Optical-Microwave Synergy for Estimating Surface Sensible Heat Flux over a Semi-Arid Rangeland

This study reports the first results of the Walnut Gulch' 92 experiment concerning the combined use of radar backscattering (ERS-1) and thermal infrared (Landsat TM) data to estimate surface sensible heat flux. The purpose is to use the radar/thermal synergy to retrieve both vegetation and soil temperatures required by a two-layer type model. The first step investigates the potential use of ERS-1 SAR images for surface soil moisture monitoring of the watershed using five calibrated images acquired during the year 1992 (dry to wet conditions). Results show that despite the typical low level of biomass of semi-arid rangeland, an attenuation of the soil backscatter (up to 2 dB) can occur during the rainy season mainly due to the vegetation characteristics. A statistical relationship is then used to retrieve the volumetric surface soil moisture from ERS-1 backscattering (sensitivity of 0.23 dB/% moisture) with a resulting root mean square error (RMSE) of 1.3% of soil moisture. In a second step a semi-empirical approach based on energy balance relates soil temperature T_s to this estimated surface soil moisture with an accuracy of 1.3°C. Vegetation temperature is then deduced from both T_s and Landsat TM composite temperature T_r in order to estimate sensible heat flux according to the two-layer model. To extenture measuremend the validation data set, additional T_s and T_r values are also obtained from ground soil moists and thermal aircraft flights respectively. The overall low RMSE of 35 W/m2 obtained between ground and remote sensible heat flux confirms the potentiality of radar/thermal synergy over semi-arid sparse vegetation for energy fluxes estimate.