Assessment and Modeling of the Hydrological Response of Extensive Green Roofs Under High-Intensity Simulated Rainfalls

Rainfall retention and runoff detention are the key hydrological processes that reduce runoff from green roofs. This study aims to quantify and model the hydrological response of nine combinations of growing substrates and drainage layers for extensive green roofs. Retention and detention capacities were evaluated using laboratory column experiments under two extreme initial moisture conditions—air-dried (D) and field capacity (W)—and three rainfall intensities (30, 60, and 100 mm h−1). Regardless of the substrate–drainage combination, retention capacity, WR, was significantly higher under dry conditions than under wet ones. Under wet conditions and rainfall intensity of 30 mm h−1 (30 W tests), the mean WR value (5.2 mm) was significantly lower than those recorded at higher intensities (14.3 and 14.2 mm, for 60W and 100Wtests, respectively). Detention capacity,WD, was less influenced by initial moisture and rainfall intensity, with mean values ranging from 7.4 to 10.9 mm. The distinct hydrological responses of green roof columns in the two antecedent moisture conditions were attributed to contrasting infiltration mechanisms: capillary flow dominated under dry conditions, while gravity-driven preferential flow prevailed under wet conditions. The application of a simple reservoir-routing model revealed that the AgriTerram (AT)—expanded perlite (EP) combination achieved the greatest reduction in total outflow volume and peak runoff. Under wet initial conditions, no single configuration clearly outperformed the others. This study highlights how the combined use of simulated rainfall experiments and a reservoir-routing model enables the identification of the most effective combination of substrate and drainage system to improve the hydrological performance of green roofs.