Retrieving surface soil moisture over a wheat field: Comparison of different methods
Résumé
The main objective of the Soil Moisture and Ocean Salinity (SMOS) mission over the land surface is to monitor surface soil moisture (SM) from measurements of the microwave emission at L-band. These measurements will be done at many incidence angles and two polarizations. The multiconfiguration system of SMOS provides the possibility to retrieve one or several surface parameters in addition to soil moisture. Such an approach, referred to as N-parameter retrievals (where N is the number of retrieved parameters), could make the retrieval process more efficient. In this study, this possibility was investigated using an experimental dataset acquired over a wheat field. First, a detailed analysis of the attenuation properties of the wheat canopy, a vertical-stem dominated crop, was made. The results showed that the polarization and incidence angle dependence of attenuation was very significant and could be accurately modeled using only two parameters: the optical depth τ at nadir (τ(θ=0)) and a polarization parameter (Cpol). The need for this latter parameter results from the difference in the dielectric properties of wheat for the vertical and horizontal polarizations. Based on these results, a detailed comparison of different soil moisture retrieval approaches was made. One-, two-, and three-parameter retrieval approaches were tested. In this latter method, soil moisture and the two vegetation parameters (τ(θ=0) and Cpol) were retrieved simultaneously. This method was found to be the most efficient: it provided the best accuracy in the SM retrievals and it did not require ancillary information on the vegetation characteristics, which cannot be easily estimated for operational SM mapping applications.