A network of GPS stations in the western United States is revealing the severity of the drought in that region.
Compared to the nine years before the drought, the GPS 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.
Investigating ground positioning data from GPS stations throughout the west, researchers at the Scripps Institution of Oceanography at the University of California, San Diego, found that the water shortage is causing an uplift effect up to 15 millimeters (more than half an inch) in California’s mountains and on average four millimeters (0.15 of an inch) across the west.
Results of the study, which was supported by the U.S. Geological Survey (USGS), appear in the August 21 online edition of the journal Science.
The measurements have a much better resolution — 200-300 kilometers — than data provided by NASA’s GRACE satellites, which shows how the water mass has changed on and below the Earth’s surface.
Researchers Adrian Borsa, Duncan Agnew, and Dan Cayan used data from more than 700 GPS stations in the National Science Foundation’s Plate Boundary Observatory to measure the rising and falling of Earth’s surface due to the presence of water. Water in lakes or aquifers weighs down on the Earth causing the surface to sink, while during a drought the surface rises. Each station provides time and position information that is used to calculate the station’s location to within 1-2 millimeters horizontally and 3-5 millimeters vertically.
The current drought began in 2012, when data shows that in general, the ground rose by about four millimeters. The rise in the Sierra Nevada mountain range was 15 millimeters.
While poring through various sets of data of ground positions from highly precise GPS stations within the National Science Foundation’s Plate Boundary Observatory and other networks, Borsa, a Scripps assistant research geophysicist, kept noticing the same pattern over the 2003-2014 period: All of the stations moved upwards in the most recent years, coinciding with the timing of the current drought.
Agnew, a Scripps Oceanography geophysics professor who specializes in studying earthquakes and their impact on shaping the earth’s crust, says the GPS data can only be explained by rapid uplift of the tectonic plate upon which the western U.S. rests (Agnew cautions that the uplift has virtually no effect on the San Andreas fault and therefore does not increase the risk of earthquakes).
For Cayan, a research meteorologist with Scripps and USGS, the results paint a new picture of the dire hydrological state of the west.
“These results quantify the amount of water mass lost in the past few years,” Cayan said. “It also represents a powerful new way to track water resources over a very large landscape. We can home in on the Sierra Nevada mountains and critical California snowpack. These results demonstrate that this technique can be used to study changes in fresh water stocks in other regions around the world, if they have a network of GPS sensors.”
The study was supported by USGS National Earthquake Hazards Reduction Program.