SMOSREX: A long term field campaign experiment for soil moisture and land surface processes remote sensing

The primary goal of the SMOS mission is to deliver global fields of sea surface salinity and surface soil moisture using L-band (1.4 GHz) radiometry. Within the context of the preparation of SMOS activities over land, a field campaign, SMOSREX (Surface Monitoring Of the Soil Reservoir EXperiment), has been in operation since January 2001 in Mauzac, near Toulouse in France. Continuous ground measurements of meteorological variables, soil moisture and temperature profiles have been taken over bare soil and a grass plot left fallow. Since January 2003, SMOSREX has been providing accurate field measurements of dual polarized L-band brightness temperature up-welling from both bare soil and fallow plots, together with multi-spectral (from visible to infrared frequencies) remote sensing surface data. The scientific objectives are presented in this paper and the corresponding experimental design is described. The experimental concept is totally new since (i) SMOSREX combines land–surface–atmosphere observations, passive microwave measurements and VIS to NIR remote sensing, (ii) SMOSREX is based on highly accurate L-band measurements carried out by a radiometer specifically designed for the experiment, and (iii) SMOSREX provides a unique continuous data set of L-band measurements over several years. The characteristics of the L-band emission are presented at diurnal, seasonal and annual temporal scales, and the emissions are compared over bare soil and natural grass. The surface emissions over bare soil and fallow area are shown to be counter-phased at the diurnal scale due to small variations in vegetation water content and bare soil surface moisture. Innovative long term results using L-band measurements for both bare soil and natural grass are presented in this paper, and the relationship between the surface emission at L-band and surface bare soil moisture is shown to be suitable for a long term period (19 months). Soil freezing is shown to be drastically different for bare soil and vegetation covered plots, with a large threshold effect on microwave surface emission.