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Determining the Components of Impervious Surfaces in Urban Watersheds

Research Activity: Land Characterization Research
Duration: Two years (FY 2002-2003)
Cooperative Research: U.S. Geological Survey and Environmental Protection Agency for Federal Highway Administration, Department of Transportation
Coordinator: Janet Tilley
Regional Investigations Team, Eastern Regional Geography
Study Area: One watershed per quadrant of the United States

Study Areas
Final Analysis and Conclusions
Disclaimer and Privacy Statement


One of the emerging areas of scientific interest in Photograph of a built-up area the control of nonpoint-source pollution (NPS) is the detection and analysis of impervious surfaces within watersheds. NPS runoff from urban surfaces is now a leading threat to water quality, and the percentage of impervious surface within a particular watershed has been recognized as a key indicator of the effects of nonpoint runoff and of future water and ecosystem quality (Arnold and Gibbons, 1996; USEPA, 1994). The imperviousness issue has even been suggested as a unifying theme for overall study of watershed protection (Schueler, 1994) and as part of an urban ecosystems analytical model (Ridd, 1995).

Impervious surfaces can be generally defined as any material of natural or anthropogenic source that prevents the infiltration of water into soil, thereby changing the flow dynamics, sedimentation load, and pollution profile of storm water runoff. The growth of impervious surfaces is directly related to human activity and habitation through the construction of buildings, roads, parking lots, sidewalks, and so on. As precipitation is diverted from possible soil infiltration, the unfiltered flow over Images of urban storm water runoffthe impervious surface allows significant increases in water runoff, as well as a rise in the acquisition and retention rate of chemical contaminants and sediments from anthropogenic sources. The subsequent surge in the in flow rate and volume in the receiving stream brings about an enlargement of bank-full and stream scour events and significantly influences the morphological structure.

The in-stream and riparian ecology is thus altered owing to changes in structural habitat and the related increases in sedimentation and pollution loadings (Arnold and Gibbons, 1996).

Although the effect of land use, population, and impervious surface cover on water quality has been generally known for 30 years, a basic problem exists in quantifying the detailed spatial extent and distribution of various classes of impervious surface phenomena. The level of imperviousness in a watershed is recognized as a key indicator of ecological condition; however, accurate and quantifiable measurements of impervious area remain elusive (Slonecker and others, 2001). Further research is needed to determine the best methods of mapping impervious surfaces and the appropriate scales for mapping and analysis. A recent report (GAO, 2001) by the General Accounting Office (GAO) recommends that both the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Transportation (DOT) should devise strategies to help State and local governments assess the impacts of land use on air and water quality.


The purpose of this research is to demonstrate scale-dependent methods for mapping impervious areas and to determine, by using remote sensing, land use and land cover, and impervious surface data, the individual contributions of the various components of impervious surfaces to the overall storm water runoff issue.

This research seeks to answer three critical questions:

  1. What are the scale issues involved with the existing methods in determining the extent of impervious surfaces?
  2. What are the individual components of impervious surfaces, and how might these be mapped with high resolution imagery data?
  3. How does the transportation infrastructure, at the State and local level, affect the overall impervious issue in urban watersheds?


Four individual studies are proposed, one per geographic quadrant of the conterminous United States. Different study sites across the United States were selected to obtain areas with unique land surface characteristics that have different impacts upon the watersheds. Within each of these quadrants, one watershed will be selected on the basis of size, degree of urbanization, the availability of land use/land cover (LU/LC), and impervious geographic information systems (GIS) data. Currently, the first watershed model is being completed for all three tasks, which are described below. Methods and initial results of this first watershed model will be presented to various organizations with interests in this type of research for their input, comments, and potential modifications to the methods. Additionally, time to complete each task in the first watershed will be documented and eventually compared with the time required to complete tasks in the other watersheds and will be available for improved project production.

This research seeks to determine the components and percentages of impervious surface in typical urban watersheds and will use existing remote sensing and GIS data in a three–phase approach that will utilize increasing levels of detail to help find the best method, in terms of accuracy, cost, and time, of determining impervious surface from existing data sources.

Image of National Land Cover Data

Three primary tasks will be completed for each watershed:

Task 1.  Generating Broad Statistical Analysis. This initial task will utilize Multiresolution Land Characteristics/National Land Characteristics data (fig. 3) and U.S. Geological Survey (USGS) digital line graph (DLG) data and will offer a coarse regional view and statistics of the impervious surface extents. Information collected will be at an Anderson level one and two. This task is designed to produce rapid results for a quick overview of impervious surfaces amounts.

Task 2.   Detailed Statistical Analysis. Using a more detailed GIS dataset acquired from State DOTs, counties, and other data sources, this task will encompass a more detailed mapping of the land use for each watershed. The total area of impervious surface will be determined either through direct mapping of impervious surfaces or though statistical calculations of land use category. The area of urban land use attributable to State-DOT maintained highway systems will be determined by analyzing DLG or other digital transportation databases and will be computed for each watershed. The mapping detail could extend to the Anderson level 3 classification. This task is dependent upon data availability.

Task 3.   Detailed Mapping of Random Samples. Three random 1-square-mile urban areas within the watershed will be mapped in GIS format from very high resolution imagery sources and coded to very specific impervious surface infrastructure components (see figure 4 for an example). The following features will be compiled into separate GIS layers:
Rooftops Private driveways
Private roads Parking lots
Federal, State, and county roadways Trails and recreation areas
Sidewalks Pools/patios
Quarries Driveways
Natural Outcrops  

compelation of pictures representing Figure 4.
Figure 4. (A) Data for task 3 were compiled by using 1-square-mile of 1-foot resolution imagery. (B) Featured area enlarged showing compiled area. (C) Data collected for all features in the one square mile area.

These samples, the compiled 1-square-mile area, will be used to statistically characterize the relative contribution of each land use category and to validate the component estimates being made from coarser source materials. High-resolution imagery and aerial photographs will be used for this task.

Study Areas

The study area consists of one watershed per quadrant of the United States. The first of the four study areas is located in the Goose Creek watershed of Virginia. The other areas have just been selected: Seattle, Washington, Tampa Florida, and Lincoln, NE.

Map of Goose Creek WatershedBefore the work continues to the other study areas across the United States, the researchers will complete all three tasks on the first study area. This will ensure that the researchers are achieving the desired objective . Once the initial results have been analyzed, the researchers will discuss these results with colleagues who have interest and expertise in this area of the research process.

Modifications to methodology will be made as appropriate and incorporated into the project to accommodate any missing concepts.


1. Report: A comprehensive report presenting measured components of impervious surfaces will be submitted to the DOT for review. As the research progresses, periodic status reports will be provided to the DOT. Additionally, reports will be submitted to appropriate peer-reviewed journals (due September 2003).

2. Web Site: The Web site will offer information about the project, study areas, progress, participants and contacts, data used, illustrations, and so on (basic information available in summer 2002 and more comprehensive Web site due September 2002).

3. Database: The database will offer access to GIS data utilized for this research (due with metadata September 2003).

4. Miscellaneous Map and Poster Products: These will show the level of impervious surfaces in watershed study areas, with specially featured subwatershed views and detailed illustrations of select areas showing the different categories of impervious surfaces (first in series due summer of 2002).


Federal Highway Administration, Department of Transportation

Environmental Protection Agency

U.S. Geological Survey


Janet Tilley
Cartographer, USGS

Terry Slonecker
Environmental Scientist, EPA

Gail Winters
GIS Specialist, USGS

Final Analysis and Conclusions

Research is currently underway.


Arnold, C.L. and Gibbons, C.J., 1996, Impervious surface coverage: American Planning Association Journal, v. 62, no. 2, p. 243-258.

General Accounting Office, June 2001, Water Quality: Better Data and Evaluation of Urban Runoff Programs Needed to Assess Effectiveness, Report GAO-01-679, Washington, D.C.

Ridd, M.K., 1995, Exploring a V-I-S (vegetation-impervious surface-soil) Model for Urban Ecosystems Analysis Through Remote Sensing: Comparative Anatomy for Cities, International Journal of Remote Sensing, v. 16, no.12, p.2165-2185.

Schueler T.R., 1994, The Importance of Imperviousness: Watershed Protection Techniques, v. 1 no. 3, p. 100-111.

Slonecker, E.T., Jennings, D.B., and Garofalo, D., 1994, Remote Sensing of Impervious Surfaces: A Review, Remote Sensing Reviews, v. 20 no.3, p. 227-255.

U.S. Environmental Protection Agency, 2001, The Quality of Our Nation's Water, 1992, United States Environmental Protection Agency, EPA-841-S-94-002, Washington, D.C., USEPA, Office of Water.

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Last modified: 17:48:54 Mon 14 Jan 2013 — Accessibility