Measuring soil moisture and snow depth

A collaboration of scientists that includes UCAR's John Braun is pioneering a new technique for using GPS satellite signals to measure snow depth as well as soil and vegetation moisture. The team, which includes researchers from CU-Boulder and NOAA, expects the technique to benefit meteorologists, climate modelers, water resource managers, and farmers.

"Water scarcity is going to be a problem for the western United States in the coming century," says Braun, one of the project's principal investigators. "Having improved observations of water in all of its phases is going to be an important step as we monitor changes in the environment, which is the most intriguing part of this project for me."

The new technique uses interference patterns created when GPS signals arriving at antennas directly from satellites are combined with "multipath" signals, which reflect off the ground. Multipath signals have been viewed largely as noise by scientists. However, the researchers have been able to successfully correlate changes in these signals to snow depth, using data collected at NCAR's Marshall Field Site during two large snowstorms in spring of 2009. They are now collaborating at the Marshall site with another soil moisture project that uses deep space neutrons to measure moisture.

The team is also analyzing how GPS signals traveling through alfalfa, corn, and grass correlate with the amount of water in the vegetation at a farm outside Boulder. The data could give farmers a more comprehensive view of soil moisture than single sensors placed in fields.

The study is the first time that traditional GPS receivers, which were designed to measure plate tectonics and geological processes, have been used to assess snowpack, soil moisture, and vegetation moisture. Although the GPS technique samples at a more shallow depth than individual soil moisture sensors, it covers a larger footprint than in situ measurements.


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