A year and a half of drought has depleted 63 trillion gallons of water across the Western United States, according to a new study that documents how the parched conditions are altering the landscape.
The loss of groundwater, as well as surface water such as reservoirs, has been so extreme that it lifted the West an average of one-sixth of an inch since 2013, according to researchers from UC San Diego's Scripps Institution of Oceanography and the U.S. Geological Survey.
The situation is even worse underneath the snow-starved mountains of California, where the Earth rose up three-fifths of an inch. Groundwater is very heavy, and its weight depresses the Earth's upper crust. Remove the weight, and the crust springs upward.
The study, published online Thursday by the journal Science, showed how a lack of rain and snow cut water levels first in the U.S. Southwest and Central and Southern California before spreading into Oregon and Washington state. Water naturally evaporates, is absorbed by plants and is pumped by humans, so levels go down if the water is not replenished.
"The thing that is exceptional about this drought is that it really covers the entire region" of the Western U.S., said Scripps assistant researcher Adrian Borsa, the study's lead author. "I can't tell you whether this is as big as earlier droughts, but I would say within the last 10 years, this is definitely an unprecedented change with this drought."
The lost water is equal to a 4-inch layer of water across the United States west of the Rocky Mountains, according to the study.
Scientists came to this conclusion by studying data collected from hundreds of GPS sensors across the Western U.S., installed primarily to detect small changes in the ground due to earthquakes.
But the GPS data can also be used to show very small changes in elevation.
The study specifically examined GPS stations on bedrock or very thin soil because it provides the most accurate measurement of groundwater loss, said Duncan Agnew, professor of geophysics at the Scripps Institution of Oceanography. Areas with thick soil, such as farms, can see the ground sinking as the soil dries out. But Agnew said the bedrock underneath that soil is actually rising.
The highest uplift of the Earth occurred in California's mountains because there is so much water below them, Agnew said. The uplift was less in Nevada and the Great Basin.
"You can only lose water where there's water to lose," Agnew said.
Scientists analyzed GPS data between 2003 and 2014. In spring 2011, the last rainy season in which Los Angeles received above-average rainfall, GPS sensors showed most of the Western United States sank by a few millimeters, compared with the average. That indicated groundwater was being replenished even in desert states like Nevada and Utah.
The pattern reverses in 2013, when the Southwest starts rising. The situation spread to the entire West by March 2014.
"By the time this spring has rolled around, it's everywhere, and it's much more dramatic," Borsa said.
The U.S. Drought Monitor has declared much of California to be in a state of "exceptional drought."
According to the Western Regional Climate Center in Reno, 2013 was California's driest in 119 years of records, and Los Angeles and other cities around the state recorded their lowest precipitation amounts for any calendar year.
"It calls attention to the severity of this drought," said Colin B. Amos, an assistant geology professor at Western Washington University, who was not affiliated with the study. "This is another piece of evidence that this drought is a big problem."
U.S. Geological Survey researcher Daniel Cayan co-wrote Thursday's study with Borsa and Agnew.
The findings come as California debates whether to impose groundwater management rules. It's the only Western state that does not regulate how much property owners can draw from their wells, and experts worry that without restrictions, the supply could be drained.
The new report also addresses an issue that has received attention in recent months: whether loss of groundwater could be triggering more earthquakes on the San Andreas fault.
These researchers didn't find cause for alarm. They said the difference in pressure on the fault due to shrinking groundwater levels over the last two years was very small compared with the seismic forces.
An earlier study in the journal Nature, which focused on a smaller area and a longer time period, suggested that more quakes were triggered in part by the pumping of groundwater in the Central Valley.
That report suggested that shrinking groundwater levels throughout California's modern history reduced the forces that kept this section of the San Andreas fault clamped together, leading to more small earthquakes during dry periods.