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