Natural impervious areas are defined herein as land covers that can contribute a substantial amount of stormflow during small and large storms, but commonly are classified as pervious areas.[2] These areas are not commonly considered as an important source of stormflow in most highway and urban runoff-quality studies, but may produce a substantial amount of stormflow. These natural impervious areas may include open water, wetlands, rock outcrops, barren ground (natural soils with low imperviousness), and areas of compacted soils. Natural impervious areas, depending on their nature and antecedent conditions, may produce stormflow from infiltration excess overland flow, saturation overland flow, or direct precipitation. The effects of natural impervious areas on runoff generation are expected to be more important in areas with low TIA than highly-developed areas. The NLCD[12] provides land-cover statistics that can be used as a qualitative measure of the prevalence of different land covers that may act as natural impervious areas. Open water may act as a natural impervious area if direct precipitation is routed through the channel network and arrives as stormflow at the site of interest. Wetlands may act as a natural impervious area during storms when groundwater discharge and saturation overland flow are a substantial proportion of stormflow. Barren ground in riparian areas may act as a natural impervious area during storms because these areas are a source of infiltration excess overland flows. Seemingly pervious areas that have been affected by development activities may act as impervious areas and generate infiltration excess overland flows. These stormflows may occur even during storms that do not meet precipitation volume or intensity criteria to produce runoff based on nominal infiltration rates. Developed pervious areas may behave like impervious areas because development and subsequent use tends to compact soils and reduce infiltration rates. For example, Felton and Lull (1963)[13] measured infiltration rates for forest soils and lawns to indicate a potential 80 percent reduction in infiltration as a result of development activities. Similarly, Taylor (1982)[14] did infiltrometer tests in areas before and after suburban development and noted that topsoil alteration and compaction by construction activities reduced infiltration rates by more than 77 percent.