Hydrological performance of Sedum species compared to perennial herbaceous species on a full-scale green roof in Italy

Although the ability of green roofs as stormwater source control technologies has been the topic of many recent studies, most conclude that a better understanding of full-scale hydrological performance and of the impact of different plant cover types is still needed. Based on these considerations, this research presents the results of 15 months of monitoring activity in Bologna (Italy) studying the hydrological performance of two adjacent full-scale extensive green roof plots with a different plant cover composition. One of the plots (SR) was planted with Sedum species (CAM) and the other (NR) with native herbaceous perennial species (C3). Continuous rainfall, runoff and weather data were collected and used to compare the effect of changing plant cover type on the green roof retention rate. Over the 15 months of this study, NR presented a 59% retention rate (111 valid events) whereas SR had a 52% retention rate (91 valid events). For 24 common events, with retention lower than 100%, the average difference between the NR and SR plot retention was 10%, confirming the higher water storage capacity, and therefore the higher retention rate, of the NR plot.