Runoff volumes from small urban watersheds

An empirical equation estimates the runoff volume from rainfall on small urban watersheds. If the rainfall is less than 1.5 inches, the runoff is predicted by the relationship between the percent impervious area in the watershed and the percent excess rainfall. An adjustment is made for the initial ion combined with the initial surface depression storage. For rainfall amounts greater than 1.5 inches, an additional increment of runoff is added for the pervious areas by using the hydrologic soil class, the vegetative cover, and the controlling parameters. Detailed 1‐ or 5‐min rainfall and runoff records from four small urban watersheds for 77 storms were used to develop the procedure. The method was tested on 17 additional storms on these watersheds. The maximum prediction error was 37% for over 80% of the test events. The median error was 17% of the actual runoff.