Simulation of willow short-rotation forest evaporation using a modified Shuttleworth-Wallace approach

Evaporation from a willow short-rotation forest was analysed using a modified version of the Shuttleworth-Wallace model. The main modification consisted of a two-layer soil module, which enabled soil surface resistance to be calculated as a function of the wetness of the top soil. Introduction of the threshold value of the leaf area index when scaling up from the leaf to the canopy resistance resulted in improvement to the simulated evaporation. The analysis was concentrated mainly on the 1988 season (May-October) when total evaporation was measured by the energy balance/Bowen ratio method throughout the growing season, covering all stages of canopy development. At the beginning of the 1994 season, soil evaporation were also measured with a ventilated chamber system. The general seasonal dynamics of the evaporation were fairly well simulated with the model. The largest deviation between measured and simulated evaporation occurred in June, when the model underestimated evaporation by about 1 mm day(-1). The model underestimated also in May but not as much as in June. In September and October the performance of the model was very good. For 130 days of the period May-October the cumulated measured evaporation was 364 mm and the simulated evaporation for the same days was 362 mm. It should be pointed out that this result was obtained without calibrating the model against the measured evaporation: The total simulated evaporation for the season was 450 mm with transpiration constituting 298 mm (66%), soil evaporation 102 mm (23%) and interception evaporation 50 mm (11%). The sensitivity analysis showed, in general, that simulated evaporation was most sensitive to changes in resistances when the leaf area index was smallest, i.e. under non-closed canopy conditions. Changes in stomatal resistance, which is one of the most sensitive parameters, with associated changes in canopy transpiration, resulted in a negative feedback effect on soil evaporation. This reduced the total evaporation's sensitivity to stomatal resistance. This type of interaction between canopy and soil or undergrowth fluxes has been observed in other studies as well. Copyright (C) 2001 John Wiley & Sons, Ltd.