Disaggregation of SMAP radiometric soil moisture measurements at catchment scale using MODIS land surface temperature data

Satellite soil moisture observations often require the enhancement of spatial resolution prior to being used in climatic and hydrological studies. This study employs the thermal inertia theory to downscale the 36 km radiometric data of the NASA's Soil Moisture Active/Passive Mission (SMAP) into 1 km resolution. Regressions between daily temperature difference and daily mean soil moisture were established over Krui River catchment. The values of daily surface temperature difference were derived from MODIS Terra and Aqua, while the soil moisture data is collected from the Scaling and Assimilation of Soil Moisture and Streamflow (SASMAS) network. In this study, the regression analysis was conducted for each season separately and further classified into six classes based on the type of vegetation cover and clay content. SMAP data covering the Merriwa River catchment was disaggregated by using the algorithms formulated at the Krui River catchment to evaluate the applicability of using pre-defined algorithms on Merriwa River catchment, a catchment with similar characteristics. A comparison between downscaled soil moisture data and in situ data at the Krui and Merriwa River catchments shows a reasonable match with RMSE 0.136 and 0.146 cm3/cm3 respectively. The study shows promising results towards developing a general model to downscale SMAP soil moisture data in semi-arid regions using multiple variables.