Estimation of the Latent Heat Flux over Full Canopy Covers from the Radiative Temperature

Abstract

This paper examines the bulk aerodynamic method and proposes an alternative algorithm for estimating evapotranspiration from radiative temperature over unstressed full-canopy cover crops. Both approaches are studied using calibration and validation datasets over four complete different canopies: grass, wheat, alfalfa, and maize. The former method gives acceptable results for grass, wheat, and alfalfa crops but is very discouraging for maize. The parameter (kB?1)s is evaluated to adjust the bulk aerodynamic method. Different expressions for (kB?1)s over homogeneous crops are tested. The latent heat flux estimations do not improve significantly for the different (kB?1)s values studied for the four crops. A simple and more accurate semiempirical method is proposed. The new approach, based on the Bowen ratio energy balance method, introduces the relation between the radiative Bowen ratio and the traditional Bowen ratio. A linear relationship is found between the two Bowen ratios over grass, wheat, alfalfa, and maize crops. Errors under 10% are achieved for latent heat flux estimations. When such small accuracies are not required and input data are available, the bulk aerodynamic approach is preferred for grass, wheat, and alfalfa because it can be applied without any empirical adjustment. The major advantage of this regression-based approach is that no a priori assumptions concerning the resistances are required. The results are promising for remote sensing applications, although they need further verification in other climatological conditions.