Multi-resolution soil moisture retrievals by disaggregating SMAP brightness temperatures with RADARSAT-2 polarimetric decompositions

Abstract

• Radar surface scattering power P s was analyzed to disaggregate SMAP TB. • Soil moisture was retrieved across multiple spatial resolutions (50–10 km). • Impacts of spatial resolutions on retrieval accuracies were investigated. • Optimal spatial resolution was obtained for soil moisture retrieval. Mapping soil moisture (SM) at high spatial resolution assists to trigger important agricultural management, such as irrigation, to enhance crop yields. This study investigates disaggregation of SMAP brightness temperature ( TB ) using RADARSAT-2 polarimetric decompositions to retrieve high-resolution SM. Compared to Sentinel-1 backscattering coefficients used in the SMAP baseline active–passive SM retrieval algorithms, the RADARSAT-2 surface scattering power P s with a reduced vegetation influence was hypothesized to be more relevant to disaggregate the SMAP TB . Different polarimetric decompositions were evaluated to extract an optimal P s , followed by an incidence angle normalization. Then, the optimal P s parameter was aggregated to the same spatial resolution as the SMAP TB to develop empirical relationships between P s and TB . Furthermore, the airborne TB data collected by Passive Active l -band Sensor (PALS) were analyzed in terms of the P s across multiple spatial resolutions, to account for the scale effect on the P s /TB relationships. Finally, the τ-ω emission model was used to retrieve SM at multiple spatial resolutions (10 km, 1 km, 500 m, 100 m, and 50 m). The impacts of spatial resolution on retrieval accuracy were analyzed to determine the best spatial resolution for SM retrievals. The results indicated that the An polarimetric decomposition with the de-orientation provided the highest surface scattering powers, which may benefit the SM estimation. In contrast to the traditional cosine algorithms, the incidence angle normalization of P s with span resulted in a temporally decreasing surface scattering power, because of the increasing vegetation attenuation as the crop grows. The sensitivity of TB to P s decreases as the resolution scale varies from 36 km to 50 m. The SM retrievals across multiple resolutions obtained marginal differences in retrieval accuracy. Although slightly better results were obtained with 1 km spatial resolution which is close to the nominal size of agricultural fields in the study area (R = 0.68–0.8 and RMSE = 0.039–0.062 m 3 /m 3 ), the retrievals at 50 m spatial resolution (R = 0.63–0.76 and RMSE = 0.046–0.067 m 3 /m 3 ) capture the spatial heterogeneity of SM within and across different fields which could be very helpful for the precision agriculture.