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

Abstract

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 km2) and Akatani (area: 21 km2) 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.

The Role of the Spatial Distribution of Radar Rainfall on Hydrological Modeling for an Urbanized River Basin in Japan

“…Control specifications are designed to control the time interval of the simulation of any event. The details of the model structures are given in the technical reference manual [37] and the user's manual [39] of the HEC-HMS, which has been used for a variety of analysis and engineering studies due to its flexibility and applicability in other regions to simulate rainfall-runoff processes [5,[17][18][19][20]22,23,[40][41][42][43][44][45][46]. The model looks applicable for predicting hydrological responses using radar rainfall during extreme storm events over Japanese river basins [19,20].…”

“…In the United States, S-band radars have mostly been used operationally, with resolutions from 1000-m and >5-min, and most European countries have used C-band radars with spatial resolutions of about 1000-m and 5-min temporal resolutions. The quality of radar rainfall data has improved substantially recently, allowing the application of weather radar for hydrometeorological research, including the use of radar rainfall data in hydrological modeling [9,[15][16][17][18][19][20][21][22][23].…”

“…One of these is the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS), which has been widely used worldwide. The accuracy and performance of this model using radar rainfall has also been widely recognized [4,5,17,19,22,23]. Since many types of hydrological models have been developed for research and operational purposes, no standard exists for selecting the type of hydrological model used.…”

“…Since many types of hydrological models have been developed for research and operational purposes, no standard exists for selecting the type of hydrological model used. In many cases, semi-distributed or lumped models have been used [5,17,19,[22][23][24][25][26][27][28], whereas physically distributed models have been used in certain other cases [20,21,29]. Each type of hydrological model has its merits and demerits depending on performance and the objectives for which it is used.…”

“…Several researchers agree that the use of accurate spatiotemporal rainfall variation data in hydrological models is essential for monitoring river discharge and may help to improve our understanding of water balances [8,22,24,25,30,31]. Therefore, the use of high-resolution weather radar data in hydrological applications has grown significantly as an alternative to traditional rainfall observations with rain gauges [5,8,17,[19][20][21][22]31], and several studies suggest that the use of high-resolution rainfall data in hydrological models may offer a more realistic output [4,8,16,21,22,24,25,30,31]. However, there is no clear guideline on the optimum spatiotemporal radar rainfall resolution for this purpose.…”