First evaluation of the simultaneous SMOS and ELBARA-II observations in the Mediterranean region

The SMOS (Soil Moisture and Ocean Salinity) mission was launched on November 2, 2009. Over the land surfaces, simultaneous retrievals of surface soil moisture (SM) and vegetation characteristics made from the multi-angular and dual polarization SMOS observations are now available from Level-2 (L2) products delivered by the European Space Agency (ESA). Therefore, first analyses evaluating the SMOS observations in terms of Brightness Temperatures (TB) and L2 products (SM and vegetation optical depth TAU) can be carried out over several calibration/validation (cal/val) sites selected by ESA over all continents. This study is based on SMOS observations and in situ measurements carried out in 2010 over one of the main SMOS cal/val sites in Europe: the VAS (Valencia Anchor Station) site in the region of Utiel–Requena, close to Valencia, Spain. The main vegetation types in the region are vineyards, orchards and natural Mediterranean vegetation. The SMOS observations were analyzed in conjunction with those carried out by the L-band ELBARA-II radiometer over a vineyard which is considered as representative of the main land use of the VAS site. Time series of TB and retrievals of SM and TAU based on both the SMOS (L2 products) and the ELBARA-II observations were compared and evaluated against in situ measurements. A good agreement was found between the time variations in TB and in the retrieved SM values computed over the site from the SMOS and ELBARA-II observations (the determination coefficient R2 was > 0.88 for the TB values and R2 > 0.64 for the retrieved SM values). However, it was found that the SMOS L2 SM products underestimated the SM values retrieved from ELBARA-II by ~ 0.2 m3/m3. It is likely this offset can be partially explained by differences between the observed scenes: while the ELBARA-II footprint includes a single vineyard, the heterogeneous SMOS footprint includes not only a large number of vineyards but also a natural Mediterranean vegetation with persistent leaves overlaying rocky soils. The time variations in TAU retrieved from the ELBARA-II observations were found to be closely related to those of the NDVI (Normalized Difference Vegetation Index) vegetation index obtained from MODIS (Moderate Resolution Imaging Spectroradiometer) (R2 = 0.61) and revealed the vegetation cycle over the year. Conversely, the time variations in the SMOS Level-2 TAU product did not reveal any trends in relation to the vegetation development over the site.