Measuring Soil Moisture and Snow Depth with GPS
Both soil moisture and snow pack are important storage pools in the hydrologic cycle, but current measurements of both are sparse. A new technique shows great promise for creating a network of both measurements. This technique uses the signal to noise ratio (SNR) recorded by high precision GPS receivers, and these receivers are already in place for geodetic applications. Some of the noise in a GPS signal comes from reflections of the GPS signal off of the ground or other surrounding objects. These reflected signals come in and out of phase with the direct signal as a GPS satellite rises, and as a result, the recorded SNR data rise and fall over this some time period, creating a sine curve. The shape of this SNR sine curve (primarily the phase and frequency) changes depending on the amount of snow on the ground, and the amount of water in the soil. By recording these changes over time, it is possible to develop a measure of soil moisture and snow depth.
Measurements of soil moisture from traditional soil moisture probes (grey) and from a GPS system (red)
Both soil moisture and snow depth are being examined at NCAR's Marshall test site, Niwot Ridge, and the Manitou Experimental Forest. To further expand the regions in which it is possible to apply this technique, data are being examined from the Manitou Experimental Forest, where the SNR data are complicated by GPS signal reflections/absorptions from the surrounding trees.
Measurements of snow depth from ultrasonic snow depth sensors (grey), manual observations (black) and multiple GPS satellites (colors)