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. 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.