Green roof retention performance under extreme current and future climate conditions in Italy

Due to the combination of intense urbanization and climate changes, cities are becoming more prone to pluvial floods. Among the different nature-based solutions proposed to ensure sustainable urban development, green roofs have shown high potential in mitigating the runoff generation during rainfall events. Beyond the design structure, the retention performance of these solutions is strongly influenced by the climate variability and could change when considering future climatic conditions. In this context, this work aims to evaluate the capacity of green roofs in mitigating runoff generation under current and future climate conditions in Italy, with a special focus on intense rainfall events. Three Italian cities (i.e., Cagliari, Palermo and Torino), characterized by different rainfall and temperature regimes, have been chosen as case studies. Two future scenarios have been investigated (RCP4.5 and RCP8.5), considering climate projections derived by dynamically downscaling the global climate model, proposed by the Euro-Mediterranean Research Center on Climate Changes, bias corrected with a quantile mapping approach. A standard extensive green roof has been modeled using a conceptual ecohydrological model at hourly scale, testing the performance variability across different design structures. Results confirm the full hydrological efficiency of green roofs under current climate conditions, with a median retention rate equal to 1 in all cities. Under future climate conditions, due to the foreseen intensification of rainfall events, the green roofs performance tends to slightly decline. The overall high retention capacity and the multiple benefits confirm green roofs as a key solution for the creation of sustainable and resilient cities.