Evaluating the Influence of Topographic and Process Representation on Runoff and River Flow in Small-Valley Scenarios Using a Coupled Rainfall–Runoff–Inundation and River-Flow Model

In Chiba Prefecture, Japan, there are numerous yatsu valleys topographies that have local elevation differences that greatly affect the rainfall-runoff process. In this study, we aimed to explore the impact of these topographic features on runoff and river flow. In addition, we also evaluate the effects of unsteady and advection terms in the equation of motion on river flow simulation, which are sometimes omitted for reasons such as improving computational efficiency. We developed a coupled rainfall–runoff–inundation and river-flow (RRI-RF) model, which combines a hydrological model and a hydraulic model, and applied it to the Ichinomiya River basin. Our findings revealed that the flow discharge is calculated to be lower in the simulation using coarser grid resolution in which the elevation differences due to yatsu valleys are smoothed. Moreover, the advection term has a significant influence on the river flow, particularly in the RRI-RF model with lateral inflow at multiple locations. Therefore, inputting elevation data of high resolution and the calculation of the full equation of motion is essential for precisely assessing river water level and discharge.