In this study, we developed a distributed rainfall-runoff and sedimentation model based on one-dimensional kinematic wave equations. Physically-based rainfall-runoff and erosion-sediment processes were coupled and solved for each spatial grid, whilst the spatially distributed grids were connected to each other to allow for space-and-time movements of water and sediment. The model was applied to the Akatani River basin of the Chikugo River in Kyushu, Japan using a 10 m high-resolution digital elevation model and eXtended RAdar Information Network (XRAIN) data as a time-and-space distributed rainfall input of the northern Kyushu heavy rainfall event in July 2017. Our results indicate that the rainfall-runoff hydrograph and sediment flow results are in agreement with the collected field data, and elevation of the river bed after the disaster was successfully reproduced by applying a sediment theory to estimate river bed variation. In addition, we found that sediment transport results are sensitive to model spatial resolution. Our simulation model is intended for use with basins that feature steep slopes and are prone to erosion and shear strength reduction after heavy rainfall events. Hence, this model can be applied to give early warnings by identifying critical erosional areas during forecasted heavy rainfall events.
A distributed hydrologic model based on a kinematic wave approximation with surface and subsurface flow components is applicable to basins that have temperate climatic conditions similar to those in Japan. However, it is difficult to present long-term river discharge using the existing model structure in basins with different climatic conditions. This study aims to improve the model structure for better estimates of long-term discharge in the Nam Ngum River, the main tributary of the Mekong River, by incorporating bedrock aquifers as part of the slope flow component of the original model structure. Three bedrock groundwater structures are configured to incorporate the original model structure. The results show that a combination of the original model component and one unconfined aquifer structure are the best representations of the river flow regime from the original model structure, in which the rate of infiltration from the layer into the bedrock aquifer was calculated using vertical hydraulic conductivity. The Nash–Sutcliffe efficiency coefficient of the original and improved models increased from 0.80 to 0.86 during the calibration period and from 0.56 to 0.62 during the validation period. The results of this study show that the improved model structure is applicable for long-term hydrologic predictions in Southeast Asian catchments with distinct dry and rainy seasons.