SURFACE SOIL MOISTURE ESTIMATION FROM MODIS APPARENT THERMAL INERTIA: A COMPARISON WITH SMOS AND SMAP SOIL MOISTURE PRODUCTS

L-band radiometry has been considered the preferred technique for global soil moisture (SM) remote sensing, as the Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) missions proved. Owing the limited spatial resolution of current L-band radiometers, several downscaling algorithms have been developed to enhance the SMOS and SMAP SM resolutions. In this regard, the apparent thermal inertia (ATI) may be used for indirectly estimating SM, not only for disaggregation. In this study, 1-km ATI-derived SM was obtained from March 31, 2015 to December 31, 2016, using Moderate Resolution Imaging Spectroradiometer (MODIS) as an alternative to L-band radiometry. The ATI-derived SM was validated over the Soil Moisture Measurement Stations Network of the University of Salamanca (REMEDHUS). Comparisons with in situ SM showed slightly lower correlation (similar to 0.62), but similar error (similar to 0.043 m(3).m(-3)) and lower bias (similar to 0.027 m(3).m(-3)) than the existing SMOS and SMAP products, whilst improving the resolution.