Estimation of Watershed Scale Soil Moisture from Point Measurements in SMEX02

Understanding watershed scale soil moisture distributions is necessary to validate current remote sensing, such as the Advanced Microwave Scanning Radiometer (AMSR). Unfortunately, remote sensing technology does not currently resolve the land surface at a scale that can be easily validated with ground observations. One method of validation uses existing soil moisture measurement networks and scales up to the resolution of these remote sensing footprints. Soil Moisture Experiment 2002 (SMEX02) was an excellent opportunity to implement one such soil moisture gaging system which, when calibrated, provided robust estimates of the watershed scale soil moisture throughout the summer of 2002. Twelve fields distributed across the Walnut Creek watershed were instrumented with in situ soil moisture probes and were intensively sampled during the experiment, between June 25 and July 12, 2002. The sampling sites were analyzed for temporal stability and scaling relationships were developed. These point measurements were scaled up to the field scale (~ 800 m) and then to the watershed scale (~ 25 km) for the field experiment period and were shown to be accurate indicators of the large-scale soil moisture distribution. Point measurements were then used as a basis for a watershed estimate for several months beyond SMEX02, thereby providing a long record of watershed scale soil moisture which can be used for validation. The ability to estimate the soil moisture is measured by a variety of techniques, including split sample verification. This analysis is a first step in the implementation of large-scale soil moisture validation utilizing networks such as the Soil Climate Analysis Network (SCAN) as a basis for calibrating soil moisture satellite products.