EC-measured evapotranspiration (E) and climate data were used to calculate bulk surface conductance (gs) using the inverted Penman-Monteith equation and were compared to gs estimates derived from a biophysical model that permits the partitioning of E into transpiration (ET) and evaporation from the soil (ES). The dependence of leaf stomatal conductance (gstom) on attenuated incoming short-wave radiation through the canopy as a function of leaf area index (LAI) was derived and parameterized using EC data to create a canopy conductance (gc) model (Saugier and Katerji, 1991. Agric. For. Meteorol., 54: 263-277), and was further modified by incorporating gstom dependence on VPD (Leuning et al., 2008. Water Resour. Res., 44, W10419). ES was estimated using the equilibrium evaporation rate. The resulting gc and calculated climatological conductance values were input into a bulk gs model originally derived by Kelliher et al. (1995, Agric. For. Meteorol., 73: 1-16) and later modified using a multiplier (fs) to account for soil moisture effects on ES by Leuning et al. (2008). After applying fs (derived from soil water content data), initial modelled half-hourly values of gs showed excellent diurnal and seasonal agreement with EC-calculated gs, but results indicated the need for a separate canopy soil water function (fc) for gc.
At HP11 measured and modelled growing season (May1-September 31) E values during the 2nd year (2012) were 208 and 223 mm, respectively, where modelled values of ES and ET were 140 and 83 mm, respectively. During the 3rd year of growth, measured and modelled growing season E at HP11 were 235 and 240 mm, respectively, where modelled values of ES and ET were 185 and 55 mm, respectively. Values of water use efficiency (WUE) at HP11 during the growing season, calculated as gross ecosystem photosynthesis (GEP) divided by E, were 1.53 and 2.54 g C kg-1 H2O in the 2nd and 3rd years, respectively. At HP09 growing season EC-measured E of 376, 401, 399 mm exceeded growing season precipitation of 312, 385, 225 mm during the 3rd, 4th and 5th years, respectively, and the respective values of growing season WUE were 1.90, 2.31, 2.03 g C kg-1 H2O. Results suggest that ES dominates plantation E during the first 3-4 years of HP development, and that growth of HP planted on highly productive former agricultural soils in Canada's aspen parkland can become water limited.