Role of trees as part of the nature-based solutions in cities and their effects on stormwater runoff generation
Authors: Alivio Mark Bryan , Bezak Nejc 
Published: February 2023

As a vital element of public spaces, trees in urban settings are acclaimed to offer numerous social and environmental benefits, making them a quintessential nature-based solution for a more sustainable city. While carbon sequestration, air quality, and urban heat island mitigation benefits have long been acknowledged, less emphasis is directed to utilizing the hydrologic function of trees in terms of stormwater runoff reduction in the urban environment and this benefit is often underutilized. For urban areas with high proportions of impervious surfaces, increasing the percentage of tree canopy cover and green spaces is crucial in restoring the natural functioning of the ecosystem and water cycle. Within the framework of our ongoing research, we are investigating the positive impacts of trees (i.e., single tree elements, forests) as nature-based solutions on the urban water cycle using field measurements of rainfall partitioning, runoff, soil moisture, and infiltration from experimental catchments in the city of Ljubljana, Slovenia which started on August of 2021. Preliminary results revealed that open-grown birch (deciduous) and pine (coniferous) tree canopies intercepted a relative amount of gross rainfall with pine trees having a greater interception capacity. The following trees also modified the drop size distribution (e.g., drop number, diameter, fall velocity) of below-canopy rainfall before reaching the ground, thus attenuating the mean and maximum 10-minute rainfall intensities by 42-50% and 40-44%, respectively, depending on canopy phenoseasons. Such reduction in the intensity of rainfall has a significant effect on the peak water level of event runoff which could provide important information for understanding the runoff generation process. Moreover, this benefit with the root system of trees has a positive impact on the condition and structure of soils in urban areas promoting infiltration, preferential flow, and soil water recharge. In addition to this, tree canopies also dampen the average kinetic energies of rainfall to cause soil erosion by 34%. These initial findings suggest that the hydrological benefits of trees in the urban environment are adequate to warrant a further investigation into their potential to regulate the flow mechanisms of stormwater runoff and reduce urban pluvial flooding. Thus, it is also imperative to explore how the integration of trees interacts with other stormwater control measures and how this interaction could leverage their functions. This will deliver invaluable information to urban planners, landscape designers, stormwater management experts, and decision-makers on the need to expand the efforts of urban greening to address the associated adverse impacts of rapid urbanization and various environmental challenges.&#160;Acknowledgments: Results are part of the CELSA project entitled &#8220;Interception experimentation and modelling for enhanced impact analysis of nature-based solution&#8221; and research programmes and projects P2-0180, J6-4629, and N2-0313 financed by the Slovenian Research Agency (ARRS).