Unit Hydrographs for Small Watersheds in Texas Using Particle-Tracking

The unit hydrograph is a well-documented method for predicting the runoff hydrograph of a watershed given an excess rainfall hyetograph. Watershed-specific unit hydrographs are parameterized by some conceptual time such as time to peak runoff or time of concentration. Estimation of time parameters when paired rainfall and runoff observations are available is reasonably straightforward. In the absence of paired observations, the parameters are estimated from regression equations that relate unit hydrograph behavior to watershed physical characteristics such as area, slope, and land use. This study performed an exploratory assessment of a particle tracking (PT) method to estimate unit hydrograph timing parameters directly from a digital elevation model (DEM) instead of the regression approach. The PT method was used to estimate the unit-hydrograph timing parameters of 135 selected small watersheds in Texas using a DEM of each watershed, and an assumed characteristic velocity to generate a watershed-specific unit-hydrograph. The study watersheds have drainage areas ranging from approximately 0.25 to 150 square miles, main channel lengths ranging from approximately 1 to 50 miles, and dimensionless main channel slopes between approximately 0.0002 and 0.02. The selected watersheds include coastal plains as well as inland hilly regions. Paired rainfall-runoff data for over 2,600 storms were compiled; these provide actual observations to test the performance of the PT model.