Modelling evapotranspiration at three boreal forest stands using the CLASS: tests of parameterizations for canopy conductance and soil evaporation

Abstract

AbstractThe performance of the Canadian Land Surface Scheme (CLASS) was evaluated in off‐line runs, using data collected at three boreal forest stands located near Thompson, Manitoba: young jack pine, mature jack pine, and mature black spruce. The data were collected in the late spring through autumn of 1994 and 1996, as part of the Boreal Ecosystem–Atmosphere Study (BOREAS).The diurnal range in modelled soil heat flux was exaggerated at all sites. Soil evaporation was modelled poorly at the jack pine stands, with overestimation common and a step change to low evaporation as the soil dried. Replacing the soil evaporation algorithm, which was based on the estimation of a surface relative humidity value, with one based on soil moisture in the top soil layer reduced the overestimation and eliminated the step changes. Modelled water movement between soil layers was too slow at the jack pine stands. Modifying the soil hydraulic parameters to match an observed characteristic curve at the young jack pine stand produced a soil water suction that agreed more closely with measurements and improved drainage between soil layers.The latent heat flux was overestimated and the sensible heat flux underestimated at all three stands. New Jarvis–Stewart‐type canopy conductance algorithms were developed from stomatal conductance measurements. At the jack pine stands, stomatal conductance scaled by leaf area index reproduced canopy conductance, but a reduction in the scaled stomatal conductance by one half was necessary at the black spruce stand, indicating a nonlinearity in the scaling of stomatal conductance for this ecosystem. The root‐mean‐squared error for daily average latent heat flux for the control run of the CLASS and for the best test run are 49 W m−2 and 14 W m−2 respectively at the young jack pine stand, 50 W m−2 and 15 W m−2 respectively at the old jack pine stand, and 48 W m−2 and 13 W m−2 respectively at the old black spruce stand. Copyright © 2003 Royal Meteorological Society