Performance of the Canadian land surface scheme at temperate aspen‐birch and mixed forests, and a boreal young jack pine forest: Tests involving canopy conductance parametrizations

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Abstract
The Canadian Land Surface Scheme (CLASS) was evaluated using off‐line tests, with data collected at temperate aspen‐birch and mixed forests located near Chalk River, Ontario, and at a young jack pine forest located near Thompson, Manitoba. Measurements of stomatal conductance and leaf area index were used to develop canopy conductance parametrizations for the aspen‐birch and the jack pine stands, and these were incorporated into CLASS in a modified run. The behaviour of the residuals from the unmodified run with respect to environmental variables at the mixed stand were used as a guide to producing a modified ad‐hoc canopy conductance parametrization for use at that site. It was found that CLASS exaggerates the diurnal range in soil heat flux. This leads to an underestimation of the diurnal range in available energy, and it is argued that this has implications for its partitioning into the turbulent fluxes of sensible and latent heat. Over time, systematic errors will accumulate which, in the case of the latent heat flux, can affect the long‐term water balance. Errors in the size of the turbulent fluxes over short time periods can limit the usefulness of CLASS for resolving mesoscale circulations in a regional model. These problems are believed to be related to CLASS's assumption of zero wind speed under the canopy, and the lack of an insulating litter layer at the soil surface. An unmodified CLASS produced estimates of sensible and latent heat fluxes that broadly agreed with measured values at the temperate stands but not at the jack pine stand, where the latent heat flux was significantly overestimated. The modified canopy conductance parametri‐zations improved the representation of the latent heat flux at all of the sites, particularly at the jack pine stand. Overestimation at the aspen‐birch stand may be related to site heterogeneity not being adequately represented in the measurement of stomatal conductance, or to difficulties in defining the vertical distribution of leaf area index for scaling stomatal conductance to the canopy level. As expected, a positive or negative bias in the modelled latent heat flux is associated with an opposite bias in the modelled sensible heat flux. The tendency towards positive intercepts and slopes less than unity in linear regressions relating modelled and measured fluxes is a symptom of the underestimation of the diurnal range in available energy. Recommendations are made to allow for vertical discontinuities in texture within a soil layer and a variable permeable depth, and to allow transpiration when the soil water suction exceeds –1.5 MPa. This is particularly important when soil moisture or texture is heterogeneous. Finally, it is suggested that volumetric soil moistures should be allowed to fall below the originally prescribed limiting value of 0.04. This is necessary for evaporation to continue in very sandy, dry soils.