Indicator ES.2.e Proportion of impervious ground surfaces

Ground Cover Distribution by Land Use in San Francisco
ES.2.e Proportion of impervious ground surfaces
Percentage of ground cover distribution by land-use in San Francisco
Land use Bare Soil Duff/Mulch Herbs Water Impervious* Building
Commercial/Industrial 1.2 3.9 3 5 54.8 32.2
Institutional 6.6 1.7 28.2 0 41.6 22
Street/ROW** 2.9 0.1 13.4 6.7 66.5 10.3
Open space 8.5 6.9 43.6 13.7 26.3 1
Residential 4.8 0.9 14.4 0.1 30.5 49.2
Vacant 6.3 0 31.7 5.2 28.9 27.9
*impervious surfaces include: streets, driveways, sidewalks, parking lots
*pervious surfaces include: bare soil, duff/mulch, herbs and water
**ROW = right of way.

Data Source

Ground cover figures and chart from San Francisco Department of the Environment, USDA Assessing Urban Forests Effects (UFORE) and Values: San Francisco's Urban Forest, Accessed on July 19, 2006: http://www.sfenvironment.org/downloads/library/sfuforefinal.pdf

Environmental Protection Agency, Terms of Environment. Accessed on July 19, 2006: http://www.epa.gov/OCEPAterms/

Explanation and Limitations

According to the Environmental Protection Agency, "impervious or impermeable surfaces" are defined as surfaces that either do not allow, or allow only with great difficulty, the movement or passage of water.

Although porous pavement is generally recommended in most areas for its benefits related to storm water runoff and decreased heat retention, porous pavement should be considered carefully in areas heavily traveled by idling cars and trucks. For more information on porous pavement, see June 2005 EPA-commissioned report on Cool Pavement. Available at: http://www.epa.gov/heatisland/resources/pdf/CoolPavementReport_Former%20Guide_complete.pdf

To help understand the urban forest resource better and its numerous values, the USDA Forest Service, Northeastern Research Station, developed the Urban Forest Effects (UFORE) model to use standardized data from randomly located plots, and local hourly air pollution and meteorological data to quantify urban forest structure and numerous urban forest effects. In addition to measuring urban forest structure, the model also estimates ground cover and land-use. To determine ground cover distribution, a total of 200 one-tenth acre field plots throughout the city were randomly located based on land-use and analyzed using the UFORE model.

Dominant ground cover types in San Francisco include herbaceous (i.e., grass, gardens) (34%), impervious surfaces excluding buildings (i.e., driveways, streets, sidewalks, parking lots) (43%), and buildings (26%).

Why is this a Community Health Indicator?

Pervious surfaces allow natural ground absorption of rainfall, replenishing groundwater tables and reducing the amount of storm-water runoff to the Bay. In turn, contaminant runoff to the Bay is reduced and residents swimming or fishing in the Bay have reduced exposure to oils, lead, and other toxins. In addition, pervious surfaces reduce sewage system loads by assimilating large amounts of rainwater. This reduces the potential for untreated sewage to be discharged, creating unsanitary conditions in the ocean and on the beach.