A Combined Optical-Microwave Method to Retrieve Soil Moisture Over Vegetated Areas

1  Abstract— A simple approach for correcting the effect of vegetation in the estimation of the surface soil moisture (w S) from L-band passive microwave observations is presented in this study. The approach is based on semi-empirical relationships between soil moisture and the polarized reflectance including the effect of the vegetation optical depth which is parameterized here as a function of the Normalized Vegetation Difference Index (NDVI). In a first step, the method was tested against in-situ measurements collected over a grass site from years 2004 to 2007 (SMOSREX experiment) Two polarizations (horizontal/vertical) and five incidence angles (20º, 30º, 40º, 50º and 60º) were considered in the analysis. The best w S estimations were obtained when using both polarizations at the angle of 40º. The average accuracy in the soil moisture retrievals was found to be about 0.06 m3/m3, improving the estimations by about 0.02 m3/m3 in comparison with the case when the vegetation effect is not considered. In a second step, the proposed method was applied to the microwave observations acquired from the Soil Moisture Ocean Satellite (SMOS) and optical observations acquired from the Moderate Resolution Imaging Spectroradiometer (MODIS) over Australia for 2010 in order to evaluate its applicability to spaceborne remote sensing observations. The results showed the potential interest of using information on vegetation (through a vegetation index such as NDVI), in the semi-empirical regressions which were calibrated over the Australian site.