Forests and Drinking Water
Forests capture rainfall and replenish and cleanse our water supply. Although these ecological services provided by forests are widely accepted in the scientific community, they have not really been translated into the language that most often drives planning and land-use decisions at the local level: dollars. Local government officials often make tough decisions about growth at the expense of natural resource conservation, and they must make these decisions without the benefit of economic data that measures the true costs of development and values of natural resources.
For decades, technology has replaced, to some extent, the services provided by forests but at a high price. Billions of dollars are invested in the construction and upgrade of water treatment plants to clean our public water supply that has been degraded by pollution as a result of industrialization and urban development. In fact, water utilities spend 19 times more on water treatment chemicals every year than the federal government invests in protecting lakes and rivers from pollution in the first place using techniques such as conservation of forest land (Frost & Sullivan 2004).
The Forest Service estimates that nearly 1 million acres of forest were converted to developed uses each year in the 1990s, and by 2050, an additional 23 million acres of forests may be lost due to development (Stein et al., 2005). Areas experiencing the most forest loss are often suburban and urbanizing communities where municipal staff struggle to keep up with the growth and may not have adequate tools to manage it. How does this loss of forest cover translate to costs incurred by communities for sustaining quality, long-term water supply? The answer to this question is largely unknown as few communities track increases in drinking water treatment costs (or other community services) with the loss of forest land or evaluate these possible impacts prior to approving new developments.
Research is needed to identify the specific economic connections between forests and drinking water based on the available science. This research can be used to (1) put advance planning for water supply and forest conservation at the forefront of community issues, (2) make the case for forest conservation to protect drinking water, (3) encourage the use of incentives for forest conservation and tree planting that are more reflective of their true value, and (4) factor in the costs of drinking water supply and treatment when evaluating development alternatives.
Reducing Stormwater Runoff
Stormwater runoff is rainfall that flows over the ground surface. It is created when rain falls on roads, driveways, parking lots, rooftops, and other paved surfaces that do not allow water to soak into the ground. Stormwater runoff is the number one cause of stream impairment in urban areas. Where rain falls on paved surfaces, a much greater amount of runoff is generated compared to runoff from the same storm falling over a forested area. These large volumes of water are swiftly carried to our local streams, lakes, wetlands, and rivers and can cause flooding and erosion, and wash away important habitats for critters that live in the stream.
Stormwater runoff also picks up and carries with it many different pollutants that are found on paved surfaces such as sediment, nitrogen, phosphorus, bacteria, oil and grease, trash, pesticides, and metals. These pollutants come from a variety of sources, including pet waste, lawn fertilization, cars, construction sites, illegal dumping and spills, and pesticide application. Researchers have found that as the number of paved surfaces (a.k.a. impervious cover) in the watershed increases, stream health declines accordingly.
To counteract these impacts of stormwater runoff, most municipalities have adopted regulations that require the management of stormwater for all new development. Stormwater management is the use of specific practices, constructed or natural, to reduce, temporarily detain, slow down and/or remove pollutants from stormwater runoff. Stormwater management practices are essentially designed to restore or mimic some of the natural processes provided by the vegetative cover that existed prior to land disturbance. In many regions of the country, this native vegetative cover includes trees and shrubs.
Preserving undisturbed vegetative cover during land development is a much more cost-effective approach than destroying these features and having to construct new stormwater management practices to replace the functions they originally provided. Trees, forests, and other vegetation and their associated soils are often referred to as “green infrastructure” when they are used to manage stormwater runoff instead of or in addition to pipes, pumps, storage chambers, or other “hard infrastructure.” Municipalities are beginning to realize the many stormwater benefits of green infrastructure, and are encouraging the use of stormwater management practices that conserve forests and incorporate vegetative features.
How Do Trees Reduce and Remove Pollutants from Stormwater Runoff?
Trees and forests improve stream quality and watershed health primarily by decreasing the amount of stormwater runoff and pollutants that reach our local waters. Trees and forests reduce stormwater runoff by capturing and storing rainfall in the canopy and releasing water into the atmosphere through evapotranspiration. In addition, tree roots and leaf litter create soil conditions that promote the infiltration of rainwater into the soil. This helps to replenish our groundwater supply and maintain streamflow during dry periods.
The presence of trees also helps to slow down and temporarily store runoff, which further promotes infiltration, and decreases flooding and erosion downstream. Trees and forests reduce pollutants by taking up nutrients and other pollutants from soils and water through their roots, and by transforming pollutants into less harmful substances. In general, trees are most effective at reducing runoff from smaller, more frequent storms.
In addition to these stormwater benefits, trees provide a host of other benefits such as improved air quality, reduced air temperatures in summer, reduced heating and cooling costs, increased property values, habitat for wildlife, and recreation and aesthetic value.
The City of Vancouver, Washington recognizes the important connection between forests and stormwater runoff and has included Urban Forestry in the City’s stormwater management plan, which represents a comprehensive watershed approach to improving water quality. In addition, 100% of the stormwater utility fees collected are used to support the Urban Forestry Division.
How Do We Measure and Provide ‘Credit’ for Stormwater Runoff Reduction by Trees?
So, if trees are so great, why isn’t everyone using them to meet their stormwater management needs? One of the reasons is that developers have no incentive to use them unless they can get credit for them and most municipalities do not provide these credits. Municipalities that do provide credits may not provide enough to act as a true incentive.
A stormwater credit system reduces the stormwater management requirements a developer has to meet in exchange for conserving forests or using site design techniques that reduce the number of paved surfaces created. The credit system directly translates into cost savings for the developer by reducing the size of stormwater management practices that must be constructed.
In most municipalities around the country with stormwater management regulations, site designers are required to capture and remove pollutants from a specified runoff volume and control the maximum (or peak) rate of runoff from the site for certain size storm events. Under this scenario, water quality ‘treatment’ is defined solely by the pollutant removal functions of the stormwater management practices used, and does not account for their ability to reduce the overall volume of runoff. A number of states and communities are beginning to recognize that there are benefits of shifting from this peak-based stormwater control to an approach that focuses on reducing the volume of runoff (though re-use, evapotranspiration, or infiltration) leaving a site. A volume reduction approach is most appropriate for the relatively small, frequent storms, which matches up well with the stormwater benefits provided by trees.
Reducing runoff volume using green infrastructure has benefits beyond just removing pollutants. It also recharges groundwater, provides better protection of sensitive aquatic resources and reduces the size and cost of hard infrastructure that would otherwise need to be constructed. One challenge with this approach has been how to account for the runoff reduction provided by green infrastructure in rainfall/runoff models commonly used by engineers.
What Are Some Specific Practices That Use Trees to Reduce Stormwater Runoff?
Conserving natural areas such as forests can reduce the amount of runoff that is created. The most effective way to minimize the impacts of stormwater runoff described above is to limit the number of paved surfaces that are created during the development and preservation as much as possible of the natural topography and vegetation. Specifically, existing forests can be protected during construction and permanently managed as conservation lands. In cases where there are no existing forest stands (e.g., development of former farmland), reforestation can help to offset these impacts.
This stormwater management practice slows down and filters pollutants from runoff from the adjacent rooftops and driveway. The impacts of stormwater runoff can also be minimized by increasing tree canopy over paved surfaces to increase interception of rainfall. Another way to minimize impacts is by “disconnecting” paved surfaces so that they no longer drain to the system of gutters, inlets, and pipes that make up the storm drainage system and ultimately flow to our local streams and rivers. Disconnection can involve redirecting runoff from rooftops or individual parking lots to stormwater management practices or vegetated areas and allowing the runoff to slowly soak into the ground.
Most stormwater management practices that incorporate vegetation have relied primarily on herbaceous vegetation with few trees and shrubs. The reason for this is that engineers are often concerned that tree roots, branches, and leaves will compromise the effectiveness or stability of the practice. When trees are planted in stormwater management practices, they often die because of harsh conditions, or they are removed by maintenance crews or overzealous homeowners.
Urban Tree Canopy
Urban tree canopy (UTC) is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. In urban areas, the UTC provides an important stormwater management function by intercepting rainfall that would otherwise run off of paved surfaces and be transported into local waters through the storm drainage system, picking up various pollutants along the way. UTC also reduces the urban heat island effect, reduces heating/cooling costs, lowers air temperatures, reduces air pollution, increases property values, provides wildlife habitat, and provides aesthetic and community benefits such as improved quality of life.
Why Set UTC Goals?
Researchers estimate that tree canopy cover in urban and metropolitan areas across the U.S. average only 27% and 33% respectively (Dwyer and Nowak, 2000). Additionally, the trees that are present are subject to a wide variety of stressors, which significantly shortens their lifespan. As such, it is important for urban communities to take steps to protect and enhance their urban forests through UTC goal-setting processes. Few communities have developed land cover strategies such as UTC that mitigate urbanization effects regardless of the land use type. Several recent efforts have explicitly included UTC in planning efforts to address community, environmental and human health concerns:
The Chesapeake Bay Program has included UTC in its strategies to improve water quality in the Bay by reducing sedimentation and nutrient loads. The 2003 Riparian Forest Buffer Directive states the following goal: “by 2010, work with at least 5 local jurisdictions and communities in each state to complete an assessment of urban forests, adopt a local goal to increase urban tree canopy, and encourage measures to attain the established goals in order to enhance and extend forest buffer functions in urban areas.” The 2007 Forestry Conservation Initiative has a goal of: “By 2020, accelerate reforestation and conservation in urban and suburban areas, by increasing the number of communities with commitments to tree canopy expansion goals to 120.”
Using the Urban Ecosystem Analyses, American Forests has pioneered the idea of measuring and calculating the value of UTC in our metropolitan areas. These analyses have been conducted for more than 30 cities and metropolitan areas across the country. As an example, results from the Montgomery, Alabama analysis include:
- As of 2002, 34% of the city was covered by tree canopy
- The stormwater retention capacity of the city’s urban forest is 227 million ft3
- The cost to manage this volume of runoff is estimated at $454 million
- The city’s urban forest removes 3.2 million lbs of pollutants from the air annually and this benefit is valued at $7.9 million.
- The city’s urban forest sequesters 11,263 tons of carbon each year and stores a total of 1.45 million tons of carbon
Some states are including UTC in State Implementation Plans to improve air quality by mitigating ground-level ozone formation.
Other UTC efforts have focused on individual development sites as opposed to entire cities or metropolitan areas. Because unshaded parking lots can become extremely hot and contribute to both the urban heat island effect and increased air pollution, many communities in hot climates require that newly constructed or reconstructed parking lots be shaded by incorporating tree plantings into the parking lot design. Parking lot shading provisions are sometimes enacted through a specific parking lot shading ordinance, the code may be incorporated into sections of the city code related to trees, landscaping, parking lots, or elsewhere. For example, Sacramento and Davis, California have parking lot tree shading ordinances that require 50% shading of paved areas in parking lots 15 years after development.
Conducting UTC Assessments and Goal Setting
In order to set UTC goals, communities must first have an idea of how much current canopy is present. The process for conducting UTC assessments and goal setting generally includes the following steps:
1. Measure current UTC
- Use a top-down remote sensing or bottom-up on-the-ground tree surveys to measure existing urban tree canopy.
- Identify the different types of forest in the community, including public (street trees, riparian corridors, parks, etc.) and private (residential, commercial, industrial areas, etc.)
2. Estimate potential UTC
- Use remote sensing imagery and Geographic Information Systems analyses to identify locations with potential for UTC.
- Identify priority locations where UTC increases will support identified community priorities (e.g., water quality, wildlife).
3. Adopt a UTC Goal
- Determine a goal based on the results of the assessments and specify a timeframe.
- Formal adoption of the goal is preferable to ensure that the goal comes to fruition (e.g., institutionalize UTC goals in a local ordinance, regulations, and comprehensive planning efforts).
Approaches to Achieve UTC Goals
Once the assessment and goal setting process is complete, the next logical step is to develop an implementation plan that summarizes the approaches the community will take to achieve their UTC goals. In general, a UTC plan identifies the UTC goal and timeline, describes the relationship of canopy goals to local ordinances, regulations, and the community’s comprehensive plan, and outlines the specific strategies for achieving UTC goals, including identifying a timeline and responsible party. Each community must develop an approach to achieve UTC goals that consider their internal capacity and resources, political climate, and stakeholder needs. The range of strategies to achieve UTC goals includes:
- Permanently protect priority forest tracts through acquisition, conservation easements, or other methods.
- Prevent forest loss during development by adopting or amending site development regulations (e.g., forest conservation regulations, open space design, clearing restrictions) and zoning.
- Maintain existing forest canopy by adopting regulations that restrict tree removal.
- Increase tree planting during development by adopting or revising site development regulations such as landscaping and parking lot shading.
- Reforest public lands, beginning with priority sites.
- Encourage reforestation of private land by developing education, stewardship, and incentive programs.