Hydrological Simulation of Small River Basins in Northern Kyushu, Japan, During the Extreme Rainfall Event of July 5–6, 2017

Extreme rainfall and associated flooding are common during the summer in Japan. Heavy rain caused extensive damage in many parts of Kyushu, Japan, on July 5–6, 2017. Many small mountainous river basins were subject to the core of this heavy rainfall event and were flooded, but no hydrological measurements were taken in most of these flooded basins during the event. There are few gauging stations in this mountainous region, and most that do exist are designed to monitor the larger watersheds. Consequently, it is difficult to determine the hydrological properties of the small subbasins within these larger watersheds. Therefore, to improve our understanding of the basic hydrological processes that affect small ungauged mountain river basins during periods of intense rainfall, a quasi-distributed model (i.e. the Hydrologic Engineering Center-Hydrologic Modeling System, HEC-HMS) was used in this study. The Hikosan (area: 65 km 2 ) and Akatani (area: 21 km 2 ) mountainous river basins were selected for the hydrological simulations. The model was validated using the Hikosan River basin because observational data are available from the outlet of this basin. However, there is no record of any hydrological observations for the Akatani River basin. Therefore, reference parameters from the Hikosan River basin were used for hydrological analysis of the Akatani River basin. This was possible because the basins are close to one another and have similar physiographic and topographic properties. The simulations of both basins, and the associated uncertainties, are discussed in detail in this paper. Based on the hydrological simulations, an attempt was made to analyze the maximum flood discharge caused by the event. The results generated using this approach to hydrological simulations in small ungauged basins could contribute to the management of water resources in these and other river basins during future extreme rain events.