The paper reports on some evaporation measurements made above a spruce forest (Picea abies) during late August and the beginning of September 1991. The period was dry, and the response of the trees to this condition is clearly seen in the form of the diurnal course of the evapotranspiration. The trees close their stomata to limit the water loss. This is also shown through a calculation of the canopy resistance to water vapour exchange. During periods around noon and in the afternoon the resistance becomes very high. The calculations are carried out by using a novel theory for the viscous sublayer resistance to atmosphere canopy exchange.