Runoff and vegetation stress of green roofs under different climate change scenarios

Abstract Green roofs are efficient tools to improve life quality in densely populated areas. The contribution to the reduction of stormwater runoff and the heat-island effect may even augment with climate change. In this study, impacts of climatic changes on vegetation drought stress and stormwater retention of two green roof types (one with sedum-moss vegetation, one with grass-herbs vegetation) and a standard bitumen roof were assessed using a water balance model (GreenRoof). A case study of the Flemish region in Belgium was done, but the study can be representative for temperate maritime climates worldwide. Local weather data for the 2050s were generated with the LARS-WG weather generator based on climate scenarios from two multi-model ensembles of respectively 15 global and 9 regional climate models. Under the projected differences in precipitation and evaporating power of the atmosphere, runoff was expected to decrease in summer and to increase in winter on all roof types. Grass-herb green roofs could reduce runoff more than sedum-moss green roofs, but were more sensitive to increased drought stress. Both green roof types were shown to remain a valid option for runoff reduction in urban areas as compared to bitumen roofs, mainly in summer. The benefit of green roofs can increase under climatic changes but increased vegetation stress urges reconsideration of green roof design and vegetation choice to obtain an optimal runoff reduction and vegetation survival.