The rapid expansion and intensification of urban areas has led to a proliferation of impervious surfaces – streets, parking lots, and rooftops – that reduce opportunities for natural hydrological losses like infiltration and evapotranspiration. This has resulted in modified hydrological regimes, wherein stormwater runoff is initiated at lower thresholds and routed across the landscape into centralized wastewater collection systems. Large volumes of runoff can lead to flooding, sewer system malfunction, and impairment of surface and subsurface water resources.
Traditionally, the management of stormwater has relied on gray infrastructure – pipes and sewers to convey runoff to treatment facilities or surface waters. However, this approach has limitations. Gray infrastructure is often expensive to repair, replace, and upgrade, and improvements may only partially solve problems associated with excessive runoff. In many cases, green infrastructure – which leverages the properties of soil and vegetation to enhance watershed detention capacity – offers a viable complement to gray infrastructure for managing stormwater volume.
Within the green infrastructure framework, trees are an often-overlooked component that can play a crucial role in the urban hydrologic cycle. Trees interact with stormwater through several mechanisms, including canopy interception, evapotranspiration, and improved infiltration. By routing precipitation to various components of the hydrologic cycle, trees can reduce the volume of stormwater reaching centralized collection systems, thereby mitigating flood risks and combined sewer overflows.
Hydrological Functions of Trees
Canopy Interception
Canopy interception loss is the sum of water stored in tree canopies and evaporated from tree surfaces. Interception loss protects water quality by reducing stormwater runoff volume and limiting soil erosion and pollutant washout. Canopy interception rates can range from 18-45% of total precipitation, depending on factors like tree species, phenology, and meteorological conditions.
Conifers and broadleaf evergreens generally have higher interception capacities than deciduous broadleaf trees, as they maintain substantial canopy cover year-round. Interception is also higher during the wet-up period early in a storm event, decreasing as tree surfaces become saturated. Canopy interception can contribute significantly to the protection of water quality, but its impact on flood control may be limited to smaller, more frequent precipitation events.
Evapotranspiration
Evapotranspiration (ET) – the combination of evaporation from plant and landscape surfaces, and transpiration wherein soil water is taken up by plants and lost through leaf surfaces – is a crucial component of the urban water cycle. While the role of ET in stormwater management is not well-quantified, studies have shown it can represent the largest output term (up to 81%) in urban water budgets.
Accurately estimating ET in cities is challenging due to spatial heterogeneity in vegetation cover, soil moisture status, and meteorological conditions. Differences in plant functional types (e.g., deciduous vs. evergreen, grasses vs. trees) can lead to variable seasonal patterns of ET, suggesting that incorporating a diversity of vegetation may optimize year-round stormwater cycling. Monitoring sap flux density can provide valuable insights into the seasonality of tree transpiration rates.
Maintaining adequate soil moisture is key to sustaining high levels of ET. Strategies to promote appropriate water availability to tree root systems, such as least limiting water range management, may enhance the stormwater control capacity of urban trees. While turfgrasses can exhibit higher total ET than trees in some settings, optimizing tree planting density and integrating trees with other green infrastructure can maximize their contribution to urban water cycling.
Infiltration
Trees can substantially increase stormwater infiltration through the creation of soil macropores by root growth and senescence, addition of organic matter, and stabilization of soil structure. Even small trees in experimental plots have been shown to reduce runoff by up to 62% compared to impervious controls, largely due to improved infiltration.
Integrating trees into green infrastructure designs like rain gardens and bioswales can enhance the performance of these systems. Trees may help regulate soil moisture content, providing capacity for subsequent storm inputs, and can improve water quality by reducing nutrient concentrations in leachate. Innovative approaches like structural soils and suspended pavement systems are emerging to further optimize the integration of trees and green infrastructure for stormwater management.
Benefits of Urban Tree Canopy
Beyond their direct hydrological functions, urban trees provide a suite of ancillary ecosystem services, including improved air quality, carbon sequestration, energy conservation, and increased property values. These co-benefits make trees an attractive green infrastructure component, complementing their ability to reduce stormwater runoff.
Municipalities and private landowners already plant and manage trees for these reasons, so opportunities exist to strategically incorporate stormwater control objectives into urban forestry programs. However, the loss of trees due to pests, inadequate care, structural failure, and urban development can undermine the reliability of trees as a stormwater control measure, necessitating a focus on maintaining healthy, diverse urban forests.
Regulatory Frameworks
At the national level, the Clean Water Act in the United States obligates cities to control sewer overflows and manage stormwater runoff. Cities with separate sewer systems must implement stormwater management programs and obtain discharge permits, while those with combined sewers negotiate binding legal agreements to reduce overflows.
These regulatory drivers have spurred the adoption of green infrastructure strategies, including the incorporation of trees, to complement traditional gray infrastructure approaches. Some cities, such as Philadelphia, PA, have implemented incentive programs that allow landowners to reduce stormwater fees by installing green infrastructure on their properties.
Stormwater Management Practices
Structural Interventions
Detention basins and bioswales/raingardens are examples of green infrastructure technologies designed to temporarily store and infiltrate stormwater. Integrating trees into the design of these systems can enhance their performance by regulating soil moisture, improving water quality, and providing additional evapotranspiration capacity.
Non-Structural Measures
In addition to engineered green infrastructure, urban forestry programs and public awareness campaigns represent non-structural approaches to stormwater management. By strategically planting and maintaining a healthy, diverse urban tree canopy, cities can leverage the hydrological functions of trees to reduce runoff and mitigate flood risks.
TriCounty Tree Care is a leading provider of comprehensive tree care services, including strategic plantings, proactive maintenance, and emergency response. Our team of certified arborists can help you optimize the role of trees in your stormwater management plan, ensuring the long-term health and resilience of the urban forest.
Conclusion
The rapid expansion of impervious surfaces in urban areas has led to modified hydrological regimes and increased flood risks. While traditional gray infrastructure approaches have limitations, green infrastructure strategies that incorporate trees can play a crucial role in mitigating these challenges.
By leveraging the hydrological functions of trees, including canopy interception, evapotranspiration, and improved infiltration, cities can reduce the volume of stormwater reaching centralized collection systems. This, in turn, can alleviate flooding, sewer overflows, and water quality impairments. However, realizing the full potential of trees as a green infrastructure component requires addressing key research gaps, arboricultural challenges, and institutional barriers.
Ongoing collaboration between the scientific community, urban forestry practitioners, and policymakers will be essential to overcome these hurdles and unlock the power of urban trees in sustainable stormwater management. TriCounty Tree Care is committed to supporting this interdisciplinary effort, working with communities to strategically integrate trees into comprehensive stormwater control plans.