Spatial and temporal variability of biophysical variables in southwestern France from airborne L-band radiometry

摘要

In 2009 and 2010 the L-band microwave Cooperative Airborne Radiometer forOcean and Land Studies (CAROLS) campaign was performed in southwesternFrance to support the calibration and validation of the new Soil Moistureand Ocean Salinity (SMOS) satellite mission. The L-band Microwave Emissionof the Biosphere (L-MEB) model was used to retrieve surface soil moisture(SSM) and the vegetation optical depth (VOD) from the CAROLS brightnesstemperature measurements. The CAROLS SSM was compared with in situobservations at 11 sites of the SMOSMANIA (Soil Moisture ObservingSystem-Meteorological Automatic Network Integrated Application) network ofMétéo-France. For eight of them, significant correlations wereobserved (0.51 ≤ r ≤ 0.82), with standard deviation of differencesranging from 0.039 m3 m?3 to 0.141 m3 m?3. Also, theCAROLS SSM was compared with SSM values simulated by the A-gs version of theInteractions between Soil, Biosphere and Atmosphere (ISBA-A-gs) model along20 flight lines, at a resolution of 8 km × 8 km. A significantspatial correlation between these two datasets was observed for all theflights (0.36 ≤ r ≤ 0.85). The CAROLS VOD presented significant spatialcorrelations with the vegetation water content (VWC) derived from thespatial distribution of vegetation types used in ISBA-A-gs and from the LeafArea Index (LAI) simulated for low vegetation. On the other hand, the CAROLSVOD presented little temporal changes, and no temporal correlation wasobserved with the simulated LAI. For low vegetation, the ratio of VOD to VWCtended to decrease, from springtime to summertime. The ISBA-A-gs grid cells(8 km × 8 km) were sampled every 5 m by CAROLS observations,at a spatial resolution of about 2 km.For 83% of the grid cells, the standarddeviation of the sub-grid CAROLS SSM was lower than 0.05 m3 m?3. The presence of small waterbodies within the ISBA-A-gs grid cells tended to increase the CAROLS SSMspatial variability, up to 0.10 m3 m?3. Also, the grid cellscharacterised by a high vegetation cover heterogeneity presented higherstandard deviation values, for both SSM and VOD.