By Rong-Gong Lin II

Los Angeles Times

A year and a half of drought has depleted 63 trillion gallons of water across the Western United States, according to a 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 the University of California, San Diego’s Scripps Institution of Oceanography and the U.S. Geological Survey.

The situation is worse under the snow-starved mountains of California, where the Earth rose 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, shows how a lack of rain and snow cuts 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.

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 under that soil is actually rising.

“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 to the average. That indicated groundwater was being replenished even in desert states such as Nevada and Utah.

The pattern reversed in 2013, when the Southwest started 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.”