APPLICATION OF AN ENERGY COMBINATION MODEL FOR EVAPORATION FROM SPARSE CANOPIES

This study tests the evaporation energy combination model developed by Shuttleworth and Wallace (1985) with data collected in a subarctic wetland. The modelled evaporation was compared with evaporation calculated from the Bowen ratio energy balance technique over a range of leaf area indices (LAI) from non-vegetated to fully vegetated conditions. The Shuttleworth-Wallace (SW) model was in excellent agreement with the measured evaporation for hourly and day-time totals for all values of LAI. This gives a particular advantage to the SW model compared to the simple Penman-Monteith combination equation. A comparison of measured and modelled total evaporation for all days yielded a root mean square error and mean bias error of 0.98 and -0.13 MJ m-2 day-1, respectively. The model also shows good agreement with the measured evaporation on an hourly basis. Although the results of this study are encouraging, we are cautious because this test is not truly independent. The need for additional investigation and testing of certain model parameters is recognized. In this study, we assume that eddy diffusivity within the canopy decreased exponentially and was controlled by a decay constant which varied with LAI. However, there is little information available to validate this treatment. Net radiation at the soil surface was computed from net radiation over the canopy and an exponential function of LAI, which was held constant over the course of the growing season. © 1990.