleague Duncan Agnew.
“I told him, ‘I think we’re looking at the effect of drought,'” Borsa remembers. “He didn’t believe me.”
But Borsa was right. As he, Agnew, and Daniel Cayan of Scripps report in Science, the recent uplift spike is consistent across the U.S. West, and consistent with recent declines in precipitation, streamflow, and groundwater levels. With a great weight of water removed, the crust is rebounding elastically across the whole region.
The median rise across all the western GPS stations has been four millimeters, just under a sixth of an inch. But the Sierra Nevada mountains, which have lost most of their snowpack, have risen 15 millimeters—nearly six-tenths of an inch.