ASSESSMENT OF DESIGN RUNOFF CURVE NUMBER FOR A WATERSHED

Abstract

Rainfall-runoff modeling is an integral part of water resources planning and management. The Soil Conservation Service-Curve Number (SCS-CN) method 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 applicability to watersheds with minimum hydrologic information, viz., soil type, land use and treatment, surface condition, and antecedent moisture condition (AMC). 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 also been suggested in literature. In this study, employing the data of three hydro-meteorologically different watersheds, viz. Ramganga watershed in Uttarakhand (India), Maithon watershed in Jharkhand (India), and Rapti watershed in Mid-Western Region (Nepal), a simple approach for CN derivation for three levels of AMC from long-term daily rainfall-runoff data has been suggested. It is of common experience that the SCS-CN method's parameter curve number decreases as the rain duration increases, and vice versa. It is because of the larger opportunity time available for water to loss in the watershed. In this study, this impact of rain duration on curve numbers is investigated in a rational manner, and a CN-rainfall duration relationship proposed for each watershed. In addition, there is no rational approach available in literature for derivation of curve numbers for design purposes associated with return periods. This study investigates this aspect and proposes a simple approach for design CN development and it is validated using the observed data.