A ONE-LAYER RESISTANCE MODEL FOR ESTIMATING REGIONAL EVAPOTRANSPIRATION USING REMOTE-SENSING DATA

A methodology has been developed for estimating regional evapotranspir ation using remote sensing data. The transfer of sensible and latent h eat is described by an aerodynamic resistance and a surface resistance , The aerodynamic resistance is formulated on the basis of the Monin-O bukhov similarity theory and the surface resistance is determined from the energy balance equation using remotely sensed surface temperature , The regional evapotranspiration is then calculated with a one-layer resistance model, The overall surface transfer characteristics at regi onal scale are characterized by an effective roughness length and this parameter was determined from local roughness length with the aid of remote sensing measurements. The model was validated by using the data obtained during the HAPEX-MOBILHY (Hydrologic Atmospheric Pilot Exper iment and Modelisation du Bilan Hydrique) experiment. At local scale w ith homogeneous surface conditions, good agreement (r = 0.90, RMSE = 5 0 W m(-2)) was obtained between model estimates and the measurements a t ground stations, The regional evapotranspiration from the agricultur al part of the HAPEX-MOBILHY was also calculated with the one-layer re sistance model by combining the NOAA-AVHRR with ground-based meteorolo gical data. The calculated regional evapotranspiration was compared wi th the area averages of the evapotranspiration measured by the ground- based stations. The two techniques agreed fairly well and the average difference was only 28 W m(-2).