Seasonal variation of boreal forest surface conductance and evaporation

Long term measurements (June 1994 to December 1996) of evaporation were made in a boreal forest in central Sweden. Fluxes were measured continuously with eddy-correlation systems from a 100 m tower. Surface conductance and potential evaporation were estimated using the Penman-Monteith equation. The overall average evaporation during the 947-days observation period was 1.07 mm d(-1). The average evaporation from June to December 1994 was 1.25 mm d(-1), 1.07 mm d(-1) during 1995, and 0.97 mm d(-1) during 1996. Maximum daily rates were typically 4 mm d(-1) around mid-summer in 1994 and 1995 and slightly less in 1996. During the winter period from November to March, the evaporation sometimes reached 0.5 mm d(-1). Generally, the actual evaporation followed the dynamics of the potential evaporation fairly well. As a total for the entire period, the actual evaporation accounted for 918 mm, or 69% of the potential evaporation (1332 mm). There were three major time periods with a considerable evaporation deficit: the late summer (July-August) 1994, the late summer (August) 1995, and the spring to early summer (March-June) 1996 (Table I). The first two periods were correlated with low soil moisture while the last probably was caused by low winter temperatures. A surface conductance model showed little dependence on soil moisture. Radiation and vapour pressure deficit were the most important factors. When soil water was not limiting, the diurnal courses of surface conductance showed a steep increase in the morning followed by an almost linear decrease, the coniferous trend caused by stomatal control. During periods of water stress a midday depression occurred after the morning maximum. During wintertime, the surface conductance hardly ever approached to zero. A simple model for surface conductance based on global radiation and vapour pressure deficit fitted the estimated data fairly well except for calm nights with poor turbulent mixing. This was apparently due to water vapour storage within and above the canopy. Including the friction velocity in the model parameterisation improved the fitting considerably. (C) 1999 Elsevier Science B.V. All rights reserved.