Graphic showing how development increases the volume and rate of runoff from a site and reduces groundwater recharge and evapotranspiration.

Enlarged view

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Impervious Surfaces and the Hydrologic Balance of Watersheds

Low-density residential suburbs and office parks might not seem to create much impervious surface, but they are served by roads, services such as shopping centers, recreational centers, schools, utilities and their associated parking lots, which together add up to increased impervious surfaces. Furthermore, soils that have been compacted by heavy machinery during construction, landscaping, or farming often function somewhat like paved surfaces. For example, a parking lot might be 95 percent impervious, a residential lawn might be 40 percent impervious and natural land covers are nearly 0 percent impervious (Anacostia Restoration Team 1991). Imperviousness results in fundamental changes in the characteristics of land cover. Ground water recharge, soil-based capture and retention of rainfall, vegetative growth, and the overall water balance and maintenance of the hydrologic cycle essential to environmental health can be negatively affected by increases in impervious surfaces.

In terms of the hydrologic cycle, less water is infiltrated and more runs off at the surface. This is an important point because the effective impervious surface in a watershed affects the physical structure of streams and waterbodies, as well as the diversity and abundance of aquatic life. It is also related to the amount of pollution caused by human activities that is transported directly to waterbodies in storm events, rather than being filtered through soil.

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Section 8 of 21