A Proper Method for Estimating Sensible Heat Flux above a Horizontal-Homogeneous Vegetation Canopy Using Radiometric Surface Observations

Abstract

A multilayer energy budget model for a horizontal-homogeneous vegetation canopy and a simple radiometric canopy model are developed to estimate sensible heat flux with use of the observed directional radiometric temperature of the canopy surface. Field experiments were carried out in an almost horizontal-homogeneous rice paddy under various canopy conditions to obtain boundary conditions and data to test the models. A proper method for estimating sensible heat flux is proposed, which is based on model simulations and the observational results. Key parameters required for the estimation of the flux are the aerodynamic conductance and the optimum viewing angle, which are investigated in detail using a rice paddy model. The aerodynamic conductance is a function of three parameters, namely, the leaf-area index (LAI), wind speed, and virtual temperature difference between the canopy and the surface layer. The optimum viewing angle for sensible heat is found to be between 50° and 70° of the nadir angle; its variance with LAI and the dependence on wind speed is relatively small. In the case of latent heat, the optimum angle is systematically smaller than that for sensible heat by about 10°. There is a gap in the estimation of the error in the sensible heat flux according to the viewing angle. During the daytime, an error of over 100 W m?2 would result if the viewing angle is near nadir, while it is around a 30 W m?2 error for an angle of 70° of the nadir angle in spite of LAI. This effect is found to be due to four factors: 1) the difference in the surface temperature between the leaf and the underlying ground; 2) the leaf surface, which plays a comparable role as the underlying ground surface in the thermal exchange; 3) the ratio of leaf area and the underlying ground area in the field of view of an infrared thermometer; and 4) the emissivity of the underlying wet ground surface, which varied with viewing angle, is effective for sparse canopy.