Mitigation potential of urban greening during heatwaves and stormwater events: a modeling study for Karlsruhe, Germany

Climate change is increasing the frequency and intensity of urban heat islands and stormwater flooding. In order to mitigate these threats cities are turning toward green infrastructure to restore the hydrologic cycle in a way that increases the ecosystem services provided by trees. Strategically designed green infrastructure can mitigate runoff volume by rainfall interception through tree canopies and redirect impervious runoff into bioswales that promote infiltration. In addition, urban greens mitigate extreme heat via evapotranspiration and shading. Here we applied the i-Tree HydroPlus model to the German city of Karlsruhe and its twenty-seven districts with varying initial conditions of tree cover to analyze the potential for both runoff and heat mitigation during dry and wet periods throughout a 5-year period. After analyzing initial tree cover and drainage conditions, we used the model to simulate a green infrastructure scenario for each district with restored hydrology and tree cover at 30%. Regarding trade-offs between runoff and heat mitigation, the results confirm that dry soils before storm events lead to greater runoff reduction by 10%, and wet soils prior to heatwaves resulted in a greater evaporative cooling. Compared to current conditions, the green infrastructure scenarios resulted in decreasing the number of extreme heat hours (Heat Index > 31 °C) per year on average by 64.5%, and to reduce runoff in average by 58% across all city districts. Thus, our simulation results show that investing into a greener infrastructure, has positive impacts on microclimate and hydrology. Finally, we discuss synergies and trade-offs of the investigated management options as well as the transferability of results to other cities.