A Soil Moisture Spatial Downscaling Method for Smap Using the Optical Trapezoid Model

High-resolution soil moisture plays an important role in irrigation scheduling and agriculture management. A spatial downscaling method based on the OPtical TRApezoid Model (OPTRAM) was proposed to retrieve soil moisture at 500 m spatial resolution by combining SMAP and MODIS observations. This method is less sensitive to cloud cover than the land surface temperature (LST)-based algorithms. The performance of this method was evaluated using soil moisture data derived from aircraft observations of L-band brightness temperature and compared to a typical LST-based downscaling method known as DisPATCh (DISaggregation based on Physical And Theoretical scale Change). The RMSE (Root Mean Squared Error) of the retrieved soil moisture was 0.037 m³/m³ for the proposed method, which is lower than the 0.064 m³/m³ achieved for the DisPATCh algorithm.