The role of urban trees as nature-based solutions for stormwater runoff control

The soil cover of urban areas drastically reduces infiltration rates and increases the risk of stormwater flooding. In this context, nature-based solutions (NBS) are well-acknowledged strategies for sustainable stormwater management and water cycle restoration. As NBS, urban trees provide significant benefits to hydrological balance through rainfall interception, evapotranspiration, and water storage in the underlying soil. This paper proposes an analytical-probabilistic model to evaluate the performance of urban trees for stormwater control. The study investigates the effects on the surface runoff of the main hydrological and environmental variables involved, in particular the infiltration rate into the soil, and the impermeable contributing areas. The reliability of the analytical-probabilistic approach to estimate the surface stormwater runoff (in terms of both runoff probability and average value) was validated through its application to a case study in the city of Lecco (Italy). The results confirmed that the beneficial impact of urban trees on stormwater control is greater during leaf-on season (late spring-summer at Lecco latitude) and when a proper infiltration rate into the soil beneath the tree is ensured.