Use of GPS Receivers as a Soil Moisture Network for Water Cycle Studies

Measurements of soil moisture at various spatial and temporal scales are needed to study the water and carbon cycles. While satellite missions have been planned to measure soil moisture at global scales, these missions also need ground-based soil moisture data to validate their observations and retrieval algorithms. It is demonstrated that signals routinely recorded by Global Positioning System (GPS) receivers installed to measure crustal deformation for geophysical studies could be used to provide a global network of soil moisture sensors. The sensitivity to soil moisture is seen in reflected GPS signals, which are quantified by using the GPS Signal to Noise Ratio (SNR) data. It is shown that these data are sensitive to soil moisture variations for areas of 1000 m2 horizontally and ~5 cm vertically. GPS data have been collected at multiple locations having different soil, vegetation, and surface roughness characteristics. Comparisons of GPS derived parameters to in-situ sensors collocated at these sites allow for a quantitative evaluation of the technique over a range of conditions. These results show that GPS techniques are suitable to measure soil moisture over a broad range of soil types with moderate vegetative canopies. A summary of results will be presented that outlines the capabilities and limitations of this methodology.