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Suburban evapotranspiration estimates in Vancouver from energy balance measurements Kalanda, Brian Douglas

Abstract

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.

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