An evaluation of the stormwater runoff reduction of two distinct tree species to support urban greening as nature-based solutions

Trees, as a vital element of urban greening, have been increasingly recognized for their hydrologic contributions to stormwater management. However, the representation of tree canopy hydrological processes is often simplified or overlooked in existing stormwater models. This study modelled and evaluated the stormwater runoff reduction potential of birch (Betula pendula) and pine (Pinus nigra) trees in three scenarios (i.e., birch, pine, and mixed planting) on a storm event basis using the updated Storm Water Management Model (SWMM) tree canopy module. The model effectively represents the rainfall interception process of both tree species during different phenoseasons, demonstrating strong correlations between simulated and observed throughfall (r = 0.97–0.99) and interception values (r = 0.72) across all storm events. The results indicate that implementing urban trees in the study area led to an average reduction of 20–25 % in runoff volume and 16–25 % in peak flow, depending on the scenarios and phenoseasons. The most significant runoff reduction benefits were observed in a mixed-species planting scenario and during the leafed season. This interplay between species highlights the advantages of mixed-species plantings in urban environments, where diverse tree characteristics can enhance hydrological performance. However, the effectiveness of trees is limited during intense, high-volume storm events, although they still provide tangible benefits of up to 13.2 % reduction. The relative contribution of canopy interception to runoff reduction is most pronounced during the leafed season, small to moderate storm events, and when trees are situated over directly connected impervious areas. Infiltration and storage beneath tree canopies are the dominant mechanism for managing and reducing surface runoff, accounting for over 20 % of the water balance. This study demonstrates that the stormwater reduction efficiency of urban trees depends on both above- and below-canopy processes and conditions.