Saturation-excess and infiltration-excess runoff on green roofs

Green roofs (GRs), as compared to conventional roofs, can retain a considerable amount of water in the soil layer and hence have been used in many urban areas to mitigate urban flooding. However, a simple yet physical model for describing the rainfall (P)-runoff (R) relationship over GRs is still lacking. In this study, a physically-based P–R relationship, which utilizes soil moisture measurements that are often available in field experiments, is proposed based on the water balance equation over flat and horizontally homogenous GRs and evaluated against field measurements. First, the two different runoff generation mechanisms on GRs, namely, saturation-excess (runoff is generated when the soil becomes saturated) and infiltration-excess (runoff is generated when the rainfall intensity is larger than the infiltration rate), are discussed. A water balance analysis is then performed to obtain a physically-based P–R relationship over flat and horizontally homogenous GRs, which is validated using measurements from a field experiment conducted over a GR site in Beijing, China. Results show that our P–R relationship is able to estimate the runoff generated on our GR site. The proposed P–R relationship is also tested against other observational data and empirical models in the literature and shows broad consistency with these previous studies. To further quantify the relative importance of saturation-excess runoff and infiltration-excess runoff, numerical simulations are carried out using HYDRUS-1D. The simulation results indicate that runoff at our GR site is generated by both saturation-excess and infiltration-excess. Nonetheless, the infiltration-excess runoff accounts for a small portion of the total runoff, which suggests that the saturation-excess mechanism is more important for generating runoff over GRs.