Validation of AMSR-E Soil Moisture Products Using Watershed Networks

Validation is a challenging task for passive microwave remote sensing of soil moisture from Earth orbit. The key issue is spatial scale; conventional measurements of soil moisture are made at a point, whereas satellite sensors provide an integrated area/volume value for a much larger spatial extent. A robust validation program should include as many types of comparisons as possible and must attempt to provide actual spatially representative ground based soil moisture. A ground based validation program also requires a wide range of conditions, long temporal coverage and continuous observations. As part of the AMSR-E validation activity an augmented network of dedicated validation sites at actively monitored watersheds has been developed. These provide estimates of the average soil moisture over watersheds and surrounding areas that approximate the size of the AMSR-E footprint. This is done on a continuous basis, partially in real time. A public database of the watershed data for all sites is being developed and made available. To implement this network, additional surface soil moisture and temperature sensors (0-5 cm depth) were installed at and around existing instrument locations in four watersheds located in different climate regions of the U.S. Through short term and extended field campaigns the calibration of these instruments has been established. Methods for scaling from the point measurements to the integrated watershed/footprint average have also been developed as part of the validation effort. These efforts will be of value in alternate algorithm comparisons and will benefit future missions including SMOS.