EVAPOTRANSPIRATION AND SURFACE CONDUCTANCE IN A HIGH ELEVATION, GRASS-COVERED FOREST CLEAR-CUT

Pinegrass is the dominant species in clearcuts in the dry southern interior of British Columbia and is the major competitor with coniferous seedlings. This paper examines water use in such a clearcut during two successive growing seasons and the response of surface conductance to environmental variables. Evaportranspiration was derived from eddy correlation measurements of sensible heat flux density and measurements of net irradiance and soil heat flux density. The Penman-Monteith equation was used to calculate hourly surface conductances of the clearcut. Leaf area index measurements were used to calculate mean stomatal conductances from surface conductances. Mean stomatal conductance was modelled using boundary line and non-linear least squares optimization techniques. The most successful model, developed from data from the first growing season and tested on measurements from the second growing season (R2 = 0.70), was obtained using non-linear optimization with mean stomatal conductance as a function of saturation deficit at the leaf surface (D0) and solar irradiance. Mean stomatal conductance was also modelled as a function of saturation deficit at 1.3 m (D) and solar irradiance. This model was less successful (R2 = 0.62) than that based on D0. Saturation deficit and temperature were highly correlated at 1.3 m and at the leaf surface. No relationship between mean stomatal conductance and air temperature or volumetric soil water content was found. Hourly evapotranspiration rates during the second growing season, calculated using stomatal conductance models (developed from data from the first growing season) based on either D or D0 agreed well with measured rates (R2 almost-equal-to 0.8). Hourly evapotranspiration rates were highly correlated with hourly equilibrium evaporation rates (R2 = 0.84). The mean hourly value of the Priestley-Taylor coefficient-alpha was found to be 0.81.