Five-minute unit hydrographs for selected Texas watersheds

In this thesis, three unit hydrograph methods for the estimation of watershed runoff were compared. The three methods were the traditional unit hydrograph approach, the NRCS unit hydrograph approach, and the Clark unit hydrograph approach. The hypothesis of this research is that results from the traditional method closely compare to those generated using the NRCS and Clark methods. Fifteen Texas watersheds were selected for intensive analysis. The traditional unit hydrograph approach was applied to 129 storm events; however this number was reduced to 51 storm events for application of the Clark and NRCS unit hydrograph approaches. Five-minute average unit hydrographs for each watershed were generated using the three methods. Regression analysis was applied to determine that the most important watershed characteristic for prediction of unit hydrograph peak discharge and time to peak was watershed area.

The 5-minute average unit hydrograph from each method for each watershed was compared to determine differences and similarities between the applied methods. Based on these comparisons, traditional unit hydrographs differed substantially from both the NRCS and the Clark unit hydrographs fitted to observed direct runoff hydrographs, contrary to initial thoughts.

An average dimensionless unit hydrograph appropriate for selected Texas watersheds was developed from the unit hydrographs computed using the traditional method. From the average dimensionless unit hydrograph, an average S-hydrograph was developed and applied to storm events used for optimization of NRCS and Clark unit hydrograph parameters. Direct runoff hydrographs from study events were reconstituted using the traditional method S-hydrograph, the average optimized Clark parameters, and the average optimized NRCS parameters using HEC-HMS. These model-regenerated direct runoff hydrographs were compared to observed direct runoff hydrographs. Comparisons were conducted on the peak discharge, the time to peak, the time base of runoff, and the overall hydrograph. The traditional method approach best simulated the response of the watersheds for the test hydrologic events.

Overall, the unit hydrograph derived using the traditional approach is not similar to the unit hydrograph developed using either the NRCS or the Clark unit hydrograph approach. When used to reconstitute direct runoff hydrographs for the study watersheds, the traditional unit hydrograph approach generates responses that mimic actual direct runoff hydrographs better than the NRCS and Clark unit hydrograph approaches even when using optimized parameters.