National Water-Quality Assessment (NAWQA) Program
This study examines the relation between varying levels of urban intensity in drainage basins and in-stream water quality, measured by physical, chemical, and biological factors in 11 metropolitan areas. For each metropolitan study area, stream sites are located in 28-30 basins that have minimal natural variability among them and represent a gradient of urban development intensity.
In this study, in-stream water quality is considered a composite of physical, chemical, and biological characteristics that are influenced by natural factors and by human activities; and, biological communities reflect the integration of these influences.
Map and description of individual study areas
Site selection process:
• Identified similarly sized basins in the study area (basin delineation).
• Assembled basin characterization information (land cover, infrastructure, socioeconomic variables, population, and so on).
• Calculated urban intensity index (UII) to rank sites along a gradient of urbanization.
• Identified an area with minimum natural variability (climate, elevation, ecoregion, etc.)
• Conducted site reconnaissance to assess site suitability for sampling.
• Recalculated information on basin characteristics based on sampling locations.
• Selected final sites that are distributed on the gradient of low to high urbanization.
Representing urban intensity: Urban Intensity
• The urban intensity of each basin is measured using an index that integrates information about the multiple dimensions of human influence on the urban landscape at the drainage basin scale.
• The UII is a multimetric index based on population, infrastructure, land cover, and socioeconomic factors correlated with changes in population density (McMahon and Cuffney, 2000). Some of the variables used in the index may also be important factors for explaining variations in water quality.
Determining variables to be included in the
Urban Intensity Index
• Assemble the GIS-derived variables used to characterize population, socioeconomic, infrastructure, and land cover variables for each basin in the study area.
• Run correlation of individual variables with 2000 population density.
• Variables correlated with population density are adjusted to represent a range of urban intensity from 0-100 over all sites within a study area.
What "urban" looks like in each study area -- see “Where we studied”
Controlling natural variability
• For each study area, basins were located primarily in a single ecoregion (see table below) to control variability in natural factors that influence water-quality, while allowing the degree of urbanization to vary among study basins.
Overview and sampling protocols for data
McMahon, Gerard and Cuffney, T.F., 2000, Quantifying urban intensity in drainage basins for assessing stream ecological conditions: Journal of the American Water Resources Association, v. 36, p. 1247-1261. http://water.usgs.gov/nawqa/ecology/pubs/index.html
|Predominant USEPA Level III ecoregions* where basins were located in different study areas.|
|STUDY AREA||PREDOMINANT ECOREGION|
|Birmingham, Alabama||Valley and Ridge|
|Boston, Massachusetts||NE Coastal Zone|
|Raleigh, North Carolina||Piedmont|
|Great Lakes Region:|
|Milwaukee-Green Bay, Wisconsin||Southeastern Wisconsin Till|
|Dallas-Fort Worth, Texas||Texas Blackland Prairies|
|Denver, Colorado||Western High Plains|
|Salt Lake City, Utah||Central Basin and Range|
|Portland, Oregon||Willamette Valley|
|Seattle, Washington||Puget Lowland|
*United States Environmental Protection Agency (USEPA), 2000, Level III ecoregions of the continental United States: Corvallis, Oregon, U.S. Environmental Protection Agency-National Health and Environmental Effects Research Laboratory, Map M-1, various scales.