GPS Network Weighs Drought in the U.S. West

A record-breaking drought has left California and most of the western United States parched, threatening crops and even some of the region's hydroelectric power.  Now a network of global positioning system stations scattered across the west is providing a new way to show just how dry it's become.

"The beauty of this is that, at a regional scale, you're able to put a number on how much water we've lost," says Daniel Cayan of the Scripps Institution of Oceanography at the University of California, San Diego.  Compared to the nine years before the drought, the new data show that the western United States has lost 240 gigatons of water, which is enough to flood the entire region in 10 centimeters of water.

NASA's GRACE satellites have provided the best data yet on the planet's total water storage, measuring how the changing water mass on and below the surface alters Earth's gravity.  The problem, however, is that they can only resolve differences across distances of several hundred kilometers.

The new measurements, which Cayan and his colleagues report this week in the journal Science, have a much better resolution—200-300 kilometers.  The researchers used more than 700 GPS stations that are part of the National Science Foundation's Plate Boundary Observatory to measure the rising and falling of Earth's surface due to the presence of water.  Whether in lakes or aquifers, water weighs down on Earth, causing the surface to sink ever so slightly.  During a drought, the lower water weight means the surface rises. Each station, which is anchored firmly into the ground, constantly receives signals from at least four GPS satellites overhead.  The signals contain time and position information of the satellites, and are then used to calculate the station's location to within 1-2 millimeters horizontally and 3-5 millimeters vertically.

GPS Network Weighs Drought in the U.S. West