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Comparison between SAR Soil Moisture Estimates and Hydrological Model Simulations over the Scrivia Test Site

In this paper, the results of a comparison between the soil moisture content (SMC) estimated from C-band SAR, the SMC simulated by a hydrological model, and the SMC measured on ground are presented. The study was carried out in an agricultural test site located in North-west Italy, in the Scrivia river basin. The hydrological model used for the simulations consists of a one-layer soil water balance model, which was found to be able to partially reproduce the soil moisture variability, retaining at the same time simplicity and effectiveness in describing the topsoil. SMC estimates were derived from the application of a retrieval algorithm, based on an Artificial Neural Network approach, to a time series of ENVISAT/ASAR images acquired over the Scrivia test site. The core of the algorithm was represented by a set of ANNs able to deal with the different SAR configurations in terms of polarizations and available ancillary data. In case of crop covered soils, the effect of vegetation was accounted for using NDVI information, or, if available, for the cross-polarized channel. The algorithm results showed some ability in retrieving SMC with RMSE generally <0.04 m 3 /m 3 and very low bias (i.e., <0.01 m 3 /m 3 ), except for the case of VV polarized SAR images: in this case, the obtained RMSE was somewhat higher than 0.04 m 3 /m 3 (≤0.058 m 3 /m 3 ). The algorithm was implemented within the framework of an ESA project concerning the development of an operative algorithm