Downscaling potential evapotranspiration to the urban canyon
Abstract. The future increase in urban population will lead to progressing urbanization with urban sprawl and densification. Urbanized areas show distinct changes in their hydrological behaviour, water quality and climate. In the last decades, the ability of urban hydrological models to represent the dynamic hydrological behaviour of the different surface types has been improved continuously. Dissenting from the urban surface which is mostly represented in high spatial resolution, the climatic input to these models, such as precipitation and potential evapo(transpi)ration, is usually observed at one or several reference climate stations that are representing a mesoscale urban foot print area or rural conditions. From urban climate studies it is known, that the meteorological variables that are governing potential evapotranspiration (Ep) can be highly variable even on a small spatial scale. Consequently, we expect Ep at the street level to be affected by this variability as well.
We observed the urban microclimate with a mobile climate station and a rotational principle at 16 different locations in two differently oriented street canyons with vegetated and non-vegetated sections, respectively, during three seasons (spring, summer, autumn) in Freiburg, in southwestern Germany. With these observations, we simulated Ep at the street level using FAO-56 Penman-Monteith reference evapotranspiration and compared it to reference Ep derived at a rooftop station. We found that Ep on street level is negatively influenced by changes in shortwave radiation and that it is barely sensitive to changes in the other input climate variables. Significant linear relationships between the relative differences in hourly and daily short-wave radiation input and Ep at the street level have been established. The application of these relationships allows to simulate Ep at the street level for any location in a city based on simulated (or observed) short wave time series and observations at a reference climate station. Our findings can be transferred easily to existing urban hydrologic models to improve modelling results with a more precise estimate of potential evapotranspiration on street level.
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