Tracking the Effects of Drought in the Western U.S. with GPS

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The effects of the drought that has stricken the western United States can be measured with GPS.  As anyone who has lugged around a one gallon bottle of water knows, water is heavy.  Erosion is not the only way that rivers and lakes affect the landscape, now researchers from the Scripps Institution of Oceanography have discovered that subtle changes in the earth’s crust due to the decline in water volume in the western U.S. can be measured by GPS.

The weight of water bodies indent the earth’s crust, much like the indentation you see when you sit on a sofa.  As water is lost across the landscape due to the drought, the earth is experiencing an uplift.  Much  like what happens to the seat cushion of the sofa you just got off of, the earth’s crust is rising up.  The change is subtle but the high precision GPS network set up by the National Science Foundation’s Plate Boundary Observatory is able to capture this change.  Setup to monitor the earth’s movement in order to better understand geological processes including earthquakes, the GPS instrumentation within this network can detect sub-centimeter movements in the earth.


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Geophysicist Adrian Borsa discovered the correlation when he noticed that a significant rise in the earth’s crust that started at the beginning of 2013 which was also the start of the drought.  Sensing that there was more to this than mere coincidence, Borsa and the other researchers found that the uplift occurred across the regions in the Western United States afflicted by drought and in line with the recent declines in precipitation, snowpack, streamflow, and groundwater levels.  The uplift is more intense in California’s mountains where the snowpack was in steep decline as evidenced by the side-by-side satellite images NASA released earlier this year.  In the mountains, the uplift effect was 15 millimeters (a little over a half and inch) whereas the measured uplift in the rest of the western U.S. average four millimeters (0.15 of an inch).  

GPS sites showing the measured displacement.  As the drought deepens in March of 2014, the red dots showed the greatest uplift in the Sierra Mountains.  Source: Borsa et al.
GPS sites showing the measured displacement. As the drought deepens in March of 2014, the red dots showed the greatest uplift in the Sierra Mountains. Source: Borsa et al.

The researchers have estimated the water deficit to be 240 gigatons (63 trillion gallons of water), equivalent to a four-inch layer of water spread out over the entire western U.S.  The results of this research was published in the journal Science.  

This isn’t the only study that has used geospatial technologies to understand the impact of drought in the United States.  Scientists at NASA’s Goddard Space Flight Center have used remotely sensed data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites by measuring changes in the earth’s gravity to understand the effect of the drought on ground water.  

References

Ongoing drought-induced uplift in the western United States.  Adrian Antal Borsa, Duncan Carr Agnew, and Daniel R. Cayan.  Science 1260279Published online 21 August 2014 [DOI:10.1126/science.1260279]

Severe Drought is Causing the Western U.S. to Rise.  Scripps Institute.  August 21, 2014.

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