Rainwater retention capacity of green roofs in subtropical monsoonal climatic regions: a case study of Taiwan

This study investigates how roof substrate and vegetation affect rainwater retention. The investigated parameters are lightweight substrate ratio, substrate depth, precipitation and vegetation type. A planting box was placed on the roof of a building to simulate a green roof. Scheduled artificial rainfall was used to test the rainwater retention capacity of various substrates, substrate depths and plant types. The experimental results indicate that precipitation, substrate depth, substrate ratio and vegetation type affect the rainwater retention capacity of green roofs. The rainwater retention rate is inversely proportional to precipitation intensity; that is, about 87-100% of rainwater is retained for light rain, 62-84% for moderate rain and only 26-33% for heavy rain. Different plants have different abilities to retain rainwater. In this study, Dianella ensifolia cv. 'Silvery Stripe' and Schefflera arboricola have the best water retention rates of 37-100%. Of the total amount of rainwater retained, the substrate accounts for 77-98% and vegetation accounts for 2-23%. In the sub-tropical region, the green roof water retention rate is roughly 30% of the total storm precipitation (100 mm). Thus, using a green roof is an effective strategy for managing urban stormwater.