Validation of the Downscaled Global 1km Daily SMOS Soil Moisture in 2010-2021

Soil moisture is an important parameter for many applications in earth sciences, for example, agriculture, hydrology, meteorology and ecology. In the past decades, passive/active microwave sensors onboard satellites are utilized to retrieve soil moisture values from radiometer or radar observations. In this study, the global Level 3 daily soil moisture retrievals at 25 km spatial resolution between 2010 and 2021, which are derived from the European Space Agency (ESA) launched satellite Soil Moisture and Ocean Salinity (SMOS) L-band radiometer brightness temperature data, were downscaled by a thermal inertia principle based algorithm and Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) and normalized difference vegetation index (NDVI) to 1 km resolution. The downscaled SMOS soil moisture were then validated by in situ soil moisture measurements from more than 300 sites of 8 different soil moisture networks in the world, which are acquired from the International Soil Moisture Network (ISMN). Meanwhile, the National Aeronautics and Space Administration (NASA) launched Soil Moisture Active Passive (SMAP) global Level 2 enhanced soil moisture product at 9 km in 2015-2021 was also downscaled and validated by in situ measurements from the same soil moisture sites, for comparing with SMOS validation results. The results show that the downscaled SMOS soil moisture have overall improved accuracy in terms of smaller unbiased root mean squared error (RMSE) and mean absolute error (MAE) over the original 25 km data.