AMSR2 soil moisture downscaling using multisensor products through machine learning approach

Soil moisture is important to understand the interaction between the land and the atmosphere, and has an influence on hydrological and agricultural processes such as drought and crop yield. In-situ measurements at stations have been used to monitor soil moisture. However, data measured in the field are point-based and difficult to represent spatial distribution of soil moisture. Remote sensing techniques using microwave sensors provide spatially continuous soil moisture. The spatial resolution of remotely sensed soil moisture based on typical passive microwave sensors is coarse (e.g., tens of kilometers), which is inadequate for local or regional scale studies. In this study, AMSR2 soil moisture was downscaled to 1km using MODIS products that are closely related to soil moisture through statistical ordinary least squares (OLS) and random forest (RF) machine learning approaches. RF (r²=0.96, rmse=0.06) outperformed OLS (r²=0.47, rmse=0.16) in modeling soil moisture possibly because RF is much flexible through randomization and adopts an ensemble approach. Both approaches identified T·V (i.e., multiplication between land surface temperature and normalized difference vegetation index) and evapotranspiration. AMSR2 soil moisture produced from the VUA-NASA algorithm appeared overestimated at high elevation areas because the characteristics of ground data for validation and correction used in the algorithm were different from those in our study area. In future study, AMSR2 soil moisture based on the JAXA algorithm will be evaluated with additional input variables including land cover, elevation and precipitation.