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

Elena Zakharova (1) , Jean-Christophe Calvet (1) , Sébastien Lafont (1) , Clément Albergel (1, 2) , Jean-Pierre Wigneron (3) , Mickaël Pardé (4) , Yann H. Kerr (5) , Mehrez Zribi (5)

1 CNRM - Centre national de recherches météorologiques
2 ECMWF - European Centre for Medium-Range Weather Forecasts
3 EPHYSE - Écologie fonctionnelle et physique de l'environnement
4 ESTER - LATMOS
5 CESBIO - Centre d'études spatiales de la biosphère

In 2009 and 2010 the L-band microwave Cooperative Airborne Radiometer for Ocean and Land Studies (CAROLS) campaign was performed in Southwestern France to support the calibration and validation of the new Soil Moisture and Ocean Salinity (SMOS) satellite mission. The L-band Microwave Emission of the Biosphere (L-MEB) model was used to retrieve Surface Soil Moisture (SSM) and the Vegetation Optical Depth (VOD) from the CAROLS brightness temperature measurements. The CAROLS SSM was compared with in situ observations at 11 sites of the SMOSMANIA (Soil Moisture Observing System-Meteorological Automatic Network Integrated Application) network of Météo-France. For eight of them, significant correlations were observed (0.51 ≤ r ≤ 0.82), with standard deviation of differences ranging from 0.039 m³ m⁻³ to 0.141 m³ m⁻³. Also, the CAROLS SSM was compared with SSM values simulated by the A-gs version of the Interactions between Soil, Biosphere and Atmosphere (ISBA-A-gs) model along twenty flight lines, at a resolution of 8 km × 8 km. A significant spatial correlation between these two datasets was observed for all the flights (0.36 ≤ r ≤ 0.85). The CAROLS VOD presented significant spatial correlations with the vegetation water content (VWC) derived from the spatial distribution of vegetation types used in ISBA-A-gs and from the Leaf Area Index (LAI) simulated for low vegetation. On the other hand, the CAROLS VOD presented little temporal changes, and no temporal correlation was observed with the simulated LAI. For low vegetation, the ratio of VOD to VWC tended to decrease, from springtime to summertime. For 83% of ISBA-A-gs grid cells (8 km × 8 km), sampled every 5 m by CAROLS observations at a spatial resolution of about 2 km, the standard deviation of the sub-grid CAROLS SSM was lower than 0.05 m³ m⁻³. The presence of small water bodies within the ISBA-A-gs grid cells tended to increase the CAROLS SSM spatial variability, up to 0.10 m³ m⁻³. Also, the grid cells characterised by a high vegetation cover heterogeneity presented higher standard deviation values, for both SSM and VOD.

Mots clés: télédétection; mission SMOS ; microonde passive ; modèle L-MEB

Domaines: Hydrologie; Milieux et Changements globaux

Elena Zakharova, Jean-Christophe Calvet, Sébastien Lafont, Clément Albergel, Jean-Pierre Wigneron, et al.. Spatial and temporal variability of biophysical variables in Southwestern France from airborne L-band radiometry. Hydrology and Earth System Sciences, 2012, 16 (6), pp.1725-1743. ⟨10.5194/hess-16-1725-2012⟩. ⟨hal-00661546⟩