Unit Hydrograph Peak Rate Factor Estimation for Texas Watersheds

Abstract

In many cases, estimation of the peak discharge is the primary goal of hydrologic modeling. We employed a dataset of 1,648 rainfall-runoff events in 104 watersheds in Texas to explore the peak rate factor (PRF) of 2.08 recommended by the Natural Resources Conservation Service (NRCS) for determining peak discharges with the unit hydrograph method, as well as the PRF’s dependency on watershed and storm characteristics. For each event, a unit hydrograph—assumed to follow a two-parameter gamma distribution—was determined by deconvolving the direct runoff hydrograph with the excess rainfall hyetograph. Results showed PRFs reaching up to 9.75 in Houston watersheds with a median of 0.58 and PRFs up to 11.02 with a median of 1.42 for the rest of Texas. It was also found that the recommended PRF of 2.08 falls between the 75th and 90th percentile of the 1,043 events analyzed in all regions except Houston, and is above the 95th percentile in all 605 events analyzed in Houston. Finally, statistical analysis showed that the PRF is primarily dependent on the watershed’s geomorphology but not on its slope, as was suggested earlier. This dependency, however, although statistically significant, explains only marginally the PRF variability.