Can We Use GPS as a Soil Moisture Instrument?

Soil moisture is fundamental to land surface hydrology, so global distribution and temporal variations of soil moisture are needed both for analyses and modeling purposes. To date, a global soil moisture dataset that fulfills the needs of the hydrology and climate communities does not exist: in situ measurements are sparse and useful satellite-derived methods are still being developed. Recently, it has been shown that high-precision GPS receivers can be used to estimate fluctuations in near surface soil moisture over an area of ~1000m². Given this sensitivity to soil moisture, some of the more than 1500 permanent and continuously operating GPS receivers that exist in the U.S. can be used to provide near-real time estimates of soil moisture for hydrology, climate, and ecology studies.

In order to use existing GPS stations as a continental-scale soil moisture network, the method must be developed further and errors must be evaluated to establish the underlying uncertainty in GPS soil moisture estimates. The research objectives are divided into two categories:

1. To quantify how various aspects of the physical environment influence soil moisture estimates derived from GPS data, including (a) vertical profiles of soil moisture and texture; (b) vegetation amount and structure; and (c) topography and surface roughness. The physical environment around each of the Earthscope Plate Boundary Observatory (PBO) sites will be evaluated for suitability for the GPS soil moisture technique.

2. To evaluate how antenna and receiver design/performance and satellite signals influence the soil moisture content time series derived from GPS signal-to-noise ratio (SNR) data. Because its network is large (1200 receivers), homogeneous, and well-maintained, the equipment used by PBO will be evaluated.

Results from our First Year of Work

Our GPS site at Marshall Mesa. Results reported in Geophysical Research Letters last December: Larson, K.M., E.E. Small, E. Gutmann, A. Bilich, J. Braun, V. Zavorotny, Use of GPS receivers as a soil moisture network for water cycle studies, Geophys. Res. Lett., 35, L24405, doi:10.1029/2008GL036013, 2008 (pdf).

Variation in VWC from multiple GPS satellites (colors represent different GPS satellites, which sense different portions of the ground) and water content reflectometers (WCR). The range of the five WCRs is shown in grey and their mean is the black line (Individual WCR records can be seen here). The daily precipitation totals are in blue. GPS measurements are only shown on days when there was no snow and the daily average temperature was above 3 C.

A longer time series for Marshall

We have two papers in press at IEEE JSTARS:

• Larson, K.M., J. Braun, E.E. Small, V. Zavorotny, E. Gutmann, and A. Bilich, Determination of GPS soil penetration depth and its relation to soil moisture.

• Zavorotny, V., K.M. Larson, J. Braun, E.E. Small, E. Gutmann, and A. Bilich, A physical model of GPS multipath caused by land reflections: toward soil moisture retrievals.

This material is based upon work supported by NSF ATM 0740515 (CU) and NSF ATM 0740498 (UCAR). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of NSF.