MODELING EVAPORATION FROM A HIGH SUB-ARCTIC WILLOW BIRCH FOREST

Continuous measurements of the energy balance were made during the 199 1 growing season over a dwarf willow-birch forest located near Churchi ll, Manitoba. Intensive measurements of stomatal conductance for sever al species were taken on three fair-weather days. These represented a wide range of air temperatures and leaf-to-air vapour pressure deficit s and allowed the quantification of the surface-atmosphere interaction . Modelling evaporation on a 0.5 h basis can be performed accurately u sing a modified version of the Penman-Monteith combination model coupl ed to a submodel of stomatal conductance. With a vegetated surface cov er 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 th at species composition, through differences in stomatal behaviour, has a marked effect on evaporation.