Effects of initial abstraction and urbanization on estimated runoff using CN technology

Few studies have been conducted to explore the effects of initial abstraction on estimated direct runoff despite the widespread use of the curve number (CN) method in many hydrologic models to estimate direct runoff. In this study, use of a 5 percent ratio of initial abstraction (la) to storage (S) to estimate daily direct runoff with modified CN values for a 5 percent I-a/S value was investigated using the Long-Term Hydrologic Impact Assessment (L-THIA) geographic information system (GIS). In addition, the effects on estimated runoff of altering the hydrologic soil group due to urbanization were investigated. The L-THIA model was applied to the Indiana Little Eagle Creek watershed with 5 percent and 20 percent I-a/S values, considering hydrologic soil group alteration due to urbanization. The results indicate that uses of a 5 percent I-a/S and modified CN values and Hydrologic Soil Group D for urbanized areas in model runs can improve long term direct runoff prediction.