Climate Driven Hydrological Performance of Nature-Based Solutions: An Empirical Assessment of a Blue-Green Roof

Nowadays, we must cope with the continuous modifications of stormwater runoff regimes induced by urbanization and climate change. Therefore, it is time to abandon the static approach that relies on single evaluations of events with specific return periods and to adopt a dynamic approach based on continuous data simulation and a resilience paradigm. This concept has been particularly discussed in the context of green roofs (GR). This work aims to define the hydrological performance of a specific Nature-Based Solution (NBS), namely a particular type of GR called a blue-green roof (BGR). This system couples a water storage layer beneath the soil layer to collect infiltrated rainfall and release it gradually. The pilot BGR, named Polder Roof, has been installed in Viterbo, central Italy. A proxy index of water content known as the Antecedent Wet Weather Period (AWWP) is proposed and tested over two years and two months of monitoring to verify its ability to explain BGR performance. The results demonstrate the potential usefulness of AWWP and other easily collected climatic indices for assessing NBS, especially when there is limited data availability for climatic variables. This work aims to assist designers and planners in 1) gaining a better understanding of the hydrological performance of BGR and 2) planning more efficient NBS in the Mediterranean climate.