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

Evaporation from a willow short-rotation forest was analysed using a modifi ed version of the Shuttleworth-Wallace model. The main modification consist ed 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 th e threshold value of the leaf area index when scaling up from the leaf to t he 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 thr oughout the growing season, covering all stages of canopy development. At t he beginning of the 1994 season, soil evaporation were also measured with a ventilated chamber system. The general seasonal dynamics of the evaporatio n were fairly well simulated with the model. The largest deviation between measured and simulated evaporation occurred in June, when the model underes timated 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 o f the model was very good. For 130 days of the period May-October the cumul ated 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 obtaine d without calibrating the model against the measured evaporation: The total simulated evaporation for the season was 450 mm with transpiration constit uting 298 mm (66%), soil evaporation 102 mm (23%) and interception evaporat ion 50 mm (11%). The sensitivity analysis showed, in general, that simulate d evaporation was most sensitive to changes in resistances when the leaf ar ea index was smallest, i.e. under non-closed canopy conditions. Changes in stomatal resistance, which is one of the most sensitive parameters, with as sociated changes in canopy transpiration, resulted in a negative feedback e ffect on soil evaporation. This reduced the total evaporation's sensitivity to stomatal resistance. This type of interaction between canopy and soil o r undergrowth fluxes has been observed in other studies as well. Copyright (C) 2001 John Wiley & Sons, Ltd.