Design Flood Estimation Incorporating Duration and Return Period of Design Rainfall Using NRCS-CN Method in Peninsular India

Application of Curve Number (CN) methodology in design flood estimation and forecasting studies is a well-accepted reality. The parameter CN is often regarded as a constant, but it is in fact a random variable, for it relies on a number of factors governing the process of rainfall-runoff. This paper suggests an alternative procedure to estimate design runoff (Q) using design storm (P) and design Curve Number (CN) values. To this end, design CN values for different durations and return periods have been derived from 25 years of daily annual maximum P-Q data of 10 catchments of peninsular India. When compared, the computed design Q from design CN-values was either very close to or slightly greater than the observed Q. The performance of design CN for design runoff estimation was also evaluated using Coefficient of Determination (R2), Percentage of Bias (PBIAS) and Normalized Nash-Sutcliffe Efficiency (NNSE). The results shows good performance in six catchments (0.77 R2 0.93, -0.01 PBIAS 0.18 and 0.32 NNSE 0.70). The magnitude of design CN was decreased for longer durations and smaller return periods, and vice versa. Such a behavior invoked the development of an empirical relation for estimation of design CN from given return period and duration for gauged as well as ungauged watersheds. The 50-year, 100-year and 200-year runoff estimated from the P-CN series exceeded only marginally those derived from the conventional Q-series approach.