Reliability of SWMM for Predicting Performance of Field-Scale Bioretention Systems

Modeling bioretention systems using the Storm Water Management Model (SWMM) is a common practice. However, there is limited observational evidence to determine how accurately and reliably the model performs. This study compared the measured outflow from the underdrain of four bioretention systems with SWMM modeling results, providing critical insights into the models’ applicability and limitations. Results indicated that prior to calibration, the SWMM captures peak flow characteristics and the shape of hydrographs reasonably well. However, calibration improved the performance for flow peaks, resulting in better Nash-Sutcliffe efficiency, for example, from 0.25 to 0.70 in bioretention cell S1. Also, the uncertainty in outflow predictions varied between different bioretention systems, with the width of the uncertainty band varying by up to a factor 2.5 between the systems with the most and least uncertainty in the model predictions. This study found SWMM to be a reliable tool for modeling bioretention hydrology, with reliability varying between systems and improving notably after calibration.