MODELING EVAPORATION FROM A HIGH SUB-ARCTIC WILLOW BIRCH FOREST

Continuous measurements of the energy balance were made during the 1991 growing season over a dwarf willow-birch forest located near Churchill, Manitoba. Intensive measurements of stomatal conductance for several species were taken on three fair-weather days. These represented a wide range of air temperatures and leaf-to-air vapour pressure deficits and allowed the quantification of the surface-atmosphere interaction. Modelling evaporation on a 0.5 h basis can be performed accurately using a modified version of the Penman-Monteith combination model coupled to a submodel of stomatal conductance. With a vegetated surface cover of about 90 per cent at full leafing, vegetation plays an important role in the overall moisture flux because 80 per cent is a result of transpiration. Simulating various vegetation-change scenarios shows that species composition, through differences in stomatal behaviour, has a marked effect on evaporation.