Evaluating stormwater benefits of an urban isolated tree with different ground covers through interception-infiltration-runoff processes using Storm Water Management Model

Urban trees play a significant role in reducing surface runoff through crown interception loss and root-enhanced infiltration. However, the effects of tree crown and roots on surface runoff are often simplified in the Storm Water Management Model (SWMM). This study aims to evaluate stormwater benefits of an urban isolated tree combined with different ground covers through interception-infiltration-runoff processes using SWMM. The crown interception process during 15 rainfall events was measured using a weighing lysimeter and used to obtain the net rainfall (i.e., the rainfall amount reaching the ground surface beneath tree crown) that was input into the rain gauge for simulations. Additionally, the optimization of key hydrological parameters in the bio-retention cell was performed to determine an optimal set of parameters to represent the effect of target tree roots on the infiltration process. This optimal set of parameters was validated using experimental results from two irrigation-drainage events, and the results showed that the coefficient of determination (R²) between simulated and measured results exceeded 0.9. According to the proposed simulation method that considers the effects of tree crown and roots on surface runoff, the interception-infiltration-runoff processes for different combinations of an isolated tree with tree pits, permeable pavement, and water-retaining pavement were analyzed during 15 rainfall events. The results revealed that the total surface runoff reduction from the isolated tree was 11.5%. Meanwhile, the simulation results for different combinations were compared to quantify stormwater benefits, including reductions in total and peak surface runoff. Based on rainfall characteristics (total rainfall amount and rainfall intensity) and surface runoff reduction, this study recommends optimal combination of the tree and ground covers to support urban greening design in stormwater management.