Differences in the calculation of the time of concentration using the velocity method result from different degrees of discretization along the longest flowpath in the watershed. We examined an idealized system for which an analytical solution could be derived. Next, we studied a dataset compiled from watersheds across the State of Maryland, for which the observed time of concentration was known. In both cases we show that the time of concentration estimate increases with the degree of discretization.

Two different models were developed that show good predictive agreement with the observed time of concentration. One method uses, gradually varied flow concepts to allow velocity to vary more realistically along the discretized flowpath. The other method uses a regression approach to guide the merging of GIS pixel-based flowpath elements into larger segments. Strengths and limitations of both methods are discussed in the context of future application in Maryland and elsewhere.