Developing Customized NRCS Unit Hydrographs for Ungauged Watersheds in Indiana

Abstract

The Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service, SCS) unit hydrograph (UH) is one of the most commonly used synthetic UH methods for hydrologic modeling and engineering design all over the world. However, previous studies have shown that the application of the NRCS UH method using the published approach and parameter values does not produce accurate peak discharges or time to peaks. Over- or underestimation of peak discharge using the NRCS UH is also a critical issue for hydrologic design in the State of Indiana. The objective of this work is to adapt the NRCS UH for use in Indiana by analyzing the role of its two key parameters, namely, the peak rate factor (PRF) and the lag time, in creating runoff hydrographs. Based on 120 rainfall–runoff events collected from 30 small watersheds in Indiana between 2000 and 2020, UHs are derived to extract corresponding PRF and lag time. Results show that the mean value of PRF for these UHs is 376 (in English units), which is lower than the standard PRF of 484, and the NRCS lag time equation tends to underestimate the true lag time for more than half of the study watersheds. Stepwise linear regression models were formulated to estimate NRCS UH parameters on the basis of the geomorphic attributes extracted from the study watersheds. Both the statewide and regional regression models show that the main channel slope is a major factor in determining the PRF and lag time. Finally, a customized Indiana unit hydrograph (INUH) is derived with updated PRFs using a gamma distribution. Validation results show that INUH can provide more accurate predictions in terms of the peak discharge and the time to peak (the mean relative error is reduced by around 45% and 6%, respectively) than the original NRCS UH for watersheds in Indiana.