Soil moisture in French Alpine valley Using L-BAND radiometer

Abstract

International audienceSince the launch of the satellite, a constant activity has been to compare SMOS derived soil moisture to in situ measurements to validate and improve the soil moisture retrieval. Many sites have been equipped with soil moisture probes and a few of them with L-Band radiometer placed on structures of ~10 m. height. The observed surfaces are homogeneous, i.e. one type of vegetation cover, whereas SMOS (~40km of resolution) observes more heterogeneous scenes, i.e. mix of different landscapes. Keeping this issue in mind, a new experimental site is proposed with the main objective to observe complex scenes to test the SMOS soil moisture retrieval. The LEWIS radiometer (L-Band radiometer for Estimating Water in Soil) has been placed by a cliff in St Hilaire du Touvet (French Alps) since May 2014. Its position allows us to monitor a surface 800m down below in the valley du Grésivaudan which is flat, avoiding topographic effect. The instrument has a beam width of 13.6° which implies a field of view of several hundreds of meters, covering various landscapes (agricultural fields, forest, lake). The instrument is equipped with 2 motors so that it moves in two directions (azimuth and elevation) and monitors regularly several areas of interest. It can also be pointed towards the deep sky for calibration. In addition, soil moisture and temperatures probes are placed in different fields in the Valley and a pyrometer is installed with the radiometer to measure the skin temperature of the observed scene. The first data confirm the consistency of Lewis measurements. The deep sky presents brightness temperatures of TBH = 5.4K et TBV = 4.3K, which is similar to what was observed at the SMOSREX site. This new site is planed to last 4 years to insure long time observations (L-band brightness temperatures and surface soil moisture) of heterogeneous scenes. Further works will also imply sun glint effect, snow detection and effect of urban areas