Estimation of storm event runoff is one of the major activities in applied hydrology for ungauged small watersheds. 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) methodology 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, considering 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.

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