Curve number applications in surface water management

13-23Mathematical modelling of watershed responses is employed to address a wide range of environmental and water resources related problems ranging from watershed management to engineering design. At the field scale, models are used for planning and designing soil conservation practices, irrigation, water management, wetland restoration, stream restoration, and water-table management. On a large scale, models are used for flood protection projects, rehabilitation of aging dams, floodplain management, water-quality evaluation and water-supply forecasting. Central to addressing these issues is surface water management which in turn largely focuses on the problem of rainfall-runoff modelling, which, in particular, is used in water resources assessment, flood and drought mitigation, irrigation and drainage engineering, and water resources planning and management. The Soil Conservation Service Curve Number method is one of the most widely used rainfall-runoff models. The SCS-CN method was developed in 1954 and it is documented in Section 4 of the National Engineering Handbook (NEH-4) published by the Soil Conservation Service (now called as Natural Resource Conservation Service), U.S. Department of Agriculture in 1956. The document has since been revised in 1964, 1965, 1971, 1972, 1985, and 1993. It computes the volume of surface runoff for a given rainfall event from small agricultural, forest, and urban watersheds (SCS, 1986). The passage of Watershed Protection and Flood Prevention Act (Public Law 83-566) in August 1954 led to the recognition of the method at the Federal level and the method has since witnessed myriad applications all over the world. Because of its simplicity, the SCS-CN method has been applied for handling a number of problems in hydrology and water resources, even for the problems it was not originally intended to solve. The method requires basic descriptive inputs that are converted to numeric values for estimation of direct runoff volume. Curve number which is descriptive of watershed’s runoff potential is required to be estimated as its only parameter. The method is widely used by engineers and hydrologists and watershed managers as a simple watershed model, and as a runoff estimating component in more complex watershed models. Since its inception, the SCS-CN method has witnessed myriad applications in realm of hydrology. Recently, Singh and Frevert (2002) edited a book titled ‘Mathematical Models of Small Watershed Hydrology and Applications’, in which at least 6 of the 22 chapters have mathematical models of watershed hydrology based on SCS-CN approach. This reflects the robustness and everlasting popularity of the SCS-CN technique. The SCS-CN method is a conceptual model of hydrologic abstraction of storm rainfall, supported by empirical data. Its objective is to estimate direct runoff volume from storm rainfall depth, based on a curve number CN. Its popularity is rooted in convenience, simplicity, authoritative origin and responsiveness to four readily grasped catchment properties viz., soil type, land use/treatment, surface condition and antecedent moisture condition. This paper describes the existing SCS-CN method, its theoretical background, strengths and weaknesses, its recent advancements, and applications in various fields of hydrology, water resources, environmental and sedimentation engineering.