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

Jansson PE, Cienciala E, Grelle A, Kellner E, Lindahl A, Lundblad M


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, transpiration 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 during the dry spell in July resulted in small changes in soil moisture dynamics. 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 combination with a flexibility coefficient of 0.7 for compensatory water uptake 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 obtained when comparing with all days of eddy-flux measurements, days without interception losses with eddy-flux measurements and sap-flow measurements of canopy transpiration, respectively. The total evaporation from May to the end of October was estimated to be 314 mm, of which interception losses, canopy transpiration and soil evaporation accounted for 70, 181 and 63 mm respectively. However, the eddy-flux measurements indicated that the total sum had been underestimated, and the sap-flow measurements suggested that canopy transpiration had been slightly overestimated. The high correspondence between simulated and measured fluxes was surprising considering the heterogeneity involved and simplistic parameterization of the model. In future studies the resolution of input data to the model will be higher, and spatial variation will be explicity represented when simulating evaporation from this ecosystem. (C) 1999 Elsevier Science B.V. All rights reserved

Publicerad i

Agricultural and Forest Meteorology
1999, Volym: 98-9, sidor: 621-628