The heavy rainfall event that occurred on 5–6 July 2017 in Northern Kyushu, Japan, caused extensive flooding across several mountainous river basins and resulted in fatalities and extensive damage to infrastructure along those rivers. For the periods before and during the extreme event, there are no hydrological observations for many of the flooded river basins, most of which are small and located in mountainous regions. We used the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model, a physically based model, to acquire more detailed information about the hydrological processes in the flood-affected ungauged mountain basins. We calibrated the GSSHA model using data from an adjacent gauged river basin, and then applied it to several small ungauged basins without changing the parameters of the model. We simulated the gridded flow and generated a map of the possible maximum flood depth across the basins. By comparing the extent of flood-affected areas from the model with data of the Japanese Geospatial Information Authority (GSI), we found that the maximum flood inundation areas of the river networks estimated by the GSSHA model are sometimes less than those estimated by the GSI, as the influence of landslides and erosion was not considered in the modeling. The model accuracy could be improved by taking these factors into account, although this task would be challenging. The results indicated that simulations of flood inundation in ungauged mountain river basins could contribute to disaster management during extreme rain events.

Extreme rainfall events cause severe flooding and/or landslides almost every summer in Japan. It seems that the frequency of such events and induced human/economic losses has increased in recent years. A torrential rainfall event in early July 2018 caused severe flooding in many river basins over Hiroshima and Okayama Prefectures of western Japan and resulted a number of fatalities and serious damage to property. One of them is the Oda river basin (about 498 km2), which was severely affected by the flood during the event. Different types of observation systems were used to measure or estimate rainfall for this event.Comparison of radar and satellite-based rainfall shows a good agreement with the rain gauge observations of the entire period. Basin-mean total rainfall from Japan Meteorological Agency's analyzed radar rainfall (RADJ), Water and Disaster Management Bureau's radar rainfall (RADW), Automated Meteorological Data Acquisition System (AMeDAS), and satellite-based rainfall data (GSMaP) were about 314, 322, 357 and 304 mm during July 5-7 2018.In this study, we applied Rainfall-Runoff-Inundation (RRI) to acquire a detailed understanding of flood processes in the Oda river basin. All the available rainfall data including different spatial resolution of hydrographic features were considered as the main input data in the model separately. Gridded flow and map of the possible maximum flood depth across the basins were generated and validated. Simulated results based on high resolution topographic data are more closely aligned with observed data. It is also found that the maximum flood inundation areas estimated by the RRI model using radar rainfall data appeared somewhat closer with the extent of flood-affected areas from the model with data of the Japanese Geospatial Information Authority (GSI) over the Mabi city. It is believed that modelled results can be used an important reference in the disaster mitigation and that they may be useful for further studies.

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.

Recently, the use of gridded rainfall data with high spatial resolutions in hydrological applications has greatly increased. Various types of radar rainfall data with varying spatial resolutions are available in different countries worldwide. As a result of the variety in spatial resolutions of available radar rainfall data, the hydrological community faces the challenge of selecting radar rainfall data with an appropriate spatial resolution for hydrological applications. In this study, we consider the impact of the spatial resolution of radar rainfall on simulated river runoff to better understand the impact of radar resolution on hydrological applications. Very high-resolution polarimetric radar rainfall (XRAIN) data are used as input for the Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) to simulate runoff from the Tsurumi River Basin, Japan. A total of 20 independent rainfall events from 2012–2015 were selected and categorized into isolated/convective and widespread/stratiform events based on their distribution patterns. First, the hydrological model was established with basin and model parameters that were optimized for each individual rainfall event; then, the XRAIN data were rescaled at various spatial resolutions to be used as input for the model. Finally, we conducted a statistical analysis of the simulated results to determine the optimum spatial resolution for radar rainfall data used in hydrological modeling. Our results suggest that the hydrological response was more sensitive to isolated or convective rainfall data than it was to widespread rain events, which are best simulated at ≤1 km and ≤5 km, respectively; these results are applicable in all sub-basins of the Tsurumi River Basin, except at the river outlet.

Use of easily accessible; public domain modeling software called Soil and Water Assessment Tool (SWAT) and its testing in watersheds has become essential to check developers' claims of its applicability. The SWAT model performance on Kliene Nete Watershed (Belgium) is examined. Given the watershed’s characteristic of a low lying; shallow ground water table, the test becomes an interesting task to perform. This paper presents calibration and validation of the watershed covering area of 581km2 . Flow separation is carried on using Water Engineering Time Series Processing tool (WETSPRO) and shows that around 60% of the total fow is contributed by base fow. Altogether seven SWAT model parameters have been calibrated with heuristic approach for the time frame of 1994-1998. Validation of these calibrated parameters in another independent time frame (1999-2002) is carried out. The parameter CH_k2 (Channel Effective Hydraulic Conductivity) is found to be the most sensitive. Nash Sutcliff Efficiency (NSE) values for the calibration and validation periods are found to be 74 and 67 percent-age, respectively. These ‘goodness-of-ft’ statistics, supported by graphical representations, show that the SWAT model can simulate such watershed with reasonable accuracy.Key words: SWAT; WETSPRO; Kliene Nete Watershed (Belgium); NSEDOI: 10.3126/hn.v6i0.4194Hydro Nepal Journal of Water, Energy and EnvironmentVol. 6, January 2010Page: 47-51Uploaded Date: 24 January, 2011

On 9–10 September 2015, the East Kanto region of Japan experienced a period of record-breaking heavy rainfall that caused a number of fatalities and serious property damage. The maximum 24-hr rainfall total (0600 UTC 9 September 2015 to 0600 UTC 10 September 2015), about 500 mm, was recorded over Tochigi Prefecture. Spatial and temporal variations in the meteorological and hydrological characteristics of this rainfall event were analyzed using data from the Japan Meteorological Agency’s (JMA) C-band radar network and data from the X-band polarimetric radar network (XRAIN). The rain gauge data available from the Kanto region has a temporal resolution of 10 min. The spatial and temporal resolutions of the JMA C-band radar data are 1000 m and 5 min, respectively, whereas the XRAIN radar has spatial and temporal resolutions of 250 m and 1 min, respectively. Data from the two radar networks were compared, both with each other and with data from various rain gauge networks to validate their accuracy. The 24-hr total rainfall data from both radar networks showed frequency distributions similar to those showed by the rain gauge data. However, the JMA and XRAIN data showed different distributions for the higher rainfall intensity thresholds. There was no relationship evident between rainfall and elevation in either of the radar datasets recorded during this event. The spatial distribution of rainfall over the study area derived from XRAIN showed clear variations, whereas the JMA radar did not. This is most probably related to the coarser spatial and temporal resolutions of the JMA observations. Based on a comparison of data from the rain gauge and radar networks, the XRAIN data more accurately reflected the rain gauge stations than did the JMA data. From a hydrological perspective, the Kinugawa watershed is unique in terms of its topography. The upper part of the watershed is wide and mountainous, whereas the rest is narrow and elongate north–south. The rain echo moved from south to north over the catchment, and the highest 24-hr accumulated rainfall totals were recorded mostly in the upper (northern) part of the Kinugawa watershed, whereas there was less rainfall in the lower (southern) part. This pattern suggests a high probability of serious flooding along the Kinugawa River in the days following such a rainfall event if the heaviest rainfall moves northwards over the watershed.

The frequency of severe flood events has been increasing recently in Japan. One of the latest events occurred in October 2019 and caused extensive damage in several river basins, especially in the central and northern regions of the country. In this study, we selected the Hitachi region (Hitachi-Omiya and Hitachi-Ota) within the Kuji River Basin which underwent considerable flooding due to the failure of embankments at two locations in the region. Maximum-possible flood inundation maps were generated using survey-based data and hydrological modeling for the Hitachi region. These maps incorporated the flood scenarios (embankment failures). All the generated products were compared with the reference flood mapping, i.e., Sentinel-1 data and Geospatial Information Authority of Japan (GSI) data for that region. It was observed that generated flood inundation mapping product based on the survey-data yielded results similar to those obtained with GSI data for the Hitachi region. Although each flood mapping product has advantages and disadvantages, they can be a good reference for the proper management and mitigation of flood disaster in the future. The rapid development of flood inundation mapping products that consider varying flood scenarios is an important part of flood mitigation strategies.

Two approaches to correcting the partial beam blockage of radar reflectivity in mountainous areas were evaluated using X-band dual polarization radar data from the Hakone mountain region, Kanto, Japan. The comparatively simple digital elevation model (DEM) method calculates the power loss in the received signal based on the geometrical relationship between radar beams and a DEM. The second approach, the modified DEM method, attempts to account for unknown power losses related to ground clutter, hardware calibration errors, etc. Comparison between ground data and reflectivity data corrected by both methods suggests that the DEM method alone was insufficient to correct beam blockage problems but that the modified DEM data were in generally good agreement with the ground data. The authors also estimated 10-min rainfall amounts using reflectivity corrected by the modified DEM method and compared these with data from a network of rain gauges in the mountainous region. In general, the results show good agreement between radar estimates and rain gauge measurements. On the basis of their results, the authors conclude that the modified DEM method offers a suitable solution to the problem of beam blockage in mountainous regions, provided that the beam blockage rate is less than 80%.