Assessment of design runoff curve number for a watershed Surendra Kumar Mishra a , Ajay Kumar Kansal a and Nishant Aggarwal b a Dept. WRD&M, IIT Roorkee, Roorkee-247 667 (UK), India. E-mail: skm61fwt@gmail.com; akkansalt18@gmail.com b National Institute of Technology, Kurukshetra. E-mail: na121991@gmail.com Abstract Estimation of storm event runoff is one of the major activities in applied hydrology for ungauged small water- sheds. There exist a myriad computer models in the field of water resources and irrigation engineering and the most comprehensive and popular ones use the Soil Conservation Service (SCS) Curve Number (CN) method- ology to determine the rainfall-excess from rainfall event. The SCS-CN method is one of the most popularly used techniques for simplicity reasons. Its parameter CN is a measure of water retention by a given combination of soil and vegetation and it varies from 0 (no runoff) to 100 (total effective rainfall becomes runoff). In this study, con- sidering each day’s rainfall and corresponding runoff as an event, the SCS-CN method is employed to long-term daily rainfall-runoff data of Maithon watershed located in Jharkhand (India). A simple approach has been suggested for derivation of the design runoff CN for different durations, wetness conditions, and return periods for use in the SCS-CN methodology. The derived design CN values are tested for their validity using the design runoff estimated conventionally from the observed data.The match between the design CN (for dry condition)- generated runoff and the conventional design runoff is found to be satisfactory for all return periods and rain durations for the studied watershed. Key words: design curve number, design flood, return period, SCS-CN method INTRODUCTION The Soil Conservation Service-Curve Number (SCS-CN) method (SCS 1956, 1964, 1969, 1971, 1972, 1985, 1993) is one of the most popular methods for computing the volume of surface runoff for a given rainfall event from small agricultural watersheds. The method has been the focus of much discussion in agricultural, hydrologic literature and is also widely used in continuous modeling schemes. The main reason behind the method being adopted by most hydrologists lies in its simplicity and applica- bility to watersheds with minimum hydrologic information, viz., soil type, land use and treatment, surface condition, and antecedent moisture condition (AMC). Curve Number (CN)-values are derived using limited values of rainfall-runoff events for a gauged watershed and using NEH-4 tables for an ungauged watershed for three AMCs. Of late, an approach based on the ordering of rainfall has been suggested in literature. Ponce & Hawkins (1996) critically examined this method; clarified its conceptual and empirical bases; delineated its capabilities, limitations, and uses; and identified areas of research in the SCS-CN methodology (Mishra & Singh 2003). The runoff CN method is devel- oped to estimate extreme or large event runoff volume. However, it is used in hydrologic simulation models such as CREAMS (Knisel 1980) and AGNPS (Young et al. 1987) to estimate direct runoff from daily rainfall events. Methods of selecting the runoff CN for a watershed under various conditions are available in the National Engineering Handbook, Section 4, Hydrology or ‘NEH-4’ (SCS 1972). The CN values were originally defined from annual maximum rainfall and runoff data on small agricultural © IWA Publishing 2012 Water Practice & Technology Vol 7 No 4 doi: 10.2166/wpt.2012.066