Stormwater Management

Urbanization and intensive land development have significantly changed the permeability of urban landscapes and impacted their natural hydrological cycles and processes. Cities are becoming more vulnerable to heavy rainfall events that result in rapid runoff and flooding (Kaykhosravi et al., 2020). With fewer natural forests and less natural vegetation, any reduction in tree canopy cover results in increased runoff and urban areas becoming more prone to flooding. Urban forests, with adequate canopy cover, structure, and composition, have the potential to improve urban hydrology and reduce runoff (Berland et al., 2018; Kuehler, Hathaway & Tirpak, 2017; Xiao, McPherson, Simpson & Ustin, 1998).  However, to effectively manage urban forests for improving hydrology, many cities and towns globally lack appropriate areas for future tree planting. Canadian urban areas are no exception when it comes to these issues of urbanization, hydrology, and tree canopy. 

Trees, individually and collectively as urban forests, play a crucial role in stormwater management. Strategically conserved, managed, and enhanced urban forests can provide a sustainable solution for Canadian municipalities experiencing environmental challenges. Urban trees and their canopies manage stormwater through evapotranspiration and by physically intercepting rainfall via leaves, branches, and tree trunks, thus reducing the volume of water that reaches the ground (Carlyle-Moses et al., 2020; Dowtin et al., 2023). However, interception and evapotranspiration are determined by morphological characteristics of tree species, tree size and stature, leaf area density, branching structure, and whether trees are planted in groups or individually (Berland et al., 2018; Kuehler, Hathaway & Tirpak, 2017; Xiao, McPherson, Simpson & Ustin, 1998).

Additionally, tree roots stabilize the soil, improve soil structure and organic content, and increase the soil’s ability to absorb and filter water. This reduces the load on stormwater management infrastructure, streams, and ponds, and lessens flood risks while mitigating erosion and sedimentation in waterways (United States Environmental Protection Agency, 2023). Urban trees also improve overall water quality (The Mersey Forest, 2014) by reducing runoff and toxic chemicals like metals, fuels, solvents, and other pollutants (USEPA, 2013). Urban forests with hydrological functions help protect properties and gray urban infrastructure by reducing flooding and runoff during extreme weather events, which provides significant economic benefits (Nesbitt et al., 2017).

Many Canadian cities have created green development standards to incorporate trees and integrate urban forests into land use planning (see City of Toronto, 2023b; City of Mississauga, 2012; The Town of Halton Hills, 2019). By preserving and managing urban trees, municipalities reduce overall runoff from rainfall and increase soil absorption capacity, thus creating a more resilient urban environment, urban forest, and natural ecosystem (City of Mississauga, 2023; Ministry of Municipal Affairs, 2023).

Implementation Strategies

One of the most effective strategies for managing stormwater and reducing runoff is to increase both the coverage and quality of tree canopy. This can be accomplished by strategically planting trees in areas such as streets, parks, private properties, and various types of land use. To further enhance the advantages of urban forests, it is recommended to combine tree canopy conservation and planting with other forms of green infrastructure, such as rain gardens, green roofs, and permeable pavements. This integrated approach can significantly improve stormwater management and maximize the benefits provided by urban forests (Carlyle-Moses et al., 2020; USEPA, 2024).

More recently in Canada, various levels of government have been encouraging urban tree planting to increase canopy cover and improve urban environmental conditions, as well as to improve stormwater management (Green Infrastructure Ontario Coalition, 2016; City of Toronto, 2023a; City of Toronto, 2023b). To that effect, many Canadian municipalities have set goals or guidelines for stormwater management that utilize urban forests and increase tree canopy cover. For example, as per Toronto’s Green Standard Requirements, the City of Toronto aims to minimize runoff to at least 50 percent of its annual rainfall and, for some sites, retain at least 5 mm of rainfall (through rainwater reuse, on-site infiltration, and evapotranspiration) from each rainfall event (City of Toronto, 2017). Aside from the various ecosystem services urban forests provide, urban trees also enable the saving of resources for managing gray infrastructure. The City of Surrey, for example, saved $4.8 million/year on stormwater infrastructure due to the presence of trees (City of Surrey & Urban Systems, 2023). Some municipalities, such as the City of Mississauga and the City of Kitchener, have implemented stormwater fees to encourage private landowners to reduce the hard surface on their properties. The fees are based on a percentage of impervious surfaces to encourage property owners to use green infrastructure and permeable surfaces to reduce localized runoff (Environmental Commissioner of Ontario, 2016).