Testing the SMOS-derived vegetation optical thickness over rainfed cereals

The vegetation optical thickness (τ) from the Soil Moisture and Ocean Salinity (SMOS) L2 products is compared to some vegetation parameters estimated both at field scale and by means of an upscaling procedure through the MODIS Normalized Difference Vegetation Index (NDVI). Vegetation measurements over rainfed cereals indicated that NDVI, Leaf Area Index (LAI) and Vegetation Water Content (VWC) were strongly correlated. In turn, τ is expressed as a linear function of VWC in the literature. These relationships were studied and tested during 2010, and used to upscale the MODIS NDVI series. The results afforded no definite conclusions, due mainly to the different temporal and spatial resolution of the estimations. SMOS τ varied significantly at daily scale, which did not seem to be a realistic behavior of the vegetation. Moreover, the spatial average of the cereals cover in the MODIS NDVI produced small changes in the NDVI values. Nevertheless, the relationships studied are encouraging for the identification of new parameters that describe τ, as it was found for the Fraction of Vegetation Cover (FVC), and they should be tested for other vegetation covers.