Improvement of the Variable Storage Coefficient Method with Water Surface Gradient as a Variable

Abstract. The variable storage coefficient (VSC) method has been used for streamflow routing in continuous hydrological simulation models such as the Agricultural Policy/Environmental eXtender (APEX) and the Soil Water Assessment Tool (SWAT) for more than 30 years. APEX operates on a daily time step and offer options for simulating processes on shorter time steps (e.g., hourly). However, APEX is not adequate for applications such as designing flood control structures or estimating flood damages because of a fundamental assumption in the VSC method: the normal flow condition. The storage with variable slope (SVS) method and an enhanced variable storage coefficient (VSCe) method are proposed as new routing methods for continuous simulation models that will improve flow routing and water quality simulation at subdaily time scales. This study describes the principle of the SVS method and the VSCe method and their performances against HEC-RAS unsteady flow results for various hydraulic and geometric conditions. Results show that the peak flow and the time to peak flow improved by up to 20% with SVS and VSCe on mild slopes (less than 0.0005 m m^-1) and small time steps of less than 1 h when compared to the conventional VSC method, although the difference narrowed as the channel slope and time interval increased. A case study on a small agricultural watershed in Texas indicates that both VSCe and VSC are reliable in watershed applications, but the improvement in streamflow prediction can be marginal in watersheds with steep slopes.