Impervious surface

Impervious surfaces are mainly artificial structures—such as pavements (roads, sidewalks, driveways and parking lots) that are covered by impenetrable materials such as asphalt, concrete, brick, stone—and rooftops. Soils compacted by urban development are also highly impervious. Homer and others (2007) indicate that about 76 percent of the conterminous United States is classified as having less than 1 percent impervious cover, 11 percent as having an impervious cover of 1 to 10 percent, 4 percent of the nation as having an estimated impervious cover of 11 to 20 percent, 4.4 percent of the nation as having an estimated impervious cover of 21 to 40 percent, and about 4.4 percent of the nation as having an estimated impervious cover greater than 40 percent