Does Improved Site Scale Modeling of Bioretention Translate to Better Estimating Watershed Scale Benefits?

Large scale implementation of green infrastructure, such as bioretention, has become increasingly common. The goal of these installations moves beyond reducing downstream peak flows, also aiming to provide more natural hydrology in receiving streams. Thus, the partitioning of runoff to infiltration, drainage, and overflow; and the timing of these hydrologic pathways becomes critical to holistically evaluating implementations. In this study, DRAINMOD-Urban (a robust drainage model) is coupled with SWMM to provide a watershed scale model that can also perform fine scale modeling of bioretention (and proper hydrologic partitioning). This combined model is tested in a watershed with three bioretention areas in Columbus, Ohio, to evaluate its performance. Initial results show the combined model performs well for the test watershed (NSE = 0.66 for hydrograph representation). Further, the performance for hydrograph representation, mean flow, and volume was found to exceed that of a model representing the system using SWMM LID tools. The results suggest that the model chosen for site scale representation of green infrastructure can make a difference in modeling watershed scale performance.