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 radiat... Abstract 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 radiat...