The importance of impervious surfaces in water quality and storm water pollutant loading is widely recognized by the scientific community. Research over the past several years has consistently revealed a strong correlation between the imperviousness of a drainage basin and the health of associated receiving waters. Stream degradation has been shown to occur when impervious surfaces approach 5 to 20% of the total area of a watershed. By way of perspective, imperviousness in single-family residential areas ranges from 25 percent to nearly 60 percent. Imperviousness in industrial areas is typically 60 - 70% and 80 - 90% in commercial areas and shopping centers.

The effect of impervious surfaces on the volume of storm water runoff can be dramatic. For example, a 1-inch rainstorm over a 1-acre area of natural grassland will typically produce 218 cubic feet of runoff. The same storm over a 1-acre paved parking lot would produce 3,450 cubic feet of runoff, nearly 16 times more than the natural setting. However the problems of stormwater runoff from industrial and commercial are not limited to runoff volume. Urban development creates new pollution sources and brings with it proportionately higher levels of car emissions, car maintenance wastes, litter, pesticides, and hazardous wastes. Parking lots, shopping areas, business and industrial areas often produce hydrocarbon and metal concentrations that are twice those found in the average urban area. Where commercial activities require liquids storage, equipment use or maintenance, other pollutants may also be present in runoff.

Working closely with Santa Barbara County's Project Clean Water and Assessor's Office, GDM first prepared land use parcel maps to identify areas zoned for industrial or commercial development.

Recent Landsat imagery was then classified and used to identify pixel classes which tended to correlate well with highly developed areas of the County. The locations of these pixel classes were transposed back to the parcel base map to identify the 20 most significant areas of highly-impervious development.

The County intends to use this analysis as a first step in developing a new program of structural controls designed to mitigate the effects of stormwater runoff.