Initial Validation of the Soil Moisture Active Passive Mission Using USDA-ARS Watersheds.

The Soil Moisture Active Passive (SMAP) Mission was launched in January 2015 to measure global surface soil moisture. The calibration and validation program of SMAP relies upon an international cooperative of in situ networks to provide ground truth references across a variety of landscapes. The USDA Agricultural Research Service operates several experimental watersheds as part of this project with distributed soil moisture networks, measuring soil moisture at a 5 cm depth. Comparisons of the in situ networks to the satellite products are ongoing, but initial results have shown a good correspondence between satellite estimates and the in situ soil moisture resources, once scaling functions are applied. The satellite products available are reviewed and the scaling methodologies for the in situ networks are discussed, including lessons learned. Results from the Little Washita, Fort Cobb, St. Joseph’s and Little River Experimental Watersheds show a good correspondence between the satellite products and in situ estimates. Reynolds Creek and Walnut Gulch show high accuracy, though with the caveat that these domains are semi-arid with small dynamic range. The South Fork Watershed is examined more closely for its detailed scaling function development and some apparent bias between the satellite and in situ estimates.