Evapotranspiration from a forest clearcut was measured over two growing seasons as part of a larger study of the microclimate of forest clearcuts and microclimate modification by site preparation. Pinegrass is the dominant species on clearcuts in the dry southern interior and is the major competitor with coniferous seedlings. This paper examines the water use of a pinegrass dominated clearcut and the response of surface conductance to environmental variables. Evapotranspiration was derived from eddy correlation measurements of sensible heat flux and measurements of net radiation and soil heat flux. 419 hours of daytime energy balance data from the summers of 1987 and 1988 were analyzed. A rearranged form of the Penman-Monteith equation was used to calculate hourly mean surface conductances for the clearcut. Leaf area measurements were used to calculate stomatal conductance from surface conductances. Stomatal conductance was modelled using boundary-line and non-linear optimization techniques. The most successful model (R² = 0.71) was obtained using non-linear optimization with stomatal conductance as a non-linear function of saturation deficit at the leaf surface (D₀) and solar irradiance. D₀ was calculated from measured evapotranspiration and surface conductance. Response of stomata to saturation deficit would be expected to be better correlated to D₀ than D measured at a reference height above the canopy. Stomatal conductance was also modelled as a function of D (measured at 1.3 m) and solar irradiance. The resulting model (R² = 0.50) was poor compared to that based on D₀. Saturation deficit and temperature were found to be highly correlated both at 1.3 m above the canopy and at the leaf surface. Use of air temperature in the conductance model caused R² to decrease. No relationship between stomatal conductance and volumetric soil water content was found. Hourly evapotranspiration rates calculated using modelled surface conductances agreed well with measured rates.(R² = 0.89). Evapotranspiration was also modelled using the Priestley-Taylor approach. The mean hourly a for all daylight data was found to be 0.81. This simple model was found to give comparable results to the stomatal conductance based model (R² = 0.85). / Land and Food Systems, Faculty of / Graduate

This study is concerned with the energy balance of a suburban area of south central Vancouver and in particular with the role of evapotranspiration in this balance. In the late summer - early fall of 1977 a measurement program was conducted to determine the energy balance components using the Bowen ratio - energy balance approach. The Bowen ratio was obtained from differential psychrometric measurements taken above mean roof-level. Net radiation was measured directly and the volumetric heat storage was parameterized in terms of net radiation. The results indicate that the Bowen ratio - energy balance approach is applicable to suburban environments. An error analysis developed for the reversing psychrometer system indicates that the errors in the turbulent fluxes were typically 10 - 20%. The turbulent latent heat flux was always a significant and often the dominant energy sink for this 'surface'. This is shown to be largely due to soil moisture replenishment by precipitation and irrigation (especially lawn sprinkling). The turbulent fluxes tended to be in-phase with net radiation during the day. This appears to be a result of the decreasing importance of non-radiative controls (especially the vapour pressure deficit) on evapotranspiration as the land use changes from rural to heavily urbanized. Sustained periods of positive turbulent fluxes were recorded at night, however the Bowen ratio was predominantly negative indicating that only one turbulent flux was positive. The data do not reveal any dependence on wind direction. The influence (if any) of the sea breeze could not be isolated. The equilibrium evapotranspiration rate often closely approximated the measured evapotranspiration. / Arts, Faculty of / Geography, Department of / Graduate