Comparison of Kinematic-Wave and Nonlinear Reservoir Routing of Urban Watershed Runoff

The linkage of surface runoff in urban areas to water quantity and quality has been studied extensively in the literature. Many computer models based on different routing theories have been developed to simulate the rainfall-runoff process in urban areas. Those computer models are classified into two categories: hydrologic models and hydraulic models. Hydrologic models are based on a hypothesized relation between outflow and water storage in the watershed, which is often modeled as a conceptual reservoir. The nonlinear reservoir method is a typical hydrologic routing method applied in models such as the Stormwater Management Model (SWMM). Hydraulic models are based on approximations of the real physical rainfall-runoff process. Kinematic-wave theory, utilized by many models including the Dynamic Watershed Simulation Model, is a commonly used hydraulic routing method. Previously unpublished experimental data obtained from the Watershed Experimentation System at the University of Illinois at Urbana-Champaign, Ill. were used to test the accuracy of these two routing methods. The nonlinear reservoir routing method as applied in SWMM may provide acceptable results for storms with durations longer than the watershed time of concentration (42 experiments) (average model fit efficiency E=0.88 ), but for storms with durations less than or equal to the time of concentration (26 experiments), poor results were obtained (average E=0.07 ). More accurate results generally were obtained using kinematic-wave routing (average E=0.928 and 0.807 for storm durations exceeding and not exceeding the watershed time of concentration, respectively). These results fit with the theoretical basis of the kinematic-wave theory that considers the actual physical processes in surface flow generation, while the nonlinear reservoir method does not consider the impact from the time lag needed for the flow depth to grow so that runoff can commence.