MODELING SENSIBLE HEAT FLUXES FROM A WHEAT CANOPY - AN EVALUATION OF THE RESISTANCE ENERGY-BALANCE MODEL

Sensible heat fluxes measured over a growing wheat crop are used to evaluate the performance of the resistance energy balance model for both sparse and full canopy covers. Aerodynamic temperatures (for heat and momentum) are derived from measured sensible heat fluxes and then compared to canopy radiometric temperatures which were measured using infra-red radiometers and a pyrgeometer. Agreement between measured radiometric and aerodynamic temperatures is excellent for the more dense, fully-closed wheat crop, and the resistance energy balance model also performed adequately for this canopy. However, very large differences between radiometric and aerodynamic temperatures were observed for the sparse wheat crop, especially at intermediate leaf area indexes (LAIs) (less than 2). Large differences were even found between the infra-red radiometer-based and pyrgeometer-based canopy temperatures. Under such conditions of sparse canopy cover the model performs poorly. At these times greater agreement between aerodynamic and canopy temperatures was found when the latter was determined as a 'composite temperature', combining both infra-red radiometer and pyrgeometer measurements. Modelling of sensible heat flux using the resistance energy balance model is improved with the use of this composite canopy temperature. Measured values for kB(-1) were extremely variable and so a unique value for the ratio of roughness lengths for heat and momentum could not be determined.