Controls on evaporation in a boreal spruce forest

The surface energy balance over a boreal spruce forest is analyzed using 3 yr of 30-min-averaged data collected during the 1994-96 Boreal Ecosystem-Atmosphere Study experiment 40 km west of Thompson, Manitoba, to show the climatic controls on surface evapotranspiration. The seasonal variation of evaporation is shown: lowest in spring when the ground is frozen, highest in summer (although daytime evaporative fractions are only 0.4), and lower again in fall after frost. The surface sensible heat flux in contrast is high in spring, when evaporation is low. Evaporation is much higher when the surface, including the moss layer, is wet. At all temperatures (in summer), evaporative fraction falls with increasing light level, because of the high vegetative resistance of the forest system. Using a Monin-Obukhov formulation and a bulk vegetation model, the vegetative resistance for the boreal spruce forest system is calculated. This bulk vegetative resistance decreases with increasing photosynthetic radiation, decreases sharply with relative humidity, decreases with increasing surface water storage, and is lower on cloudy days than on sunny days with the same incoming photosynthetic radiation. Vegetative resistance at its midmorning minimum is lower by a factor of 4 when the moss surface is very wet. As over grassland sites, the lower surface resistance to evaporation directly influences the diurnal cycle of lifting condensation level and cloud-base height, which are much lower on days with a wet surface. The reduction of vegetative resistance under cloudy skies at the same incoming radiation level presumably reflects the more efficient use of diffuse radiation by the canopy for photosynthesis. Vegetative resistance is roughly doubled in spring, when the ground is frozen, and is higher in fall after frost. About 63% of the observed variance in vegetative resistance can be explained in terms of meteorological variables using multiple linear regression. Some measurement issues are addressed in an appendix. The residual in the energy balance falls with increasing wind speed, which may be due to a small (10%-15%) underestimation of the sensible and latent heat fluxes at low wind speeds. During spring melt, however, this residual has a high daytime value of 30% of net radiation. The residual is also much higher on sunny days than on cloudy days.