Advances in soil moisture retrieval from synthetic aperture radar and hydrological applications

► High-resolution soil moisture is important for hydrological applications. ► Accuracy of SAR soil moisture is less important than distribution. ► Proper treatment of soil moisture generally improves model performance. ► Soil moisture can be retrieved without in situ measurement with multi-angular SAR. The sensitivity of Synthetic Aperture Radar (SAR) to soil moisture is well established, however, the retrieval of soil moisture from SAR is confounded by the effects of surface roughness and vegetation. This difficulty has resulted in limited applications of SAR as an operational source of soil moisture in hydrology despite the demonstrated benefits of high-resolution distributed soil moisture. Technical and methodological advances such as multi-configuration radar and forthcoming SAR constellations are increasingly mitigating the shortcomings of SAR with respect to soil moisture estimation at the field and catchment scale. At the same time, progress in data assimilation and a better understanding of the impact of phenomena, such as climate change, are revealing the hydrological importance of soil moisture spatial distribution. Thus, despite the currently modest retrieval accuracy, SAR is an important source of soil moisture state information for the hydrological community. Towards the end of increasing the hydrological utilization of SAR soil moisture, a comprehensive literature review was conducted to provide the state-of-the-art of SAR soil moisture retrieval methodology, its limitations and potential. Following the methodology review, a discussion of the benefits and limitations of soil moisture data retrieved from SAR is used to outline the scope of SAR derived soil moisture for hydrological applications.