Uncertainty Analyses of Watershed Time Parameters

Because of differences in roughness, flow depth, and slope, the timing of runoff from most parts of a watershed differs from that along the principal flow path, which is generally used to compute times of concentration. Thus, times of concentration computed from principal flow path information do not accurately reflect the timing of runoff from most parts of a watershed. Specifically, travel times based on the principal flow path will be shorter than times based on the entire watershed, which could result in inaccurate computed peak discharge rates. This potential bias in watershed runoff timing was investigated using both measured rainfall-runoff data from 27 watersheds and computer simulation. Multiple definitions of the time of concentration were compared, with the expected uncertainty in the alternative definitions evaluated. The difference in timing between the centers of mass of rainfall excess and direct runoff was found to be the least uncertain timing parameter while the traditional definition of time of concentration, i.e., the difference between the end of rainfall excess and the inflection point, was found to be highly uncertain. An adjustment factor that can be applied to times of concentration computed solely from the principal flow path is provided to eliminate the bias inherent to times computed from information obtained from the principal flow path.