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An evaporatranspiration-interception model for urban areas Grimmond, Christine Susan


This study presents a model to calculate evapotranspiration from urban areas for use over a wide range of meteorological conditions. The evapotranspiration-interception model is a modified version of those attributable to Penman-Monteith (1965), Rutter et al. (1971) and Shuttleworth (1978). Sub-models to calculate anthropogenic heat flux, storage heat flux, aerodynamic resistance, surface resistance and drainage are developed. The model and its sub-components, where possible, are tested against measured data from a suburban site in Vancouver, B.C. This is the first extended set of winter/spring time energy balance measurements for a city. It shows that the latent heat flux can be the most important output flux of the energy balance in wintertime. The spring energy balance is similar in form to that in summer. Construction of valid energy balances for spatially-non-uniform surfaces such as cities requires that all component fluxes represent identical source areas. Given that the radiative field is relatively uniform in space it is appropriate to match all fluxes to the source areas for the turbulent fluxes since these are very variable. A scheme to match areas is developed using the Schmid (1988) model for the turbulent source areas and a geographic information system containing the surface characteristics necessary to calculate the spatially-corresponding anthropogenic and storage fluxes. The anthropogenic heat flux at a point is more variable in space than time. Therefore the calculated flux is strongly influenced by the method used to identify the source area. A test of storage heat flux models concludes that the objective hysteresis model of Cleugh (1988) performs well, especially during the daytime. The model introduces an appropriate temporal asymmetry in the storage flux. The surface resistance model developed for the city is a modified version of that suggested by Jarvis (1976). The performance of the model is good and resembles that obtained for similar models applied to forests. Comparison of modelled and measured evapotranspiration using the complete model shows this to be a promising method which is capable of providing realistic hourly and daily estimates of the areally-averaged latent heat flux and surface water state in urban areas.

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