Soil moisture mapping over the central part of the Tibetan Plateau using a series of ASAR WS images

Abstract The wide swath (WS) and global monitoring (GM) modes of the Advanced Synthetic Aperture Radar (ASAR) provide for monitoring purposes a good compromise between the spatial coverage, spatial resolution and revisit time. Similar imaging modes will be a part of the Sentinel-1 SAR sensing system. A consequence of operating at the wider swath is, however, the rather large view angle range (about 16–43°), which needs to be corrected for if biophysical variables are to be retrieved from these data sets. A soil moisture retrieval algorithm based on the Integral Equation Method (IEM) scattering model is presented that explicitly simulates the backscatter as a function of the incidence angle instead of normalizing. This algorithm is applied to 150 ASAR WS scenes collected from April 2005 to September 2007 over the central part of the Tibetan Plateau. We selected three of these images covering the study area at different view angles during the winter season for the IEM roughness derivation. Subsequently, soil moisture is retrieved from the entire σ o data sets via inversion of the IEM model with the obtained roughness parameters as input. The retrievals are validated against soil moisture measured from July 2006 to September 2007 at four sites representative for a 10 × 10 km 2 area. Further, the uncertainties following from assumptions made for the development of the algorithm are also quantified.