Modeling the hydrological benefits of green roof systems: applications and future needs

Green roof systems (GRs) provide a promising stormwater management strategy in highly urbanized areas when limited open space is available. Hydrological modeling can predict the ability of GRs to reduce runoff. This paper reviews three popular types of GR models with varying complexities, including water balance models, the U.S. EPA's Stormwater Management Model (SWMM), and Hydrus-1D. Developments and practical applications of these models are discussed, by detailing model parameter estimates, performance evaluations and application scopes. These three models are capable of replicating GR outflow. Water-balance models have the smallest number of parameters (≤7) to estimate. Hydrus-1D requires substantial parameterization effort for soil hydraulic properties but can simulate unsaturated soil water flow processes. Although SWMM has a large number of parameters (>10), it can simulate water transport through the entire GR profile. In addition, SWMM GR models can be easily incorporated into SWMM's stormwater model framework, so it is widely used to simulate the watershed-scale effects of GR implementations. Four research gaps limiting GR model applications are identified and discussed: drainage mat flow simulations, soil characterization, evapotranspiration estimates, and scale effects of GRs. The literature documents promising results in GR simulations for rainfall events, however, a critical need remains for long-term monitoring and modeling of full-scale GR systems to allow interpretation of both internal (substrate) and external (meteorological characteristics) system effects on stormwater management.