Abstract/Summary

Green roofs can mitigate the flood risk by reducing the volume of runoff through direct interception and subsequent evapotranspiration (ET), but the planting choices can influence the extent of this service. Glasshouse experiments were carried out in spring/summer using simulated rainfall to compare the rainfall retention capacity of three physiologically active broadleaf species (Heuchera micrantha, Salvia officinalis and Stachys byzantina), which have previously shown to provide improved rooftop cooling, to an industry standard green roof species, Sedum spurium. Furthermore, the impact of varying ambient temperature and humidity conditions on the ability of these species to restore the substrate retention capacity through ET was also tested in a series of controlled-environment experiments simulating a range of potential UK summertime scenarios. Canopies alone retained up to 17% (Sedum) of the total rainfall in this study, with Salvia and Stachys also retaining in excess of 10%, and can make a substantial contribution to rainfall retention on a green roof. Rainfall retention was also strongly correlated with total ET in the preceding 72 hours (R2 = 0.94; P < 0.001). Species with high ET rates (Salvia and Stachys) were able to provide the greatest stormwater management service (up to 72% retention due to ET component). Furthermore, species ‘rankings’, in terms of ET and thus restoration of substrate retention capacity, were the same in all simulated potential UK summertime temperature and relative humidity scenarios, indicating that ‘superior’ species will be able to provide the greatest stormwater management provision in all climatic conditions.