Active-Passive Approach for NISAR High Resolution Soil Moisture Products: Retrieval and Accuracy Assessment over Indian Cropland

Soil moisture is an essential variable in agricultural applications for irrigation scheduling, crop water requirements and pest management etc. However, currently available global satellite microwave radiometer derived soil moisture products are inadequate due to coarse spatial resolution for such applications. In order to improve spatial resolution, SMAP (Soil Moisture Active Passive) mission has first time demonstrated the potential of combining microwave radiometer and radar data based on Active-Passive approach to produce 3-9 km soil moisture globally. However, this approach has not been well tested quantitatively at sub km (<1km) grid resolution using spaceborne observations. In this work, we adopted and tested active-passive approach to integrate radiometer derived coarse resolution soil moisture (SMAP L-band radiometer) with fine resolution radar backscatter (Sentinel-1 C-band SAR) to downscale soil moisture at multiple grid resolution (100 m, 500m and 1000 m). The impact of scale on the robustness of the algorithm is analyzed by assessing the soil moisture retrieval accuracy. Detailed validation was attempted using Multi-Scale Field Sampling Framework (MFSF) over selected agricultural cropland study site. The results obtained are very encouraging, showing the potential of Active-Passive approach for high spatial soil moisture with correlation of 0.75, 0.76 & 0.68 and ubRMSE of 4.94%, 6.07% & 7.21% for 100m, 500m and 1km respectively. Based on the above assessment as pre-cursor study, Active-Passive approach will be the potential candidate for utilizing NISAR data to deliver high spatial resolution soil moisture operational products along with radiometer.