The NRCS Peak Rate Factor for Severe Storms.

The Natural Resources Conservation Service's Dimensionless Unit Hydrograph is used extensively in industry by design engineers for the purpose of determining the runoff hydrograph from a rainfall event. As part of the methodology, the user is required to input a Peak Rate Factor (PRF) which describes the runoff response (volume distribution) of the corresponding unit hydrograph. Unfortunately, little guidance is given in the literature for selecting an appropriate PRF for a given watershed. Additionally, consideration has not been given to determine if the PRF should remain constant for a given watershed or be allowed to vary as a function of return period. The goal of this study was to back‐calculate site‐specific PRFs for various watersheds which have experienced two or more significant rain events since the year 2000. A total of 14 watersheds were investigated with results showing PRFs remained consistent for five of the watersheds regardless of the storm event. Nine of the watersheds showed a significant variation in PRF with an average variation of approximately 440 across all storm events within a given watershed. Additional analysis showed there is a general trend of increasing PRF with decreasing storm‐specific curve number. Finally, the complacent‐violent runoff response was shown to occur for storms with resulting PRF's in excess of 1,000. Research Impact Statement: Peak Rate Factors used as part of the NRCS unit hydrograph method are shown to vary as a function of both storm size and curve number. [ABSTRACT FROM AUTHOR]