A comparison of parametrizations of canopy conductance of Aspen and Douglas-fir forests for CLASS

Canopy conductance (g(c)) of an old boreal aspen forest and a west coast Do uglas-fir forest was calculated from the inversion of the Penman-Monteith ( PM) equation with above-canopy water vapour flux measurements. Values of as pen g(c), agreed reasonably well with those obtained by scaling up from lea f stomatal conductance measurements. Comparison of values of g(c) obtained from the CLASS (Canadian LAnd Surface Scheme) parametrization with values o f Douglas-fir g(c) in 1983 and 1984 calculated from the PM equation showed that the CLASS parametrization (based on the Jarvis-Stewart (JS) model) wor ked well at high soil water potential (psi), but underestimated g(c) at low psi. In the case of the aspen forest during a wet growing season in 1994, the CLASS parametrization underestimated g(c) ,for high values of incident photosynthetic photon flux: density. The effectiveness of three parametrizations of g(c), developed using linear or non-linear least squares analysis, was evaluted for the two forests. Th e first (based on the JS model), related g(c) to the product of several ind ependent limiting functions, the second (based on the Ball-Woo-drow-Berry ( BWB) model related g(c) to the product of canopy net assimilation rate and canopy surface relative humidity divided by canopy surface CO2 concentratio n and the third (based on a modifed form of the BWB (MBWB) model) was the s ame as the second except that the relative humidity was replaced by the rec iprocal of air vapour pressure deficit. For both forests, the JS parametriz ation gave the highest r(2) and lowest root mean square (RMS) error The RMS error of the MBWB parametrization was less than that of the BWB parametriz ation because the latter underestimated g(c) during the morning. With the i ncorporation of the new JS and MBWB parametrizations into CLASS, better est imates of the latent heat flux (Q(E)) from the aspen and Douglas-fir forest s were obtained on half-hourly and daily bases than with the original CLASS parametrization. The JS parametrization gave better estimates than the MBW B parametrization. Both models parametrized using 2994 data from the aspen forest were successfully applied to the same stand in 1996 which also had a relatively wet growing season. Both models parametrized using data from th e Douglas-fir forest were also applied to four other similar-aged Douglas-f ir forests but with different values of the leaf area index. Under conditio ns of minimal,vater stress, better estimates of Q(E) were obtained for thre e of the four forests using both parametrizations. In the case of the fourt h forest, none of the parametrizations gave satisfactory estimates. This wa s likely because the initial conditions of soil water content and yr used i n CLASS for the gravelly soil was significantly overestimated as a result o f not inking the stone content into account. For conditions of high water s tress, which occurred in two of the forests, none of the parametrizations g ave satisfactory estimates. However when the psi limiting function in the J S parametrization was replaced by that developed from measurements made in the other two forests, the JS parametrization gave reasonable estimates of Q(E) In the case of the MBWB parametrization we were unable to adjust the p si limiting function due to the lack of measurements of canopy net assimila tion rate at these two sites.