Use of in-situ soil moisture measurements to evaluate microwave remote sensing products in south-western France

A long term profile soil moisture data acquisition effort is currently under way at 12 monitoring stations across southwestern France. The spatial distribution and set-up of those sites was specifically designed for validating remote sensing and model soil moisture estimates [1], [2]. Those m-situ measurements are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter using a time constant, T, is used to compute a profile soil water index from the surface observations. Through optimisation, a unique value for T of 6 days is found to result in a satisfactory correlation between observation and estimate for all stations. The surface soil moisture observed is also used to evaluate the normalized surface soil moisture estimates derived from coarse resolution (25 km) active microwave data of the ASCAT C-band. For 11 stations, significant correlation levels are found. The best correlation between a soil water index derived from ASCAT and the in-situ observations is obtained for a T of 14 days.