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.
The GPS network was originally designed to monitor the motion of tectonic plates, says Adrian Borsa of the University of California, San Diego, who led the study. And at first, that's what he was interested in. But when the researchers looked at their data, something else caught their eyes.
"We discovered this whopping signal that started at the end of 2012 and has continued to the present," Borsa says. "That's the signature of this current drought." The data showed that in general, the ground rose by about four millimeters. "It doesn't sound like a lot, but to us, that's a huge, huge signal to see over that short period," he says.
The Sierra Nevada mountain range, which Cayan says is California's largest water source for domestic and agricultural purposes, normally sees the most precipitation and thus feels the effects of the drought most. And there, the rise was 15 millimeters. "That's stunning," Borsa says.
While the source of the data was unexpected, the results themselves are not. "This confirms what we've seen in other measures," Cayan says. What's special about the GPS measurements is that they give a quantitative, overall view of the water system at a high resolution. Hydrologists have accurate ways of measuring surface water like precipitation, stream flow, and snowpack, but determining the amount of groundwater is murkier, relying on models and estimates, he says.
"This drought signature is a very strong signal," Cayan says. "But there's other things that are buried in there, and it remains to be seen how much information can be extracted from this record."
There are GPS networks covering most of the United States and many other countries, mainly used for studying tectonic shifts and determining earthquake risk. Now, Borsa says, these networks can be used to study water systems as well.
"It does show a lot of promise," says Michael Anderson, state climatologist at the California Department of Water Resources, who was not involved in the study. But he adds that it'll take some time for hydrologists to figure out exactly how this new kind of data can inform water management.
According to Dennis Lettenmaier of the University of Washington, who also wasn't part of the team, one way the data could be used is to update and improve hydrology models that make predictions about the water supply. More work needs to be done, but these results represent a good poof-of-principle, he says. "It's intriguing and there's potential."