Potential of bias correction for downscaling passive microwave and soil moisture data

Abstract

Passive microwave satellites such as Soil Moisture and Ocean Salinity or Soil Moisture Active Passive observe brightness temperature (TB) and retrieve soil moisture at a spatial resolution greater than most hydrological processes. Bias correction is proposed as a simple method to disaggregate soil moisture to a scale more appropriate for hydrological applications. Temporal stability of soil moisture and TB was demonstrated at the Little Washita and Little River Experimental Watersheds using in situ observations and the Community Microwave Emissions Model. Decomposition of the mean square difference (MSD) between the watershed average soil moisture and TB showed that bias was a major contributor to differences between watershed average and local-scale soil moisture and TB, particularly at sites with high MSD. The mean RMSD between watershed average and local soil moisture was 0.04m3m-3 and 0.06m3m-3 at Little River and Little Washita, respectively. Following a simple bias correction the RMSD was reduced to 0.03m3m-3 at both sites. Considering multiple incidence angles at both horizontal and vertical polarization, bias correction of watershed average TBV reduced the RMSD by approximately 75% and 45% and TBH RMSD by 68% and 36% for Little River and Little Washita, respectively, at all incidence angles. Therefore, at subsatellite grid scale, bias correction can be considered a viable technique for downscaling passive microwave observations and soil moisture retrievals.