The Indicator

Perhaps the single most dramatic and pervasive impact of urbanization on the functions and values of a watershed is the replacement of the natural landscape with pavement and other water-impervious (impenetrable) material such as roads, parking lots, driveways, sidewalks, and rooftops. Increased levels of impervious surfaces interrupt the hydrologic cycle, alter stream structure, and degrade the chemical profile of the water that flows through streams. These changes affect fish and wildlife in various ways, and are cumulative within watersheds. Research indicates that when total impervious area (TIA) in a watershed reaches 10%, stream ecosystems begin to show evidence of degradation. Ecological effects become severe as TIA approaches 30% (for more discussion, see Arnold and Gibbons 1996; Booth and Jackson, 1997; Schueler 1994; Schueler and Holland 2000).

Effects that have been associated with increases in impervious area include the following:

• Increases in stream temperature, as rain runs over heated pavement. During warmer months, water flowing over impervious surfaces is often 10–12ºF warmer than water that passes through fields and forests. Higher water temperatures increase the metabolic rates of stream-dwelling plants and animals, so that an organism living in warmer water needs more oxygen than the same species in cold water. Unfortunately, warmer water cannot hold as much oxygen as cold water.
• Changes in stream flows. Greater stormwater volumes traveling over the surface and being delivered too rapidly to streams leads to increased stream flashiness and a reduction in summer base flows, sometimes causing perennial streams to become intermittent or to dry up completely. As a result, urbanized watersheds are prone to more frequent and bigger floods.
• Stream channel modification. The rapid runoff associated with increased stormwater velocity and volume quickly erodes and incises the stream channel and banks. Channels widen and straighten to accommodate higher flows. Ponds, pools, riffles, and sandbars are simplified or washed away, eliminating critical habitat for fish, waterfowl, and other species of animals and plants.
• Increased pollutant loadings. Concentrations of pollutants in streams increase with increases in impervious area. Common urban pollutants include pesticides, bacteria, nutrients such as phosphorus and nitrogen, and other contaminants, such as PCBs and heavy metals.

The percentage of impervious surfaces within a watershed is a good indicator of the degree of urbanization and the associated negative ecological impacts, but it can be very difficult to measure. Where such data are available, watershed urbanization is most often quantified in terms of the proportion of the basin area covered by impervious surfaces, or TIA.

The Data Gap

Existing data should be examined in order to develop a cost-effective way of estimating impervious area regionally and nationally. This may involve the use of new remote-sensing techniques; collation of existing local information; the use of surrogates, such as the amount of road surface; or other approaches.

References

Arnold, C.L., and C.J. Gibbons. 1996. Impervious surface coverage: The emergence of a key environmental indicator. Journal of the American Planning Association 62(2):243–258.

Booth, D.B., and C.R. Jackson. 1997. Urbanization of aquatic systems: Degradation thresholds, stormwater detection, and the limits of mitigation. Journal of the American Water Resources Association 35(5):1077–1090.

Schueler, T.R. 1994. The importance of imperviousness. Watershed Protection Techniques 1(3):100–111. Schueler. T.R. and H.K. Holland. eds. 2000. The practice of watershed protection. Ellicott City, MD: Center for Watershed Protection.