Green Roofs in the Mediterranean Area: Interaction between Native Plant Species and Sub-Surface Runoff

Abstract

Green roofs (GRs) may represent a solution for minimizing the impact of urbanization on the hydrological cycle and for sustainably managing water resources in urban environment. Several studies have shown that GRs effectively control the surface runoff conveyed into the combined sewer systems, by reducing overall volumes and peak flow rates. The hydrological/hydraulic performance of a GR is strongly dependent upon vegetation species used. In this study the influence of plants on the sub-surface runoff is investigated by monitoring two experimental GRs in wet weather conditions. It is hypothesized that although plants reduce sub-surface runoff, they may also boost the formation of preferential flow paths. The results showed that the sub-surface runoff is strongly influenced by vegetation. While for smaller storm events the GR highly retains stormwater due to the plant interception, for larger storm events the GR’s performance decreases due to the increase of sub-surface runoff. This may be due to the presence of the plant root which produces preferential macropores flow through the substrate.