One of the emerging areas of scientific interest in
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 the
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:
- What are the scale issues involved with the existing methods
in determining the extent of impervious surfaces?
- What are the individual components of impervious surfaces, and
how might these be mapped with high resolution imagery data?
- 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 threephase 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
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:
|Federal, State, and county roadways
||Trails and recreation areas
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.
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.
Before 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
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
Environmental Protection Agency
U.S. Geological Survey
Environmental Scientist, EPA
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.