Abstract: A new method is developed here to model daily evapotranspiration over large areas by combining remote sensing data and ground-based meteorological variables.However,many differences of surface parameters between transpiration and evaporation are more complex because the transpiration from vegetation is affected by the physiological characteristics of the vegetation.Thus,this model describes the parameters separately.The model is validated at large scale with homogeneous surface conditions,good agreement is obtained between model estimates and the measurements at ground stations.