An impervious area is considered connected if runoff from it flows directly into the drainage system. It is also considered connected if runoff from it occurs as concentrated shallow flow that runs over a pervious area and then into a drainage system.

Urban CNs (for more information, see "Runoff Curve Numbers for Urban Areas" on page B-899) were developed for typical land use relationships based on specific assumed percentages of impervious area. These CN values were developed on the assumptions that:

• Pervious urban areas are equivalent to pasture, in good hydrologic conditions.
• Impervious areas have a CN of 98 and are directly connected to the drainage system. Some assumed percentages of impervious area are shown in "Table B-1: Runoff Curve Numbers for Urban Areas"on page B-899.

If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions in "Table B-1: Runoff Curve Numbers for Urban Areas"on page B-899 are not applicable, use Figure 2-3 from TR-55, Urban Hydrology for Small Watersheds (U.S. Soil Conservation Service 1986b) to compute a composite CN. For example, "Table B-1: Runoff Curve Numbers for Urban Areas"on page B-899 gives a CN of 70 for a 1/2-acre lot in HSG B, with an assumed impervious area of 25 percent. However, if the lot has 20 percent impervious area and a pervious area CN of 61, the composite CN obtained from Figure 2-3 (U.S. Soil Conservation Service 1986b) is 68. The CN difference between 70 and 68 reflects the difference in percent impervious area.