Event size, substrate water content and vegetation affect storm water retention efficiency of an un-irrigated extensive green roof system in Central Texas

As green roofs continue to grow in popularity more research will be needed in new ecoregions to support development of policy, regulation, and incentives. Extensive green roofs represent the majority of new construction each year and understanding their performance expectations is critical for regulation. In our experiment we tested the ability of three monocultures of low stature CAM plants (Sedum kamtschaticum, Delosperma cooperi and Talinum calycinum syn. Phemeranthus calycinus) to improve runoff reduction efficiency of un-irrigated modular extensive green roofs exposed to a range of precipitation event sizes in a warm and dry southern US climate. Stormwater runoff data were collected for 15 rainfall events that ranged from a minimum of 4.1 mm to a maximum of 102.9 mm. An average precipitation event retention efficiency of 78% was recorded. On average, the presence of T. calycinum enhanced retention efficiency by an additional 7.5% compared to unvegetated modules. Substrate volumetric water content affected retention capacity of unvegetated modules only.