Combining green roofs and rainwater harvesting systems in university buildings under different climate conditions

Rainwater Harvesting Systems (RWHS) are an alternative solution with the potential to increase water supply security and reduce the pressure on water resources and urban stormwater drainage systems. Likewise, Green Roofs are a nature-based solution with several ecosystem services able to improve well-being in densely urbanized areas. Despite these benefits, the combination of both solutions is still a knowledge gap to be explored. To address this issue, the paper explores the potential of integrating traditional rainwater harvesting systems (RWHS) with extensive green roofs (EGR), while simultaneously evaluating the performance of traditional RWHS in buildings with high and variable water consumption patterns across different climatic conditions. The analyses were carried out assuming two university buildings hypothetically located in three distinct climates (Aw - Tropical Savanna, Cfa - Humid Subtropical, and Csa - Hot-summer Mediterranean). The results show that the relationship between available water and demand is the key factor that defines if the system is most suitable for saving water, reducing stormwater runoff rates, or dual-purpose (when there is a balance between non-potable water supply and capture of stormwater). Combined systems were most effective when there is a balanced rainfall distribution over the year, as in humid subtropical regions. Under these conditions, a combined system designed for dual purposes could potentially have a green roof coverage of up to 70% of the total catchment area. Conversely, climates that have well-defined rainy and dry seasons such as Aw and Csa, may limit the effectiveness of a combined system (RWHS+EGR), being not able to supply water demands during certain periods of the year. However, if the primary objective is effective stormwater management, a combined system should be strongly considered. Because green roofs provide other ecosystem benefits that can contribute to enhancing urban resilience in the face of climate change.