Time of Concentration Estimated Using Watershed Parameters Determined by Automated and Manual Methods

The time of concentration ( Tc ) for a watershed is a widely used time parameter to estimate peak discharges in hydrologic designs. In this study, Tc is estimated for 96 Texas watersheds using five empirical equations: Williams, Kirpich, Johnstone–Cross, Haktanir–Sezen, and Simas–Hawkins methods. The drainage areas of watersheds studied are approximately 0.88–440.3 km2 . Watershed parameters used to estimate Tc were developed by researchers at three institutions using three different methods: the automated method using digital elevation models and geographic information system software, the manual method with watershed delineation, and the manual method without watershed delineation. Tc estimated from five empirical equations using three sets of watershed parameters is compared and analyzed. Tc estimated using watershed parameters developed by the three methods is qualitatively similar and has average relative differences ranging from 6.4 to −16.9% . Differences between manual and automatic-based watershed characteristics are considered minor sources of error in relation to other uncertainties inherent in time parameter estimation. Average relative differences of Tc estimated using different empirical equations with the same set of watershed parameters range from −38 to 207% (absolute average differences range from −3.0 to 2.8 h ) and are much larger than differences estimated using three sets of watershed parameters. Kirpich and Haktanir–Sezen methods provide reliable estimates of mean values of Tc variations.