Validation of a τ-ω model with Soil Moisture Active Passive Experiment (SMAPEx) data sets in Australia

There is a strong demand for soil moisture information in establishing efficient irrigation scheduling, climate change prediction, and sustainable land and water management. However, such data are not readily available with an appropriate accuracy or spatial and temporal resolution. Addressing the need for soil moisture at high resolution globally, the European Space Agency (ESA) has developed the Soil Moisture and Ocean Salinity (SMOS) mission. Launched in November 2009, SMOS provides the first satellite dedicated to near surface soil moisture measurement at 40-km resolution. Moreover, the National Aeronautics and Space Administration (NASA) is developing the Soil Moisture Active Passive (SMAP) mission with a planned schedule for launch in October 2014. SMAP will deploy active (radar) and passive (radiometer) microwave instruments simultaneously to enhance soil moisture retrieval capabilities to a 10-km resolution. Both the SMOS and SMAP missions use a τ-ω model as the key component of their passive microwave soil moisture retrieval algorithms over moderately vegetated areas. While this model has been used in numerous L-band studies for soil moisture retrieval at various spatial scales throughout Europe, America and Australia, most studies have resulted in some “calibration” of model parameters or have proposed changes to the model physics. Consequently, this study presents an independent validation of the currently accepted version of the τ-ω model using data from the first Soil Moisture Active Passive Experiment (SMAPEx-1) airborne campaign undertaken in south-eastern Australia in July 2010. Designed specifically to support the development of soil moisture retrieval algorithms for the SMAP mission, SMAPEx-1 includes a series of intensive monitoring periods at the Yanco study area in south-eastern Australia. During these periods, data were collected from airborne radar and radiometer instruments concurrently with ground measurements of soil moisture, vegetation water content and surface roughness. In this paper, 1-km soil moisture maps of the intensive monitoring areas were derived from airborne L-band passive (radiometer) data acquired from SMAPEx-1, and validated using extensive ground measurements of soil moisture. The comparison between measured and retrieved soil moisture content showed that the τ-ω model with the currently accepted parameters worked better for crops field than grassland. The RMSE for pixels with ancillary measurements was found to be 0.13 mm, which exceeds the SMAP target accuracy of 0.04 mm. With the b parameter optimized for different land cover types, this number was reduced to 0.04 mm, and the bias reduced from 0.05 to 0.01 mm. The contribution of this study is two-fold. First, it provides an independent test of the τ-ω model for an independent set of soil and vegetation conditions. Second, the validated soil moisture data will be used as a baseline validation for many planned studies in support of SMAP, including radar-only soil moisture retrieval, active-passive soil moisture retrieval, validation of soil moisture downscaling, and testing scaling properties of the study area.