Comparison of different polarimetric decompositions for soil moisture retrieval over vegetation covered agricultural area

This study investigates and compares the potential of three model-based polarimetric decompositions, namely those developed by Freeman-Durden (1998), Hajnsek et al. (2009) and An et al. (2010), for soil moisture retrieval over agricultural fields covered by several crops. The volume scattering component was first removed from the full coherency matrix. Then, in order to reduce the effect of the incidence angle on the retrieval results, a normalization process at a reference incidence angle was conducted for the first time, on the dominant surface or dihedral scattering component from which the soil moisture was retrieved. The time series of Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data and the ground measurements of soil and vegetation characteristics collected during the Soil Moisture Active Passive (SMAP) Validation Experiment in 2012 (SMAPVEX12) were used to compare the three decomposition methods with respect to the scattering mechanisms and the soil moisture retrieval performances. The results show that the performance of each decomposition method for soil moisture retrieval depends on the crop types and the crop phenological stages. Indeed, Freeman-Durden model provided the best results for corn and wheat, Hajnsek decomposition performed well for canola, while better results were obtained for soybean using An decomposition. At the early growth stage, both the surface and dihedral scattering components contributed to retrieve the soil moisture, while at a later crop development, the surface scattering component is almost the only scattering mechanism from which soil moisture was retrieved. Thus, the best performance for soil moisture retrieval was obtained a) at the early crop development stage from Hajnsek decomposition which better integrated the dihedral component and b) at a later growth stage from An decomposition which enhanced the surface scattering. Finally, an overall soil moisture underestimation with RMSE of 0.06–0.11m3/m3 was observed from the three decompositions, and the highest retrieval rate of 33%–39% was obtained from An decomposition as a result of the enhanced surface scattering.