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 ev aluate 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 com pared to canopy radiometric temperatures which were measured using inf ra-red radiometers and a pyrgeometer. Agreement between measured radio metric and aerodynamic temperatures is excellent for the more dense, f ully-closed wheat crop, and the resistance energy balance model also p erformed adequately for this canopy. However, very large differences b etween 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-re d radiometer-based and pyrgeometer-based canopy temperatures. Under su ch conditions of sparse canopy cover the model performs poorly. At the se 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. Mo delling of sensible heat flux using the resistance energy balance mode l is improved with the use of this composite canopy temperature. Measu red values for kB(-1) were extremely variable and so a unique value fo r the ratio of roughness lengths for heat and momentum could not be de termined.