Soil Moisture Mapping Using Combined Active/Passive Microwave Observations Over the East of the Netherlands

A coarse resolution soil moisture product is downscaled to 1, 5, and 10 km using synthetic aperture radar (SAR) observations acquired over the east of the Netherlands. The combination of phased array L-band SAR (PALSAR) backscatter and VUA-NASA C-band Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) soil moisture product is adopted to mimic the radar/radiometer setup as will be available from NASA´s soil moisture active passive (SMAP) mission. The validation of retrievals is based on measurements collected by a sparse network of 20 stations distributed across 50 × 75 km study area selected as one of the key validation sites for the SMAP soil moisture products. Reasonable agreements between the measurements and soil moisture retrieved at 1-, 5-, and 10-km resolution are found that lead to coefficients of determination of 0.37, 0.36, and 0.36, respectively. The retrievals, however, severely overestimate the measured soil moisture, which is attributed to the well-known positive bias of the selected AMSR-E product. After bias-correction, root mean squared differences reach as low as 0.046 m³ m^-3 for individual locations and are 0.067, 0.068, and 0.069 m³ m^-3 on average for the soil moisture retrieved at 1-, 5-, and 10-km resolutions, respectively. These error levels do not satisfy SMAP´s targeted accuracy of 0.04 m³ m^-3, but the radar/radiometer setup as well as the characterization of the soil moisture conditions representative are not optimal. On the other hand, it is demonstrated that the sequence of soil moisture maps does capture valuable hydrological and hydrometeorological information.