Urban Stormwater Runoff

This briefing paper provides current information related to the national issue of stormwater runoff in urban areas. State foresters can provide assistance to integrate trees and forests into green infrastructure planning and implementation to address this important issue.

Stormwater runoff is a significant contributor to water quality impairments, especially in developing and urban areas. Urban stormwater runoff affects water quality and quantity, habitat and biological resources, public health, and the aesthetic appearance of urban waterways. This rapidly moving runoff:

  • Creates flooding leading to property damages;
  • Transports high levels of sediments, pollutants and dissolved contaminants into surface waters;
  • Causes accelerated erosion of streambanks resulting in degradation of aquatic habitat and accelerated deposition of sediments into receiving waters;
  • Threatens drinking water supplies by reducing groundwater recharge;
  • Impairs water-based recreation that can lead to economic impacts;
  • Causes public health concerns due to the presence of bacteria and other pathogens; and
  • Carries unsightly debris and litter into waterways (EPA, 1999).

This briefing paper will discuss the sources and effects of stormwater, green infrastructure approaches to improve water quality, and describe how urban watershed forestry practices can improve overall watershed health. Also presented is a case study of how a city in Vermont is using an urban tree canopy (UTC) study to strategically plant trees to meet water quality targets.

Stormwater: Its Source and Impacts on Water Quality and Watershed Health
In urban areas, stormwater runoff originates from a number of sources including residential areas, commercial and industrial sites, roads, highways and bridges (EPA, 1999). Essentially, any surface that does not have the capability to pond and infiltrate water will produce runoff during storm events. When a land area is changed from a natural forested ecosystem to an urbanized land use consisting of rooftops, streets and parking lots, the hydrology of the system is significantly altered. Water that was previously ponded on the forest floor, infiltrated into the soil and converted to groundwater, utilized by plants and evaporated or transpired into the atmosphere is now converted directly into surface runoff. It should also be noted that in addition to delivering pollutants to receiving waters, the volume and energy of urban stormwater runoff can cause physical changes to surface water resources.

An important measure of the degree of urbanization in a watershed is the amount of impervious surfaces. As the amount of imperviousness increases in a watershed, more rainfall is converted to runoff (Figure 1). Urban watersheds can best be described as a highly fragmented landscape as a result of development, tree canopy cover has been diminished and impervious surface is pervasive, typically considered as being greater than 10% (Barnes et al., 2009).

State and local governments across the nation must deal with this issue to meet federal regulations through the Clean Water Act and they are working hard to identify the source of pollutants, to develop methods for reducing their stream loads, and taking action to improve water quality. The method used to address impaired waters from stormwater is to meet a Total Maximum Daily Load (TMDL), which identifies the total pollutant loading that a waterbody can receive and still meet water quality standards. If a municipality's separate storm sewer system (MS4) contributes a pollutant of concern to a waterbody that is listed as impaired by the state, they are more than likely taking steps to modify their program to meet state and federal TMDL regulations (NAS, 2008). An emerging concept on the rise in TMDL implementation plans is green infrastructure. Green infrastructure strives to manage stormwater and pollutants by restoring and maintaining the natural hydrology in a watershed (EPA, 2008). As communities adapt to meet these regulations, urban and community forests can assist in maintaining the health of our urban watersheds, improve water quality (both impaired and unimpaired streams), and lower maintenance and construction costs of water storage and treatment systems.

Blending the Green with the Gray
The conventional way of collecting and conveying stormwater in developed areas consists of inlets and underground pipes commonly referred to as storm sewers. This "gray infrastructure" is man-made and immovable. A newer approach to stormwater management is "green infrastructure," which strives to eliminate or reduce runoff and pollutant loading as close to the source as possible by linking together small-scale practices to maintain or replicate the predevelopment hydrology of the site. Some sites do not have sufficient conditions to handle water collected from surrounding impervious surfaces. In addition, sites that are largely paved usually cannot support large trees, and thus are unable to benefit from tree canopy interception and the influence of roots on soil hydrology. What's often needed on these sites is green engineering that harnesses the ability of vegetation and soils to mitigate urban runoff.

Green engineering with trees requires thinking below ground to give trees what they need - soil and space. With new technologies and strategies-including using engineered soils under pavement, underground frameworks to suspend pavement for soil and tree box filters-trees can play an important role in managing stormwater in our most urban areas.

In addition to the small-scale stormwater management approach, green infrastructure has also been employed on a larger scale by strategically planning and managing networks of natural lands, working landscapes and other open spaces that conserve ecosystem values and functions and provide associated benefits to humans. The concept of large-scale green infrastructure goes beyond traditional conservation practices and represents a major shift for greenspace planning and protection. Unlike traditional conservation efforts that focus on restoration and preservation, this new concept considers pace, shape, and location of development in relationship to important natural resources and amenities. Green infrastructure planning accepts continuing development and land use changes, and strives to simultaneously integrate greenspace with building development early in the planning process to inform smarter land use decisions.

When planning for green infrastructure, one needs to consider elements from all scales: state-region-watershed-community-site-parcel. At the largest scale, the preservation and restoration of natural landscape features such as forests, floodplains and wetlands are critical components. At the site and parcel level, stormwater management practices such as tree box filters, vegetated swales, green roofs and rain gardens are part of the system. At all scales, trees and forests are recognized for positively influencing water quality. Some of the best and cost effective green infrastructure practices naturally involve forests and trees, as they can reduce the amount of runoff and pollutant loading to receiving waters in several ways:

  • Capturing and storing rainfall in the canopy and releasing water into the atmosphere through evapotranspiration;
  • Intercepting and storing rainfall on leaf and branch surfaces thus reducing runoff volumes and delaying the onset of peak flows;
  • Increasing infiltration capacity of soil enhanced by tree roots, leaf litter and organic matter; and
  • Reducing pollutants by taking them up through their roots and by chemically transforming them into less harmful substances.

Urban Watershed Forestry: The Forestry Component of Green Infrastructure
Green infrastructure recognizes the services provided by all natural systems and often involves engineering systems to replicate them, while the emerging field of urban watershed forestry focuses on the role that urban trees and forests play in providing for overall watershed health. Urban watershed forestry involves a multi-disciplinary approach by incorporating principles and practices of forestry, hydrology, engineering, landscape architecture, mapping, planning, and soil science (Cappiella et al.). The underlying principle stresses the importance of reducing urban forest loss and maximizing urban and community forest gains over time through watershed analysis, planning and management. Foremost among its core principles, is the recognition that forest cover is the highest and best use of land in a watershed in terms of water storage, groundwater recharge, stormwater runoff, pollutant reduction and wildlife habitat.

It is well documented that the amount of forest cover in a watershed is a key indicator of water quality in a region. Watersheds with a large proportion of forest cover are more likely to be associated with good water quality. The crucial link between watershed health and trees and forests has been documented by many. Trees and forests benefit the watershed by:

  • Reducing stormwater runoff and flooding;
  • Improving regional air quality;
  • Reducing stream channel erosion;
  • Improving soil quality;
  • Providing habitat for terrestrial and aquatic wildlife; and
  • Reducing summer air and water temperatures.

Based upon this precept, urban watershed forestry has three goals to:

PROTECT undeveloped urban and community forests and the impacts of land development by creating and applying various inventory and planning techniques, regulatory tools and incentives.

ENHANCE the health, condition and function of urban forest fragments.

REFOREST open land through active replanting or allowing for natural regeneration to regain some of the functions and benefits that forests and trees provide for watershed health.

As forestry professionals, it is important to be involved in the dialogue of how forests and trees are being protected, managed and incorporated to address water quality, and in providing assistance to both state and local governments as they strive to meet regulatory requirements. The goals and principles of Urban Watershed Forestry offer the means to integrate trees and forests into Green Infrastructure planning and implementation.

A Case Study: Rutland, Vermont
Rutland is home to an urban stormwater impaired waterway, Moon Brook. In 2009, a TMDL was established to reduce the impacts from stormwater runoff. The city believes its long-standing urban forestry program and tree canopy are vital components to helping them reduce pollutant loads. To understand its current green infrastructure, the city worked with the Vermont Division of Forests, the University of Vermont Spatial Analysis Laboratory, and the U.S. Forest Service to conduct an urban tree canopy assessment (NASF, 2009). The analysis found that more than 1,784 acres of the city was covered by tree canopy (termed Existing UTC) representing 37% of all land in the city (Figure 2). An additional 51% (2,470 acres) of the city could theoretically be improved (Possible UTC) to support tree canopy. In the Possible UTC category, 14.7% (711 acres) of the city were Impervious Possible UTC (asphalt or concrete surfaces, excluding roads and buildings that are theoretically available to the establishment of tree canopy) and another 36.4% (1,760 acres) were Vegetated Possible UTC (grass or shrub area that is theoretically available for the establishment of tree canopy).

Further analysis showed that more than 80% of the parcels in the Moon Brook watersheds have less than average canopy cover of 37%. To improve water quality using trees, the city is currently reviewing the data to identify which specific parcels in the watersheds will benefit the most from tree planting, i.e. low tree canopy, high impervious area and high possible vegetated tree canopy (Figure 3). This spring it will offer free trees through water supply bills to those landowners.

City officials understand that they will have to implement other solutions to address stormwater and that water quality improvement from the new trees will not be evident in the first few years. Yet, they also recognize and stand behind the fact that by increasing tree cover in the city, they are doing more than just addressing water pollution and that the trees are working double duty for them to provide a myriad of other benefits such as increased property values, energy efficiency, economic development and improved air quality. The investments they make in trees today will pay them back as they work hard to create a community that is more livable, healthy, and sustainable.


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Cappiella, K., Schueler, T., & Wright, T. (2005). Urban Watershed Forestry Manual Part 1: Methods for Increasing Forest Cover in a Watershed. United States Department of Agriculture Forest Service, Northeastern Area State and Private Forestry NA-TP-04-05.
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USDA Forest Service, Pacific Northwest Research Station. (2009). Private Forests, Public Benefits: Increased Housing Density and Other Pressures on Private Forest Contributions.
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