GPS Offers New Way to Measure Drought

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GPS Offers New Way to Measure Drought

ResearchersHave a Fuller Picture of the Extent of Damage Caused by Drought, byUsing GPSMeasurements

Using existingGPSmeasurements of ground uplift in the western U.S. from 2003 to 2014, researchers found that the land surface has risen approximately 0.15 inches (4 millimeters), and as much as 0.6 inches (15 mm) in California's mountains.

The total water loss across the region is about 240 gigatons, which is equivalent to the amount of mass the Greenland Ice Sheet loses every year, according to the study in the journal Science.

Hydrologists canmeasure precipitationand the amount of surface water in streams and reservoirs, but measures of ground water are few and far between, Borsa told Live Science. "What's going on in aquifers is completely unknown."

To remedy the situation, Borsa and his colleagues set out to repurpose existing data from the Plate Boundary Observatory, a network of GPS stations throughout the western U.S. that were built to measure the movement of tectonic plates. The team realized that the same measurements could be used to determine how much water is in the ground, as a gauge ofdrought.

Water trapped in the ground pushes down on soil, but when water near the surface evaporates, the ground springs upward. "It's the same as if you had a block of rubber and you pushed down on it with your finger," Borsa said. "If you take your finger off, it comes back up." However, the movements of the ground are very subtle, so they are only visible with GPS.

The monitoring stations can measure both vertical and horizontal motion, but the vertical motion is what corresponds to the amount of water locked in the ground. Earthquakes primarily cause horizontal movements, unless they are very large, but the western United States did not experience a major quake during the decade studied, Borsa said. Volcanic activity, such as the Yellowstone hotspot (a volcanic region above a superheated patch of Earth's crust that feeds volcanoes in Oregon, Nevada, Idaho and Wyoming) can also cause vertical motion, so the researchers removed those regions from their data.

The GPS measurements show that the crust in California was actually subsiding in 2011, but started to rise in 2013, coinciding with the recentCalifornia drought.

Source: LiveScience

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