How Green Infrastructure Measures Up to Structural Stormwater Services
Quantifying the contributions of trees and vegetation
Joni Mitchell’s classic lyrics, “You don’t know what you’ve got ’til it’s gone,” aptly describes the aftermath of the Cedar Fire that swept into the city of San Diego in the fall of 2003. The devastating fire not only destroyed homes and businesses but also burned a significant portion of the vegetation, diminishing nature’s ability to manage stormwater and related water-quality issues. The California Department of Forestry and Fire Protection wanted to quantify the loss in terms of ecosystem services and posed this question to American Forests, a not-for-profit conservation organization.
Ironically, just four months before the fire, American Forests had prepared a digital GIS-based map of the area’s green infrastructure—trees and other vegetation together with their complex interaction with soil, air, and water. Green infrastructure is a significant asset that was not previously documented as part of the city infrastructure. These data, along with American Forests’ CITYgreen GIS software tool that quantifies ecosystem values, provided local elected officials with tools to better gauge the benefits of San Diego’s green infrastructure for future planning decisions.
 |
Photo: Rusty Rozzelle, Mecklenburg County, NC |
| McDowell Creek, Carolina Piedmont Region |
“The dramatic and sudden loss of vegetation warranted an update to the digital GIS map or what we refer to as a ‘green data layer,’” explains Gary Moll, senior vice president at American Forests. “These data and analysis tools give city leaders a quantitative measure of the lost ecosystem services, including the dollar value for providing stormwater management.”
The Cedar Fire affected 28,466 acres of land within the San Diego city limits, about 13% of the entire city. Comparing pre- and post-fire conditions in the Cedar Fire area, American Forests reported a loss of 49% tree canopy and 73% each of chaparral and shrub. This loss in vegetation resulted in decreased ecosystem services: Within the Cedar Fire area, stormwater runoff increased by 12,674,490 cubic feet. The value of retaining this additional stormwater, replacing what the trees did for free, is estimated at $25,349,000. The ability of the lost canopy to remove air pollutants decreased by 314,870 pounds per year, a loss in value estimated at $798,000 annually.
High-resolution satellite images collected after the fire were classified into Level 1 landcovers, which identify trees, shrubs, chaparral, open space, and impervious surfaces. CITYgreen was provided to San Diego’s city government along with training in its use.
As San Diego’s leaders make decisions about the restoration of their green infrastructure amid the challenges of an arid climate, across the country, the lush green environment of the central Carolinas faces a very different stormwater challenge—development. This 15-county, 4.5-million-acre region spread over parts of North and South Carolina is mostly in private ownership.
 |
| Cedar Fire Area Within San Diego City Boundary Pre-Fire (2002) |
“People in rural counties see a lot of wooded areas and don’t think they need to protect them,” says Rick Roti, chair of Charlotte, NC’s Tree Advisory Commission, as quoted in American Forests magazine. “Developers are gobbling it up at an astonishing rate. Overnight, rural spaces can become suburban USA.” American Forests is collaborating with Roti and local governments on a Carolina Piedmont Green Initiative to quantify the environmental and economic impacts of tree loss. The goal is for local leaders to develop strategies for incorporating green infrastructure into future development.
Trees Slow Stormwater Runoff and Improve Water Quality
Whether tree canopy across the country is declining due to fire, development, or other reasons, the resulting cleared or urban landcover produces much more stormwater runoff than the natural landscape. The Natural Resources Conservation Service, historically, and the Center for Watershed Protection, more recently, have deemed forest cover to be the best use of land for water storage, recharge, runoff reduction, pollutant reduction, and habitat. Tom Schuler, director of watershed research and practice for the center, sees percent forest cover—rather than impervious surface—as a leading indicator of watershed health.
Trees and soils function together to reduce stormwater runoff. Trees reduce stormwater flow by intercepting rainwater on leaves, branches, and trunks. Some of the intercepted water evaporates back into the atmosphere and some soaks into the ground, reducing the amount of runoff that must be managed in urban areas. Trees also slow storm flow, reducing the volume of water that a containment facility must store.
When stormwater hits impervious surfaces in urban areas, it increases the water temperature and also picks up various pollutants, everything from excess lawn fertilizers to oils on roadways. This translates into water-quality problems when large volumes of heated stormwater flow into receiving waters, posing a threat to temperature-sensitive species, such as trout and small invertebrates, as well as providing conditions for algal blooms and nutrient imbalances. Tree cover helps intercept rainwater, thus reducing the amount and speed of stormwater as it filters pollutants that eventually flow to receiving waters.
American Forests’ studies show that impervious surfaces in urban areas have increased by 20% over the past two decades at a cost exceeding $100 billion nationally. Local governments are increasingly looking toward non-built stormwater management strategies, including trees, to reduce the cost of constructing stormwater control infrastructure.
Tree Canopy, Stormwater Quality, and Decision Making
Obtaining reliable calculations of increased stormwater volumes and water quality as landcover becomes more urbanized is an important environmental and economic issue––but one that is poorly understood by decision makers. Not all landscape change occurs as dramatically or instantly as with fire or flood. Community growth and development occurs over several years or decades, and the impact of the changes is difficult to put into perspective. As a result, thousands of urban areas have grown rapidly and displaced natural systems entirely without attracting much attention.
 |
| Cedar Fire Area Within San Diego City Boundary Post-Fire (2004) |
Recognizing the disconnect that has occurred between rapidly growing urban areas and the natural landscape, American Forests packaged the expertise of scientists and engineers into a user-friendly GIS software product called CITYgreen to help local elected officials make decisions that connect nature with growth. Communities need tools to measure the effect of development on the natural cycles of air, water, and energy so they can make better decisions about how their communities will look in the future. While the engineers understand the technical aspects of calculating stormwater movement over various landcovers, urban decision makers need to have this knowledge packaged in a way that links green infrastructure to urban environmental issues the voting public responds to like economics and public health. Calculating the benefits of a green infrastructure meets this need (Moll 2005).
Creating a Green Data Layer
Those who work on stormwater and water-quality issues can add a green data layer to their GIS—landcover. This digital, green data layer provides an accurate measurement of features such as trees, impervious surface, shrubs, bare soil, water, and wetlands. The green data layer is created from aerial or satellite imagery (Lillesand and Keifer 1994). Aerial or satellite imagery with a high enough resolution to identify individual trees must first be obtained from either archived or newly collected images. The imagery must then be stratified or classified into different landcovers so it can be used in stormwater or air-quality calculations.
American Forests has found two types of imagery useful for communicating the values of a healthy green infrastructure. Landsat satellite images, which are 30-meter resolution, are very useful for documenting change in tree cover over time. This provides decision makers with data that show the rate and costs associated with their changing landcover at a scale suitable for public policy decisions. Landsat images for the Charlotte, NC, area covering a time span from 1984 to 2003 revealed a 20% loss in tree cover and open space, while urban surfaces increased by 127%. The loss of green infrastructure, valued at $5.3 billion, dramatically increased the volume of stormwater that the county manages.
However, this moderate-resolution satellite image does not provide enough detail to show individual trees (Schowengerdt 1997) and provide decision makers with the detail they need to manage growth and development on a daily basis. Recent technological advances in high-resolution satellite and aerial imagery allow more accurate representation of landcover. The high-resolution multi-spectral imagery with a 1-meter spatial resolution is well suited for local planning and management activities (Chou et al. 2001).
| Water Quality (Contaminant Loading) in San Diego |
 |
| Percent Change in Contaminant Loadings From 1984 to 2003 Due to Landcover Change |
After using the Landsat satellite images to document the disturbing loss in green infrastructure in the Carolina Piedmont Region, local officials, planners, and engineers are focused on the specific growth and development issues of the Mountain Island Lake Watershed (MIL). This 70-square-mile watershed provides 80% of the drinking water for the 700,000 people who live in the area, and the water quality is severely threatened by development. One of the larger subwatersheds within MIL is McDowell Creek, which covers some 30 square miles. Thousands of homes have already been built in the watershed, and many more are scheduled for construction. Already, the water entering Mountain Island Lake from McDowell Creek is unhealthy for swimming.
Even more disturbing is that the creek delivers polluted water into the lake. Although the lower third of McDowell Creek, closest to where the water enters the lake, has watershed protections, the upper 20% has no restrictions to growth and development. Sediment washes into the creek after a minor storm event acts as a magnet and attracts more pollutants, which are carried downstream into the lake. Another problem with sediment loading that settles in behind the dam is that it displaces the lake’s total water volume. The area’s stormwater engineers and most public officials recognize the need to take action. Developing a green data layer so they can analyze the effects of proposed development before it occurs has become an urgent need. Now that the ecosystem services of landcover can be measured, stormwater engineering should include green infrastructure, along with nonstructural measures, to ensure the quality of drinking water.
Image Analysis
Once the satellite or aerial imagery is obtained, it must be classified into different landcover types. A technical field of study, called digital image analysis, has developed around the classification of remotely sensed imagery. Highly accurate descriptions of the land can be produced from this approach. Different landcovers like trees, water, and roads reflect light in different wavelengths or bands.
“Our eyes can see the visible bands of the spectrum, while a digital image can record beyond the visible to infrared and other bands,” explains Russ Congalton, professor of remote sensing at the University of New Hampshire. “For example, trees look green to our eyes because they reflect green light. They also reflect infrared light that our eyes don’t see. The more wavelengths that an image can sense in, the easier it is to separate the different landcover types. In this case, the infrared portion of the spectrum increases our ability to tell landcover such as trees and agricultural crops apart.”
To fully understand the relationship between green infrastructure and ecosystem services, this digital landcover map must be connected to the ecological functions of the natural systems. “American Forests developed CITYgreen software to help people connect the structure of the land with the functions of the ecosystem,” says Moll. “People working in local agencies can analyze specific areas of interest within their community when planning, zoning, or development issues are relevant. Enabling local communities to conduct these analyses will ensure that they accurately reflect local issues and priorities. Agency staff will also attain a level of acceptance and usage within planning and management circles.”
First released in 1996, CITYgreen software is an extension of ESRI’s popular software products. American Forests currently offers CITYgreen 5 and CITYgreen for ArcGIS, corresponding to ESRI’s two platforms, ArcView and ArcGIS. Peer-reviewed CITYgreen uses algorithms developed by scientists and engineers to analyze the benefits of the green infrastructure for mitigating air pollution, storing and sequestering carbon, reducing stormwater, and improving water quality. The software builds an elaborate model of the landscape by combining classified image data with existing tabular datasets on the soil types and rainfall measures. These ecological models are critical for environmental monitoring systems (Bruns and Wiersma 2004). The analysis report generated provides quantifiable results in hard numbers and colorful graphs.
CITYgreen Incorporates TR-55 for Calculating Stormwater Runoff and Water Quality
The surface-water component of CITYgreen is based on the single event concepts documented by the widely used and time-tested Technical Release-55, Urban Hydrology for Small Watersheds (TR-55), developed by the Natural Resources Conservation Service (NRCS). TR-55 evaluates the impact of urbanization on runoff volume and peak flow. TR-55 formulas are used in most engineering firms, soil conservation districts, and municipalities around the country. The NRCS methods used in TR-55 are very effective in evaluating the effects on direct runoff of landcoverchanges and conservation practices (Sanders 1986). The TR-55 model uses NRCS curve numbers that represent the relative amount of imperviousness and water-infiltration properties of soil and landcover. Curve numbers range from 30 to 98; the smaller the number, the less the runoff (Woodward 2005).
McDowell Creek Watershed
Urban Ecosystem Analysis Findings
(Using 2001 classified high-resolution multi-spectral imagery) |
| | Acres | Tree Canopy | Stormwater Mgt. Value (cu. ft.) | Stormwater Mgt Value ($) |
| McDowell Creek Subwatershed | 20,780 | 51% | 52,671,000 | $105,343,000 |
| Planners can use this digital "green data layer" to model and analyze how the landcover will change ecologically with additional development and decide how to best manage it. |
The CITYgreen stormwater module incorporates several ancillary datasets to establish the curve number and runoff generated. The State Soil Geographic Database is used to determine the soil distribution of a specified analysis area. Data obtained from the National Weather Service provides rainfall amounts for a two-year, 24-hour storm event.
Using the ancillary data, the landcover dataset, and the location and size of the area of interest, CITYgreen uses algorithms (American Forests 2004) to calculate curve number and stormwater runoff volume. The software models the percent change between two landcover scenarios for the hydrologic conditions previously mentioned. For example, one scenario represents all the landcovers on the existing site. Another scenario would replace tree canopy with impervious surface. Percent change in runoff volume is determined automatically by comparing two different scenarios and calculating the additional stormwater that must be managed. The dollar benefit is calculated by multiplying this additional volume of stormwater by a local, user-defined cost per cubic foot mitigation (for example, the cost for building retention ponds, building additional stormwater management facilities, or treating water). The CITYgreen stormwater runoff analysis is not intended to be used to design stormwater management facilities, culverts, or ditches. Rather, the program is used to estimate the effects of landcover, especially trees, on runoff volume (Ray 2005).
The quantity of runoff from a storm event was the first water component of CITYgreen. In 2004, a water-quality element was added to the program with the release of CITYgreen for ArcGIS. The water-quality component was developed by Don Woodward of the NRCS, using values from the USEPA and Purdue University’s Long-Term Hydrologic Impact Assessment (L-THIA) spreadsheet water-quality model (Harbor 1994). The model calculates the effect of landcover on the amount of pollutants and suspended solids in surface-water runoff.
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Using NRCS curve numbers just as in the stormwater runoff component, this model determines the effect of landcover type on the event mean concentrations (concentration of the pollutants in runoff during a typical storm event) of nitrogen, phosphorus, suspended solids, zinc, lead, copper, cadmium, chromium, chemical oxygen demand, and biological oxygen demand. “The model works with TR-55 by making use of the curve number system: matching curve numbers with specific pollutant loadings during a storm event,” explains Woodward. “Values for the pollutants are given as a percentage of change in their concentrations.” (Visit www.americanforests.org/downloads/graytogreen/value
_waterquality.pdf for loading details).
Putting Urban Ecosystem Analyses Into Action
American Forests has conducted urban ecosystem analyses in more than 25 metropolitan areas over the last decade (see reports at www.americanforests.org/resources/urbanforests/analysis.php). The real value of this work is in the transfer of data, tools, and knowledge to local communities so that they can make informed planning decisions based on their green infrastructure and the ecosystem benefits it provides. Planners, stormwater managers, natural resource specialists, and GIS technicians have learned how to incorporate these data and tools into their GIS. Communities like the Carolinas’ Piedmont region and San Diego are taking the analysis findings into community planning meetings, rewriting local ordinances, setting citywide tree canopy goals, and incorporating green infrastructure into stormwater management plans to comply with Phase II of the National Pollutant Discharge Elimination System (NPDES). Quantifying the ecosystem benefits of green infrastructure provides the metrics to see how local landscape ordinances and stormwater management plans measure up to state and federal regulations and municipal goals. These metrics can also be used as incentives for development certification programs like LEED (Leadership in Energy and Environmental Design). Most importantly, by reconnecting communities to nature, as Joni Mitchell says, we will all know what we’ve got—before it’s gone. And this will enable community leaders to better plan for the future.
Author's Bio: Cheryl Kollin is vice president, Urban Ecosystem Center, American Forests, in Washington, DC.
July-August 2006
How Green Infrastructure Measures Up to Structural Stormwater Services
Quantifying the contributions of trees and vegetation
Joni Mitchell’s classic lyrics, “You don’t know what you’ve got ’til it’s gone,” aptly describes the aftermath of the Cedar Fire that swept into the city of San Diego in the fall of 2003. The devastating fire not only destroyed homes and businesses but also burned a significant portion of the vegetation, diminishing nature’s ability to manage stormwater and related water-quality issues. The California Department of Forestry and Fire Protection wanted to quantify the loss in terms of ecosystem services and posed this question to American Forests, a not-for-profit conservation organization.
Ironically, just four months before the fire, American Forests had prepared a digital GIS-based map of the area’s green infrastructure—trees and other vegetation together with their complex interaction with soil, air, and water. Green infrastructure is a significant asset that was not previously documented as part of the city infrastructure. These data, along with American Forests’ CITYgreen GIS software tool that quantifies ecosystem values, provided local elected officials with tools to better gauge the benefits of San Diego’s green infrastructure for future planning decisions.
|
Photo: Rusty Rozzelle, Mecklenburg County, NC |
| McDowell Creek, Carolina Piedmont Region |
“The dramatic and sudden loss of vegetation warranted an update to the digital GIS map or what we refer to as a ‘green data layer,’” explains Gary Moll, senior vice president at American Forests. “These data and analysis tools give city leaders a quantitative measure of the lost ecosystem services, including the dollar value for providing stormwater management.”
The Cedar Fire affected 28,466 acres of land within the San Diego city limits, about 13% of the entire city. Comparing pre- and post-fire conditions in the Cedar Fire area, American Forests reported a loss of 49% tree canopy and 73% each of chaparral and shrub. This loss in vegetation resulted in decreased ecosystem services: Within the Cedar Fire area, stormwater runoff increased by 12,674,490 cubic feet. The value of retaining this additional stormwater, replacing what the trees did for free, is estimated at $25,349,000. The ability of the lost canopy to remove air pollutants decreased by 314,870 pounds per year, a loss in value estimated at $798,000 annually.
High-resolution satellite images collected after the fire were classified into Level 1 landcovers, which identify trees, shrubs, chaparral, open space, and impervious surfaces. CITYgreen was provided to San Diego’s city government along with training in its use.
As San Diego’s leaders make decisions about the restoration of their green infrastructure amid the challenges of an arid climate, across the country, the lush green environment of the central Carolinas faces a very different stormwater challenge—development. This 15-county, 4.5-million-acre region spread over parts of North and South Carolina is mostly in private ownership.
|
| Cedar Fire Area Within San Diego City Boundary Pre-Fire (2002) |
“People in rural counties see a lot of wooded areas and don’t think they need to protect them,” says Rick Roti, chair of Charlotte, NC’s Tree Advisory Commission, as quoted in American Forests magazine. “Developers are gobbling it up at an astonishing rate. Overnight, rural spaces can become suburban USA.” American Forests is collaborating with Roti and local governments on a Carolina Piedmont Green Initiative to quantify the environmental and economic impacts of tree loss. The goal is for local leaders to develop strategies for incorporating green infrastructure into future development.
Trees Slow Stormwater Runoff and Improve Water Quality
Whether tree canopy across the country is declining due to fire, development, or other reasons, the resulting cleared or urban landcover produces much more stormwater runoff than the natural landscape. The Natural Resources Conservation Service, historically, and the Center for Watershed Protection, more recently, have deemed forest cover to be the best use of land for water storage, recharge, runoff reduction, pollutant reduction, and habitat. Tom Schuler, director of watershed research and practice for the center, sees percent forest cover—rather than impervious surface—as a leading indicator of watershed health.
Trees and soils function together to reduce stormwater runoff. Trees reduce stormwater flow by intercepting rainwater on leaves, branches, and trunks. Some of the intercepted water evaporates back into the atmosphere and some soaks into the ground, reducing the amount of runoff that must be managed in urban areas. Trees also slow storm flow, reducing the volume of water that a containment facility must store.
When stormwater hits impervious surfaces in urban areas, it increases the water temperature and also picks up various pollutants, everything from excess lawn fertilizers to oils on roadways. This translates into water-quality problems when large volumes of heated stormwater flow into receiving waters, posing a threat to temperature-sensitive species, such as trout and small invertebrates, as well as providing conditions for algal blooms and nutrient imbalances. Tree cover helps intercept rainwater, thus reducing the amount and speed of stormwater as it filters pollutants that eventually flow to receiving waters.
American Forests’ studies show that impervious surfaces in urban areas have increased by 20% over the past two decades at a cost exceeding $100 billion nationally. Local governments are increasingly looking toward non-built stormwater management strategies, including trees, to reduce the cost of constructing stormwater control infrastructure.
Tree Canopy, Stormwater Quality, and Decision Making
Obtaining reliable calculations of increased stormwater volumes and water quality as landcover becomes more urbanized is an important environmental and economic issue––but one that is poorly understood by decision makers. Not all landscape change occurs as dramatically or instantly as with fire or flood. Community growth and development occurs over several years or decades, and the impact of the changes is difficult to put into perspective. As a result, thousands of urban areas have grown rapidly and displaced natural systems entirely without attracting much attention.
|
| Cedar Fire Area Within San Diego City Boundary Post-Fire (2004) |
Recognizing the disconnect that has occurred between rapidly growing urban areas and the natural landscape, American Forests packaged the expertise of scientists and engineers into a user-friendly GIS software product called CITYgreen to help local elected officials make decisions that connect nature with growth. Communities need tools to measure the effect of development on the natural cycles of air, water, and energy so they can make better decisions about how their communities will look in the future. While the engineers understand the technical aspects of calculating stormwater movement over various landcovers, urban decision makers need to have this knowledge packaged in a way that links green infrastructure to urban environmental issues the voting public responds to like economics and public health. Calculating the benefits of a green infrastructure meets this need (Moll 2005).
Creating a Green Data Layer
Those who work on stormwater and water-quality issues can add a green data layer to their GIS—landcover. This digital, green data layer provides an accurate measurement of features such as trees, impervious surface, shrubs, bare soil, water, and wetlands. The green data layer is created from aerial or satellite imagery (Lillesand and Keifer 1994). Aerial or satellite imagery with a high enough resolution to identify individual trees must first be obtained from either archived or newly collected images. The imagery must then be stratified or classified into different landcovers so it can be used in stormwater or air-quality calculations.
American Forests has found two types of imagery useful for communicating the values of a healthy green infrastructure. Landsat satellite images, which are 30-meter resolution, are very useful for documenting change in tree cover over time. This provides decision makers with data that show the rate and costs associated with their changing landcover at a scale suitable for public policy decisions. Landsat images for the Charlotte, NC, area covering a time span from 1984 to 2003 revealed a 20% loss in tree cover and open space, while urban surfaces increased by 127%. The loss of green infrastructure, valued at $5.3 billion, dramatically increased the volume of stormwater that the county manages.
However, this moderate-resolution satellite image does not provide enough detail to show individual trees (Schowengerdt 1997) and provide decision makers with the detail they need to manage growth and development on a daily basis. Recent technological advances in high-resolution satellite and aerial imagery allow more accurate representation of landcover. The high-resolution multi-spectral imagery with a 1-meter spatial resolution is well suited for local planning and management activities (Chou et al. 2001).
| Water Quality (Contaminant Loading) in San Diego |
|
| Percent Change in Contaminant Loadings From 1984 to 2003 Due to Landcover Change |
After using the Landsat satellite images to document the disturbing loss in green infrastructure in the Carolina Piedmont Region, local officials, planners, and engineers are focused on the specific growth and development issues of the Mountain Island Lake Watershed (MIL). This 70-square-mile watershed provides 80% of the drinking water for the 700,000 people who live in the area, and the water quality is severely threatened by development. One of the larger subwatersheds within MIL is McDowell Creek, which covers some 30 square miles. Thousands of homes have already been built in the watershed, and many more are scheduled for construction. Already, the water entering Mountain Island Lake from McDowell Creek is unhealthy for swimming.
Even more disturbing is that the creek delivers polluted water into the lake. Although the lower third of McDowell Creek, closest to where the water enters the lake, has watershed protections, the upper 20% has no restrictions to growth and development. Sediment washes into the creek after a minor storm event acts as a magnet and attracts more pollutants, which are carried downstream into the lake. Another problem with sediment loading that settles in behind the dam is that it displaces the lake’s total water volume. The area’s stormwater engineers and most public officials recognize the need to take action. Developing a green data layer so they can analyze the effects of proposed development before it occurs has become an urgent need. Now that the ecosystem services of landcover can be measured, stormwater engineering should include green infrastructure, along with nonstructural measures, to ensure the quality of drinking water.
Image Analysis
Once the satellite or aerial imagery is obtained, it must be classified into different landcover types. A technical field of study, called digital image analysis, has developed around the classification of remotely sensed imagery. Highly accurate descriptions of the land can be produced from this approach. Different landcovers like trees, water, and roads reflect light in different wavelengths or bands.
“Our eyes can see the visible bands of the spectrum, while a digital image can record beyond the visible to infrared and other bands,” explains Russ Congalton, professor of remote sensing at the University of New Hampshire. “For example, trees look green to our eyes because they reflect green light. They also reflect infrared light that our eyes don’t see. The more wavelengths that an image can sense in, the easier it is to separate the different landcover types. In this case, the infrared portion of the spectrum increases our ability to tell landcover such as trees and agricultural crops apart.”
To fully understand the relationship between green infrastructure and ecosystem services, this digital landcover map must be connected to the ecological functions of the natural systems. “American Forests developed CITYgreen software to help people connect the structure of the land with the functions of the ecosystem,” says Moll. “People working in local agencies can analyze specific areas of interest within their community when planning, zoning, or development issues are relevant. Enabling local communities to conduct these analyses will ensure that they accurately reflect local issues and priorities. Agency staff will also attain a level of acceptance and usage within planning and management circles.”
First released in 1996, CITYgreen software is an extension of ESRI’s popular software products. American Forests currently offers CITYgreen 5 and CITYgreen for ArcGIS, corresponding to ESRI’s two platforms, ArcView and ArcGIS. Peer-reviewed CITYgreen uses algorithms developed by scientists and engineers to analyze the benefits of the green infrastructure for mitigating air pollution, storing and sequestering carbon, reducing stormwater, and improving water quality. The software builds an elaborate model of the landscape by combining classified image data with existing tabular datasets on the soil types and rainfall measures. These ecological models are critical for environmental monitoring systems (Bruns and Wiersma 2004). The analysis report generated provides quantifiable results in hard numbers and colorful graphs.
CITYgreen Incorporates TR-55 for Calculating Stormwater Runoff and Water Quality
The surface-water component of CITYgreen is based on the single event concepts documented by the widely used and time-tested Technical Release-55, Urban Hydrology for Small Watersheds (TR-55), developed by the Natural Resources Conservation Service (NRCS). TR-55 evaluates the impact of urbanization on runoff volume and peak flow. TR-55 formulas are used in most engineering firms, soil conservation districts, and municipalities around the country. The NRCS methods used in TR-55 are very effective in evaluating the effects on direct runoff of landcoverchanges and conservation practices (Sanders 1986). The TR-55 model uses NRCS curve numbers that represent the relative amount of imperviousness and water-infiltration properties of soil and landcover. Curve numbers range from 30 to 98; the smaller the number, the less the runoff (Woodward 2005).
McDowell Creek Watershed
Urban Ecosystem Analysis Findings
(Using 2001 classified high-resolution multi-spectral imagery) |
| | Acres | Tree Canopy | Stormwater Mgt. Value (cu. ft.) | Stormwater Mgt Value ($) |
| McDowell Creek Subwatershed | 20,780 | 51% | 52,671,000 | $105,343,000 |
| Planners can use this digital "green data layer" to model and analyze how the landcover will change ecologically with additional development and decide how to best manage it. |
The CITYgreen stormwater module incorporates several ancillary datasets to establish the curve number and runoff generated. The State Soil Geographic Database is used to determine the soil distribution of a specified analysis area. Data obtained from the National Weather Service provides rainfall amounts for a two-year, 24-hour storm event.
Using the ancillary data, the landcover dataset, and the location and size of the area of interest, CITYgreen uses algorithms (American Forests 2004) to calculate curve number and stormwater runoff volume. The software models the percent change between two landcover scenarios for the hydrologic conditions previously mentioned. For example, one scenario represents all the landcovers on the existing site. Another scenario would replace tree canopy with impervious surface. Percent change in runoff volume is determined automatically by comparing two different scenarios and calculating the additional stormwater that must be managed. The dollar benefit is calculated by multiplying this additional volume of stormwater by a local, user-defined cost per cubic foot mitigation (for example, the cost for building retention ponds, building additional stormwater management facilities, or treating water). The CITYgreen stormwater runoff analysis is not intended to be used to design stormwater management facilities, culverts, or ditches. Rather, the program is used to estimate the effects of landcover, especially trees, on runoff volume (Ray 2005).
The quantity of runoff from a storm event was the first water component of CITYgreen. In 2004, a water-quality element was added to the program with the release of CITYgreen for ArcGIS. The water-quality component was developed by Don Woodward of the NRCS, using values from the USEPA and Purdue University’s Long-Term Hydrologic Impact Assessment (L-THIA) spreadsheet water-quality model (Harbor 1994). The model calculates the effect of landcover on the amount of pollutants and suspended solids in surface-water runoff.
Using NRCS curve numbers just as in the stormwater runoff component, this model determines the effect of landcover type on the event mean concentrations (concentration of the pollutants in runoff during a typical storm event) of nitrogen, phosphorus, suspended solids, zinc, lead, copper, cadmium, chromium, chemical oxygen demand, and biological oxygen demand. “The model works with TR-55 by making use of the curve number system: matching curve numbers with specific pollutant loadings during a storm event,” explains Woodward. “Values for the pollutants are given as a percentage of change in their concentrations.” (Visit www.americanforests.org/downloads/graytogreen/value
_waterquality.pdf for loading details).
Putting Urban Ecosystem Analyses Into Action
American Forests has conducted urban ecosystem analyses in more than 25 metropolitan areas over the last decade (see reports at www.americanforests.org/resources/urbanforests/analysis.php). The real value of this work is in the transfer of data, tools, and knowledge to local communities so that they can make informed planning decisions based on their green infrastructure and the ecosystem benefits it provides. Planners, stormwater managers, natural resource specialists, and GIS technicians have learned how to incorporate these data and tools into their GIS. Communities like the Carolinas’ Piedmont region and San Diego are taking the analysis findings into community planning meetings, rewriting local ordinances, setting citywide tree canopy goals, and incorporating green infrastructure into stormwater management plans to comply with Phase II of the National Pollutant Discharge Elimination System (NPDES). Quantifying the ecosystem benefits of green infrastructure provides the metrics to see how local landscape ordinances and stormwater management plans measure up to state and federal regulations and municipal goals. These metrics can also be used as incentives for development certification programs like LEED (Leadership in Energy and Environmental Design). Most importantly, by reconnecting communities to nature, as Joni Mitchell says, we will all know what we’ve got—before it’s gone. And this will enable community leaders to better plan for the future.