Quantifying the stormwater runoff reduction potential of two distinct urban tree species

Urban trees are essential components of urban greening efforts and in the concept of “sponge cities”, providing a multitude of ecosystem services. In recent years, there has been a renewed interest in the practical contribution of trees to stormwater management in cities. However, the representation of trees in most conventional urban stormwater models remains inadequate. Often, these models implicitly lump specific parameters of tree species as part of the general vegetation categories or pervious accounting processes. In this study, we utilized the updated SWMM tree canopy module to model and evaluate the stormwater runoff reduction potential of birch (Betula pendula Roth.) and pine (Pinus nigra Arnold) trees in three scenarios (i.e., birch, pine, mixed-species planting) on a storm event basis. The model allows for the definition of individual trees, as the added canopy module introduces several key parameters to characterize different tree species. Modelling results demonstrated that the interception routine implemented in the updated SWMM model effectively captured the temporal evolution of throughfall + stemflow (T_f + S_f) under both trees in different phenoseasons. There is also a strong correlation between the simulated and observed throughfall (r = 0.97-0.99) and interception values (r = 0.72) across all storm events. The model tends to overestimate T_f + S_f, particularly for the pine tree, resulting in an underestimation of canopy interception by 3.1% for the birch and 19.6% for the pine. Thus, the reduction in runoff volume and peak flow across all scenarios and phenoseasons in an event-based is between 20-25% and 16-25%, respectively. The mixed-species tree planting scenario performed better in reducing both runoff volume and peak flow than the single-species scenarios. However, the stormwater reduction efficiency of both trees becomes limited during intense, high-volume storm events, but they continue to provide tangible benefits. Water balance analysis further emphasizes the relative contribution of canopy interception in the stormwater runoff reduction benefits of urban trees, particularly during the leafed season, small to moderate storm events, and when trees are in directly connected impervious areas. This underscores the importance of considering rainfall interception as a critical hydrological process, especially when modeling nature-based solutions in urban environments. Moreover, infiltration and storage in the soil play a dominant mechanism in managing net rainfall under the tree canopy before it contributes to runoff, accounting for over 20% of the water balance. Importantly, the findings from our study offer valuable insights and guidance for urban planners and stormwater engineers on appropriately crediting the stormwater reduction benefits of urban trees within urban planning frameworks and policy development. 

 

Acknowledgment: This work was supported by the P2-0180 research program through the Ph.D. grant to the first author, which is financially supported by the Slovenian Research and Innovation Agency (ARIS). Moreover, this study was also carried out within the scope of the ongoing research projects J6-4628, J2-4489, and N2-0313 supported by the ARIS and SpongeScapes project (Grant Agreement ID No. 101112738) and NATURE-DEMO (Grant Agreement ID No. 101157448), which is supported by the European Union’s Horizon Europe research and innovation programme.