Using GIS to Assess Stormwater Benefits of Urban Trees

By Mark C. Dwyer and Robert W. Miller
Stevens Point, WI (March 1999)- CITYgreen, a geographic information system (GIS)-based program, was used to evaluate selected benefits provided by the tree canopy in the city of Stevens Point, Wisconsin. The distribution of open space in and around the greater Stevens Point area, energy savings from lowered air-conditioning costs, and the reductions in stormwater runoff as a partial function of existing tree canopy were assessed. Estimated annual energy savings for residential areas in Stevens Point and surrounding communities was $126,859.

Identifying and describing the benefits of the urban forest to a community is the first step in gaining support for an urban forestry program of tree planting, maintenance, and replacement. Similarly, community planners require information regarding current land-use and open-space distributions in order to direct future patterns of growth and greenspace development. Urban forest structure, defined by the size, species composition, and location of urban trees, is typically expressed as a percentage of tree canopy cover over a city, and is an indicator of the contributions of the urban forest (Nowak 1991).
Urban tree canopies reduce summer cooling (air-conditioning) costs by shading structures and air conditioners. The magnitude of cooling energy savings due to direct tree shade depends on tree placement, crown shape, crown density, growth rate, and longevity (McPherson and Rowntree 1993), as well as the age of building construction, occupant behavior, window location, and seasonal solar angles (Simpson and McPherson 1996). The individual homeowner can obtain substantial savings from optimal tree shade, but greater benefits will accrue for the community as a whole due to the combined effect of the overall urban canopy on the local climate. Reduced demand for air-conditioning energy will lower the amount of fossil fuel and/or water consumed by power plants, resulting in both conserved water and reduced emissions of carbon dioxide and other pollutants from smokestacks (McPherson 1991; Nowak 1995).
Urbanization increases the land area that is covered with impermeable surfaces such as streets, sidewalks, driveways, and building rooftops. Rain falling on these surfaces flows quickly into sewers, increasing the incidence and severity of flooding (McPherson 1990). Tree canopies intercept rainfall, thereby reducing peak discharge into stormwater sewers. This interception allows for groundwater recharge, reduces the cost of stormwater disposal, and averts flooding and sedimentation of waterways. Slope, soil type, amount of rain, and other factors also affect stormwater runoff rates.
A storm delivering 6.6 cm (2.6 in.) of rain in 24 hours will deposit just under 2 billion L (530 million gal) of water on Stevens Point, of which 400 million L (106 million gal) will run off into the Wisconsin River. Approximately 6% of Stevens Point is covered by impervious surfacing, which accounts for 24% of the city’s total stormwater runoff volume. Orthophotographs were digitized on screen, and land surrounding Stevens Point was classified based on vegetation cover, land use, and current zoning. Land use in the greater Stevens Point area (22,250 ha [55, 000 ac]) is 20.7% developed, 24.1% agriculture, 46.8% undeveloped, and 8.4% surface water. Planners, managers, elected officials, and other interested parties in land-use planning for the region are using the results of this study for open-space planning.
The amount of tree canopy in urban environments, however, is a “controllable” element that significantly and measurably affects stormwater runoff rates and volumes. To ensure future contributions of the urban forest, urban foresters must frequently look beyond the community at larger systems of forest and other open space that has potential to be preserved or conserved as land is converted for residential, commercial, and industrial use. A comprehensive planning approach, frequently encompassing multiple jurisdictions, could plan development patterns so as to maintain natural areas and greenspace while allowing for growth.
Recent developments in technology have allowed for an accurate and comprehensive assessment of the structure and benefits provided by urban forests. Aerial photographs combined with remotely sensed spatial data provide an overview of natural resources and land use. When used in conjunction with GIS technology, information on urban forests can be analyzed and updated. GIS reduces the time needed for map production, revisions, and information storage while allowing for the combination of data “layers” and the timely analysis of spatial variables.
For the full article, visit the Journal of Arboriculture.