The combined performance of green roof and rainwater harvesting: retention capacity and water saving

Population growth and migration to urban spaces are influencing the management of the urban water cycle, both in terms of supply and drainage, driving a more holistic management of the urban water cycle, combining hard centralized engineering infrastructures with soft decentralized nature-based solutions. Among the latter, rainwater harvesting (RWH) and green roofs are solutions that have been most encouraged by some municipalities around the globe at the building level. The present study describes a methodology to assess the combined performance of RWH and green roofs in terms of water savings and rainwater retention. The methodology can be easily applied and is compatible with the format in which the required data are usually available. The methodology is applied to the city of Turin, in Italy, and is used to develop generalized relations that allow estimating the performance of RWH systems with traditional and green roofs, based on two ratios that contain all the relevant characteristics of the rainwater harvesting system (water consumption, collection area and tank capacity).