Response of a Sedum green-roof to individual rain events

Precipitation and runoff data from several controlled experiments (with dry and wet initial conditions) on a Sedum album green-roof have been analysed by means of linear programming in order to estimate a unit hydrograph (UH). The obtained UH was able to accurately predict peak flows and runoff volumes for any rain input. Results from the experiments indicated that roof slope had no effect on the direct runoff hydrograph, i.e., on peak flows and stormwater volumes. Whether conditions were dry or wet affected the retention capacity of the green-roof; for dry conditions, between 6 and 12 mm of rain were required to initiate runoff, while for wet conditions the response was almost straight.