Surface soil moisture estimation over the AMMA Sahelian site in Mali using ENVISAT/ASAR data

This paper focuses on different methods for estimating soil moisture in a Sahelian environment by comparing ENVISAT/ASAR and ground data at the same spatial scale. The analysis is restricted to Wide Swath data in order to take advantage of their high temporal repetitivity (about 3–4 days) corresponding to a moderate spatial resolution (150 m). On the one hand, emphasis is put on the characterization of Surface Soil Moisture (SSM) at a spatial scale compatible with the derivation of the backscattering coefficients, and a transfer function is developed for up-scaling local measurements to the 1 km scale. On the other hand, three different approaches are used to normalize the angular variation of the observed backscattering coefficients. The results show a strong linear relationship between the HH normalized backscattering coefficients and SSM. The best result is obtained when restricting the ASAR data to low incidence angles and by taking into account vegetation effects using multi-angular radar data. For this case, the rms error of the SSM retrieval is 2.8%. These results highlight the capabilities of the ASAR instrument to monitor SSM in a semiarid environment.