Development of An Algorithm to Estimate Canopy Temperature for Tall Vegetation.

In the METRIC satellite based surface energy balance model it is assumed that the radiometric surface temperature and the exchange of energy between the ground and the atmosphere can be estimated a originating from a single source. This assumption is normally reasonable for dense and uniform agricultural crops. For vegetation that does not cover the surface completely the surface temperature as sensed by the satellite originates partially from the vegetation, and partially from the sunlit and shaded portions of the ground. This paper describes the procedure that has been developed to estimate the canopy temperature for tall vegetation. The canopy temperature is estimated as a function of the temperature of the non-vegetated fractions of the ground, the radiometric surface temperature as sensed by a satellite and the fraction of canopy cover. The fraction of canopy cover (f_c) when viewed from nadir is estimated based on LAI and a clumping factor. Based on vegetation geometry and the sun zenith angle the non-vegetated fraction of the surface (1-f_c) is divided into the fraction of soil that has a shadow cast on it (f_sh) and the fraction that is sunlit (f_su). The temperature of shaded soil is estimated as a function of the surface temperature of an alfalfa field meeting the requirements of a reference crop and wet bulb temperature. A function based on the temperature of an alfalfa field under reference conditions and of a field with dry bare soil is used to estimate the temperature of sunlit soil. Field measurements from several orchards with stands at varying development and fraction cover are used to test the procedure. Refinements in the estimation of shaded and sunlit temperatures will improve the agreement between model predictions and measurements. The model is being further developed to include aerodynamic transfer between soil, vegetation and atmosphere.