Simulated evapotranspiration from the Norunda forest stand during the growing season of a dry year

A SVAT model was used to simulate evaporation from the Norunda forest stand during the dry growing season of 1994. Daily mean values of meteorological data were used as input and compared with actual data on soil moisture, tr anspiration and total evaporation. The soil moisture data of TDR were used as a basis for defining combinations of parameters values that resulted in acceptable agreement. Different threshold for reduction of transpiration du ring the dry spell in July resulted in small changes in soil moisture dynam ics. Using also sap-how measurements the final estimated value of -150 hPa as the threshold value below which water uptake is reduced was found in com bination with a flexibility coefficient of 0.7 for compensatory water uptak e between soil horizons. The agreement with daily rates of evaporation was good, and coefficients of determination of 0.44, 0.78 and 0.89 were obtaine d when comparing with all days of eddy-flux measurements, days without inte rception losses with eddy-flux measurements and sap-flow measurements of ca nopy transpiration, respectively. The total evaporation from May to the end of October was estimated to be 314 mm, of which interception losses, canop y transpiration and soil evaporation accounted for 70, 181 and 63 mm respec tively. However, the eddy-flux measurements indicated that the total sum ha d been underestimated, and the sap-flow measurements suggested that canopy transpiration had been slightly overestimated. The high correspondence betw een simulated and measured fluxes was surprising considering the heterogene ity involved and simplistic parameterization of the model. In future studie s the resolution of input data to the model will be higher, and spatial var iation will be explicity represented when simulating evaporation from this ecosystem. (C) 1999 Elsevier Science B.V. All rights reserved.