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.

1. [1] S. Kusunoki, J. Yoshimura, H. Yoshimura, R. Mizuta, K. Oouchi, and A. Noda, “Global Warming Projection by an Atmospheric Global Model with 20-km Grid,” J. Disaster Res., Vol.3, pp. 4-14, 2008.
2. [2] IPCC, “Summary for policy makers, Climate Change 2014: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,” Cambridge University Press, Cambridge, UK, pp. 1-32, 2014.
3. [3] S. Kreft, D. Eckstein, and I. Melchior, “Global Climate Risk Index 2017. In: Who Suffers Most from Extreme Weather Events? Weather-related Loss Events in 2015 and 1996 to 2015,” Germanwatch e.V., Bonn, pp. 1-32, 2016.
4. [4] S. Kotsuki and K. Tanaka, “Impacts of mid-rainy season rainfall on runoff into the Chao Phraya river, Thailand,” J. Disaster Res., Vol.8, pp. 397-405, 2013.
5. [5] B. B. Shrestha, T. Okazumi, M. Miyamoto, S. Nabesaka, S. Tanaka, and A. Sugiura, “Fundamental analysis for flood risk management in the selected river basins of Southeast Asia,” J. Disaster Res., Vol.8, pp. 858-869, 2014.
6. [6] H. Oshikawa, A. Hashimoto, K. Tsukahara, and T. Komatsu, “Impacts of recent climate change on flood disaster and preventive measures,” J. Disaster Res., Vol.3, No.2, pp. 131-141, 2008.
7. [7] M. Nakamura, S. Kaneda, Y. Wakazuki, C. Muroi, A. Hashimoto, T. Kato, A. Noda, M. Yoshizaki, and K. Yasunaga, “Effects of global warming on heavy rainfall during the Baiu season projected by a cloud-system-resolving model,” J. Disaster Res., Vol.3, No.1, pp. 15-24, 2008.
8. [8] A. Tominaga, “Lessons learned from Tokai heavy rainfall,” J. Disaster Res., Vol.2, No.1, pp. 50-53, 2007.
9. [9] K. Toda, “Urban flooding and measures,” J. Disaster Res., Vol.2, No.3, pp. 143-152, 2007.
10. [10] S. P.C., R. Misumi, T. Nakatani, K. Iwanami, M. Maki, T. Maesaka, and K. Hirano, “Accuracy of quantitative precipitation estimation using operational weather radars: a case study of heavy rainfall on 9–10 September 2015 in the East Kanto region, Japan,” J. Disaster Res., Vol.11, No.5, pp. 1003-1016, 2016.
11. [11] Y. Morooka and T. Yamada, “Study on the characteristics of rainfall runoff in the Kinugawa river basin and the evacuation behavior of the residents at the time of Kanto and Tohoku flood disaster in September, 2015,” J. Disaster Res., Vol.12, pp. 176-186, 2017.
12. [12] A report from Disaster management Agency on damages due to the heavy rainfall and typhoon, Report No.68 (in Japanese), http://www.fdma.go.jp/bn/【第68報】6月30日からの大雨及び台風３号の被害状況等.pdf [accessed October 30, 2017].
13. [13] DRI Survey Report No.49, 2017 (in Japanese), http://www.dri.ne.jp/wordpress/wp-content/uploads/No.49-kyusyuhokubu.pdf [accessed August 26, 2017]
14. [14] Activity status for early recovery from the heavy rain disaster in northern Kyushu, July, 2017 (in Japanese), http://www.qsr.mlit.go.jp/site_files/file/H29hokubugouu_topicset_1.pdf [accessed August 17, 2017]
15. [15] D. Kuribayashi, M. Ohara, T. Sayama, A. Konja, and H. Sawano, “Utilization of the flood simulation model for disaster management of local government,” J. Disaster Res., Vol.11, pp. 1161-1175, 2016.
16. [16] H. Uyeda, “Mesoscale precipitation systems along the Meiyu/Baiu front and future expectation for research radar and weather radar network,” J. Disaster Res.. Vol.3, pp. 61-68, 2013.
17. [17] S. P. C., M. Maki, S. Shimizu, T. Maesaka, D. Kim, D. Lee, and H. Iida, “Correction of reflectivity in the presence of partial beam blockage over a mountainous region using X-band dual polarization radar,” J. Hydrometeor., Vol.14, pp. 744-764, 2013.
18. [18] S. P. C., “Quantitative precipitation estimation and hydrological modeling in Japan,” J. of Japan Soc. of hydrol. and Water Resour., Vol.30, No.1, pp. 6-17, 2017.
19. [19] http://maps.gsi.go.jp [accessed August 10, 2017]
20. [20] USACE, “Hydrologic Modeling System HEC-HMS, Technical Reference manual,” Hydrologic Engineering Center, Davis, CA, CPD-74B, p. 145, 2000.
21. [21] USACE, “Hydrologic Modeling System HEC-HMS, Application Guide,” Hydrologic Engineering Center, Davis, CA, CPD-74C, p. 167, 2015.
22. [22] USACE, “Hydrologic modeling system HEC-HMS. User’s manual version 4.1,” Hydrologic Engineering Center, Davis, CA, CPD-74A, p. 606, 2015.
23. [23] Y. Gao, Y. Chen, L. Zhang, and T. Peng, “Radar-rainfall estimation from S-band radar and its impact on the runoff simulation of a heavy rainfall event in the Huaihe river basin,” J. Meteor. Soc. Japan, Vol.94, No.1, pp. 75-89, 2016.
24. [24] H. A. K. M. Bhuiyan, H. McNairn, J. Power, and A. Merzouki, “Application of HEC-HMS in a Cold Region Watershed and Use of RADARSAT-2 Soil Moisture in Initializing the Model,” Hydrology, Vol.4, No.1, pp. 1-19, 2017.
25. [25] R. Mahmood, S. Jia, and M. S. Babel, “Potential impacts of climate change on water resources in the Kunhar river basin, Pakistan,” Water, Vol.8, No.1, pp. 1-24, 2016.
26. [26] H. L. Zhang, Y. J. Wang, Y. Q. Wang, D. X. Li, and X. K. Wang, “The effect of watershed scale on HEC-HMS calibrated parameters: a case study in the Clear Creek watershed in Iowa, US,” Hydrol. Earth Syst. Sci., Vol.17, pp. 2735-2745, 2013.
27. [27] V. Montesarchio, F. Lombardo, and F. Napolitano, “Rainfall thresholds and flood warning: an operative case study,” Nat. Hazards Earth Syst. Sci., Vol.9, pp. 135-144, 2009.
28. [28] S. Ghavidelfar, S. R. Alvankar, and A. Razmkhah, “Comparison of the lumped and quasi distributed Clark runoff models in simulating flood hydrographs on a semi-arid watershed,” Water Resour. Manage., Vol.25, pp. 1775-1790, 2011.
29. [29] M. M. G. T. De Silva, S. B. Weerakoon, and S. Herath, “Modeling of event and continuous flow hydrographs with HEC-HMS: Case study in the Kelani River Basin, Sri Lanka,” J. Hydrol. Eng., Vol.19, No.4, pp. 800-806, 2014.
30. [30] USACE, “Hydrologic Modeling System HEC-HMS, Quick Start Guide: Version 4.2” Hydrologic Engineering Center, Davis, CA, CPD-74D, p. 56, 2016.
31. [31] Water level increased about 3 meters within 2 hours in the Hikosan river of Fukuoka prefecture (in Japanese), https://www.nishinippon.co.jp/nnp/f_chikuhou/article/341581/ [accessed August 7, 2017]
32. [32] K. Asahiro, M. Tani, and H. Kanekiyo, “Support for farmland restoration through mutual assistance after flood disasters in Hilly and Mountainous areas – Cases of the cities of Yame and Ukiha affected by the torrential rainfall in Northern Kyushu in July 2012 –,” J. Disaster Res., Vol.10, No.5, pp. 794-806, 2015.
33. [33] T. Maesaka, M. Maki, K. Iwanami, S. Tsuchiya, K. Kieda, and A. Hoshi, “Operational rainfall estimation by X-band MP radar network in MLIT, Japan,” 35^th Conf. on Radar Meteorology, Pittsburgh, USA, 2011.
34. [34] http://www.data.jma.go.jp/fcd/yoho/wxchart/quickdaily.html?show=20170705 [accessed August 21,2017]