Cooling potential of green spaces in the Vienna metropolitan area during extended periods of drought

Heat stress during heat waves in urban areas can have devastating effects on human health. To mitigate this heat stress, major efforts are being made to plant more vegetation and remove sealed surfaces. But many green roofs and facades are not irrigated, which in turn leads to drought stress for plants during prolonged periods of heat in the summer without precipitation. As a result, evapotranspiration from vegetated areas is reduced and the desired cooling effect is not achieved. During dry periods, the agricultural environment also cannot develop its full cooling effect even during the day.

In the project Imp_DroP (Impact of longer Drought Periods on Climate in Greater Vienna: appropriate Mitigation measures), the cooling potential of green spaces in and around Vienna is to be determined via evapotranspiration and future irrigation requirements. Thus, four measurement sites were established on green roofs in different local climate zones of Vienna for two years in spring 2022. At each of the selected sites, two lysimeters were installed for extensive and intensive green roofs, as well as soil moisture sensors in the upper and lower soil layers.

These measurement data from 2022 are used in a further step for the calibration of the crop growth model AquaCrop, and the data collected in 2023 are taken for validation. Therefore, soil moisture can be simulated for selected dry periods on green roofs. For the surrounding agricultural areas of Vienna, soil moisture is simulated using the ARIS model.

All collected data are applied in a final step to initialize, run and validate the coupled WRF-TEB model. Here, the atmospheric conditions as well as the urban microclimate for current and future summer drought episodes are simulated to estimate the expected thermal stress for the Viennese population.