Ecosystems Research Division

Urban/suburban land uses are the most rapidly growing land use class generating non-point source loadings likely to seriously impair streams. Impervious cover is the amount of land cover in roads, buildings and parking lots, and turf grass cover in a watershed and can seriously impact biotic integrity in associated streams. Methods are being developed to estimate and project impervious cover and turf grass area in small watersheds based on regionally available data. These estimations and projections will be used to guide monitoring and focus educational efforts on the importance of watershed management in developing localities.

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The Importance of Impervious Cover

Nonpoint source pollution (NPS) or pollution from diffuse sources such as urban/suburban areas and farmlands is now recognized as the primary threat to water quality in the United States. NPS pollution threats from urban and suburban development are increasing as the U.S. population rises. Along with this increase in development comes an increase in impervious surfaces--areas where infiltration of water into the underlying soil is prevented. Roadways and rooftops account for the majority of this impervious area. Research in recent years has consistently shown a strong relationship between the percentage of impervious cover in a watershed and the health of the receiving stream. Scientists generally agree that stream degradation consistently occurs at relatively low levels of imperviousness (10 to 20%). Increased impervious surfaces alter stream hydrology resulting in lower flows during droughts and higher peak flows during floods. Roadways and other impervious areas channel pollutants directly into streams without being processed during transport through the soil. With advanced planning and identification of at-risk watersheds, total impervious cover can be reduced during development within a watershed and steps taken to mitigate the impacts of added impervious cover.

GIS and Spatial Data

A Geographic Information System (GIS) is a computer system capable of assembling, storing, manipulating and displaying geographically referenced information, i.e. data identified according to their locations. The way maps and other data have been stored or filed as layers of information in a GIS makes it possible to perform complex analyses.

Digital orthophoto quarter quadrangles (DOQQ) are computer-generated versions of aerial photographs that represent true map distances available throughout the nation. Ground features can be identified and categorized efficiently and accurately by a human analyst with the help of GIS software. A user looking at a good quality photograph can differentiate features using shape, spatial relationships and geographic context.

Readily available high resolution satellite imagery is providing rapidly expanding use of remote sensing techniques for impervious cover estimation. Classified land cover data is now available throughout the country. Detailed road networks and block level census data are other GIS data sources available to aid in estimating impervious cover.

Research Objective

Impervious cover is proposed as an indicator of aquatic conditions for sub-watersheds throughout the country. The focus here is on methods that would be useful in doing region-wide assessments. The utility of impervious cover as an indicator is a function of the ease and accuracy for estimating it. The question then is with what degree of accuracy can impervious cover be estimated for sub-watershed areas from data available throughout a region, e.g. categorized land use/land cover data, block level census data and roadways.

Research Summary
An impervious cover test data set for 56 14-digit sub-watersheds in Frederick County, MD was developed from interpretation of DOQQs by multiple analysts. This test data set was used to evaluate different estimation techniques suitable for application to a regional scale characterization. Methods tested include empirical relationships with population density data, estimations from land use zoning data, analysis of categorized land cover data and use of a combination of data sources. Characterized land cover data can identify commercial/manufacturing areas and high density residential development, but does not accurately identify lower density suburban development. Population density relationships identify the residential contribution to impervious cover. The most accurate estimation technique was based on use of a combination of data including block level population density from the U.S. Census Bureau, industrial/commercial areas from the national land cover data, and interstate and major US roadways from the U.S. Department of Transportation. Using a combination of data sources, we were able to estimate the percentage of impervious cover in a watershed within +/- 1%.

Additional impervious cover data sets were developed from aerial photographs for approximately 209 watersheds in the Atlanta area for two time periods. These data sets were used to confirm estimation methods and to determine the accuracy of methods to project future changes in impervious surface area. The estimation technique developed in this research can be used by states and regional offices to target watersheds for monitoring and mitigation efforts.

The overall goal of this study was the development and application of a simple, reliable method for estimating and projecting impervious cover in small watersheds applied by region or state. We accomplished that goal by applying the method for all the states in the EPA's Region 4--southeastern U.S. The report describing this research and the impervious estimates for the southeastern U.S., "Estimating and Projecting Impervious Cover in the Southeastern United States," is listed below and accessible as a pdf. A 2-page research abstract (PDF, 2 pp., 85 KB, about PDF) is available.

To access the downloadable shapefiles with estimated and projected impervious cover for the 8 U.S. Southeastern states, click here.

Participants
• U.S. EPA, Office of Research and Development, National Exposure Research Laboratory, Ecosystems Research Division, Athens, GA
• U.S. EPA, Region 4, Water Management Division, Atlanta, GA

For More Information Contact: Lourdes Prieto prieto.lourdes@epa.gov

For more information on this and other NERL science projects, visit our Website http://www.epa.gov/nerl/

Publications from this research

Exum, L., R., S. L. Bird, J. Harrison, and C. A. Perkins. Estimating and Projecting Impervious Cover in the Southeastern United States. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-05/061, 2005. Click here to access downloadable pdf.

Bird, S., J. Harrison, L. Exum, S. Alberty and C. Perkins. 2002. "Screening to Identify and Prevent Urban Storm Water Problems: Estimating Impervious Area Accurately and Inexpensively." In Proceedings for National Water Quality Monitoring Conference, May 20-23, 2002, Madison, WI.

Bird, S.L., S.W. Alberty, L.R. Exum. "Generating High Quality Impervious Cover Data." Quality Assurance. 8:91-103. 2000.

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Of Continuing Interest.......

See Estimating and Projecting Impervious Cover poster at the NERL EPA 2005 Science Forum page.

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The following pdf is a poster from the ReVA workshop held in Durham, NC, October 29-30, 2001 highlighting our method for estimating impervious cover: http://www.epa.gov/athens/highlights/le1335.pdf (PDF, 1pp., 5.8 MB, about PDF). Abstract: Estimating Impervious Cover from Regionally Available Data--Nonpoint source pollution (NPS) or pollution from diffuse sources such as urban/suburban areas and farmlands is now recognized as the primary threat to water quality in the United States. NPS pollution threats from urban and suburban development are increasing as the U.S. population rises. Along with this increase in development comes an increase in impervious surfaces--areas where infiltration of water into the underlying soil is prevented. Roadways and rooftops account for the majority of this impervious area. Population density can be used to project impervious cover in watersheds. Existing relationships tend to under predict impervious cover, however, in relatively low density (<2000 persons/sq mi) areas. Categorized satellite imagery does not adequately capture impervious cover in suburbanized areas at levels where initial degradation of water quality may be occurring. Use of block level census data to account for residential sources of impervious cover coupled with satellite imagery identifying commercial, manufacturing, mining and quarrying along with road network information appears to be a promising approach for developing a usefully accurate impervious cover indicator.