First Takiya River fieldwork in a long while

Stage = 0.719m, discharge = 0.685 m3/s

 Today three of us made the trip to Murakami to check on the Takiya River monitoring site after a long break of about 18 months. Since the maximum flood on record of August 2022, the channel had been filled with huge volumes of sediment making the channel cross-section very unstable, and making it difficult to establish a meaningful stage-rating curve. So since the flood of August 2022 we have not been able to estimate the discharge hydrograph.

Seeing the condition of the channel today, it appears that the channel still shows high levels of aggradation due to sediment loading. It may still be challenging to establish a stage-rating curve but we will give it a go this year. New logger sensors were installed to monitor the water level and the atmospheric pressure.

Velocity, depth and discharge across the section (upstream view)

 

The stage data below was salvaged from the loggers recovered at the site. Significant flooding occurred on 15 September 2024 (flash-flood) and during 20-22 September 2024 when >330 mm of precipitation occurred at the Miomote Amedas gauge and hourly precipitation reached >70 mm/h. During this flood event it is evident that the channel degraded somewhat towards the pre-2022-flood level.

 Hope to give you another update on the monitoring soon!

New paper published on flow duration curves

Analysis of flow duration curves to evaluate the influence of climate change on the flow regime in the Japan Sea region

Kaba Diallo, Andrew Charles Whitaker, Al- Shakil, Aaron Conte, Antonio Machava Junior

Hydrological Research Letters 20(2), pp 135-142

DOI: 10.3178/hrl.25-00054

Abstract

There is a need to evaluate and understand how ongoing climate change is impacting hydrological regimes and seasonal flows in Japan’s heavy snowfall region along the Sea of Japan. This study analyzed various flow duration curves from the Takiya River (19.45 km2), Ara River (240 km2), and Uono River (1,408 km2) to evaluate the distinct influence of basin characteristics, climate change, and dam regulation on flow regimes. Seasonal flow duration curves showed consistent dominance of spring flows associated with snowmelt, most pronounced in basins with higher elevations and greater snow accumulation. The moderately regulated Uono River exhibits reduced flood peaks, flattened median flows, and more persistent summer–autumn low flows, reflecting dam operations. A comparison of the periods 1961–1990 and 1991–2023 for the Uono River shows significant increases in winter runoff (Q50 +15%) and a significant shift in the spring flow regime (Q10 –16%, Q90 +23%), consistent with warming trends. In addition, the development of stochastic flow duration curves made it possible to assess the impact of climate change on flow regimes in a probabilistic manner. These findings underline the dual impacts of climate change and flow regulation, with implications for flood control and water supply.

Figure 2. Seasonal flow duration curves for the recent period 2000-2022 comparing the dam-regulated Uono River (1,408 km2), with the unregulated Ara River (240 km2), and the unregulated Takiya River (19.45 km2) for each season; (a) winter DJF, (b) spring MAM, (c) summer JJA, and (d) autumn SON

Figure 3. Seasonal flow duration curves for the Uono River showing changes between Period 1 (1961-1990) and Period 2 (1991-2023) for (a) winter (DJF) and spring (MAM), and (b) summer (JJA) and autumn (SON). The change in mean seasonal runoff is notable for DJF (12% increase) and MAM (7.5% decrease), but not notable for JJA or SON. Using a sample of water years (2003-2009), we also show seasonal flow duration curves for measured and reconstructed runoff for (c) winter (DJF) and spring (MAM), and (d) summer (JJA) and autumn (SON). Natural runoff was reconstructed by computing inflow minus outflow for each of the four dams which were operational, and adding the result to the measured runoff

New paper published on climate change impacts on seasonal runoff in the Japan Sea region

Climate change impacts on seasonal runoff in the snowy Japan Sea region of Honshu, Japan

Al- Shakil, Andrew Charles Whitaker
Hydrological Research Letters 19(1), pp 8-14
DOI: 10.3178/hrl.24-00017

Abstract
Historically, the Japan Sea region of Honshu has experienced some of the heaviest snowfalls in the world, though snowfalls are now decreasing due to global warming. This study examined changes in streamflow seasonality and trends in monthly runoff during the past 60 years at nine river basins located across Niigata, Yamagata, Akita and Aomori Prefectures. A streamflow seasonality index (Center Time, CT) was adopted, and meteorological stations paired with gauging stations to analyze the dependence of streamflow seasonality on air temperature. Overall, there is a strong tendency for winter and early snowmelt season flows to increase (December to March), while the peak snowmelt season flows in April are decreasing. In most cases, CT shows a trend for earlier seasonal runoff, and we confirm a strong linear relationship between CT and temperature during snow cover season (December to April). The relationship between CT and temperature appears to be stronger in the south of the study region (mean R2 = 0.64) than in the north (mean R2 = 0.37). In addition, the regression slope (temperature sensitivity of CT) is greater in the southern region, with an average value of –6.3 days per degree Celsius compared to –4.2 days per degree Celsius in the northern region.

Figure 1. Locations of the streamflow gauging stations in the Japan Sea region of eastern Japan

Figure 2. (a) Long-term temperature trend during snow accumulation season (DJFMA) and seasonal snowfall trend at Tokamachi FFPRI, Niigata Prefecture (Shinano River Basin), (b) Monthly streamflow fractions (monthly runoff/ annual runoff) for the Uono River (1408 km2), a major tributary of the Shinano River. Comparing the early period with the later period, dominant spring snowmelt runoff (Apr–May) is decreasing, while winter season runoff (Dec-Mar) is increasing

Figure 3. Comparison of seasonal runoff trends for the Uono River, showing significant increasing runoff in winter to early spring (Dec–Mar) is balanced by significant decreasing runoff in April, as indicated by the Sen’s slope of the Mann-Kendall trend test

Figure 4. Relationship between (y) seasonal runoff timing (CT) and (x) mean temperature during snow cover season (DJFMA) for river basins Uono (a), Shinano (b), Agano (c) and Akagawa (d) in the southern region (37.24–38.63 degrees N) and Mogami (e), Omono (f), Yoneshiro (g) and Iwaki (h) in the northern region (38.76–40.81 degrees N)

Regular fieldwork

 Much flood debris had accumulated around the gauging cross-section, which we had to remove by hand. Over two years have passed since the record flood of August 2022, but the channel is still very unstable and heavily aggraded. The left-hand bank upstream of the bridge has been engineered with concrete to protect the forest access road located above it. Partly as a result of this activity, the right-hand bank is looking very prone to erosion with the danger of large cedar trees falling into the river. 

New paper published on long-term trends in runoff in eastern Japan

Assessing characteristics and long-term trends in runoff and baseflow index in eastern Japan

Stanley N. Chapasa, Andrew C. Whitaker
Hydrological Research Letters 17(1), pp 1-8
DOI: 10.3178/hrl.17.1

Abstract
Baseflow is the portion of streamflow derived from delayed subsurface pathways. The baseflow index (BFI) is a ratio of baseflow to total streamflow, and is an important hydrological variable when linking watershed characteristics to baseflow. The ‘smoothed minima’ procedure of baseflow separation was applied to streamflow data (29–67 years) from twenty-six gauges of watersheds in eastern Japan. The Mann-Kendall statistical test and Sen’s slope estimator were used to identify trends and estimate the rate of change in annual and seasonal runoff and BFI per decade at 0.01 and 0.05 significance levels. To the best of our knowledge, this is the first study to investigate long-term trends in runoff and BFI for watersheds in the large-scale region of eastern Japan. Results showed significant trends in annual runoff and BFI, with a concentration of significant seasonal trends occurring in winter with five gauges showing trends in runoff and nine gauges showing trends in BFI. The results suggest that the response of annual and seasonal runoff and BFI to climate change can already be seen, which implies that policymakers need more information on the impacts of climate change and human activities to manage water resources sustainably.

Figure 1. Location map showing gauges (MLIT), streams, and the digital elevation model (DEM), which was processed in ArcGIS 10.5. The DEM was sourced freely online from the ALOS World 3D 30-meter (AW3D30) DEM database (Japan Aerospace Exploration Agency, 2021)

Figure 2. Relative frequency histograms for the sample of 26 basins showing the distribution of BFI in the case of (a) annual period, (b) spring season (March–May), (c) summer season (June–August), (d) autumn season (September-November), and (e) winter season (December–February)

Largest flood on record occurs August 3rd 2022

The largest flood on record (since 2000) occurred at Takiya River on August 3rd during record-setting rainfall in the region of Murakami City over August 3rd and 4th. Unfortunately there was much flood damage within Murakami City as several rivers burst their banks, and at Takiya River catastrophic bank erosion and channel aggradation took place. Damage occurred to the staff gauge and at least one of the sensors recording river stage. A second stage sensor is assumed to be buried under about 50-60 cm of fresh sediment accumulation. The whole channel reach shows a similar level of sediment accumulation (channel aggradation). The excess of sediment in the channel has led to braided channel features where multiple channels are flowing around islands of sediment accumulation.

New daily precipitation records were set at several gauges, with maximums of 397 mm/d and 396 mm/day for Ishiguro Mountain and Takane gauges respectively on August 3rd. New hourly precipitation records were also recorded, with a maximum of 149 mm/hour for Shimoseki gauge on August 4th (almost a record for the whole of Japan's Amedas database). New 10-minute precipitation records were set for two gauges, with a maximum of 30 mm/10 minute for Shimoseki gauge. The total 2-day precipitation recorded at Shimoseki was 569 mm! The data in the table below shows that the intense precipitation was extremely localized, especially on August 4th.

Daily and maximum hourly and 10-minute precipitation (mm) recorded by Japan Meteorological Agency and Miomote Dam during August 3-4, 2022

Date         Gauge         Daily  Hourly  10-min.

August 3  Takane              396     70          20
                 Ishiguro Mt      397     57
                 Miomote Dam  260    81
                 Miomote           232    95          27
                 Shimoseki        189     52          13
                 Nakajo             134     37          13

August 4  Takane              18        10          6.5
                 Ishiguro Mt      52        14
                 Miomote Dam  36        15
                 Miomote          40        12          6
                 Shimoseki        380      149        30
                 Nakajo             158       92         23.5

The first hydrograph below shows the river stage during August 3rd to 4th, although correct recording of stage is lost when the stage is rapidly rising towards 3 metres. The sensor cable was snagged by debris and the sensor forcibly pulled out from the sensor housing in the river. We can see there were three large peaks above 2 metres up to this point. Previously, the maximum stage recorded was 2.59 metres on August 11th 2005. The second hydrograph shows the period leading up to the flood from mid-July. The flood on August 3rd occurs very suddenly after a dry period of about 10 days.

The last catastrophic flood of this magnitude, which also caused severe channel aggradation and channel instability, occurred on 11 August 2005. For the near-future we expect that it will be very difficult to monitor discharge accurately for a period of months or years as the channel adjusts to the huge pulse of sediment which has been introduced by this event. Extreme channel instability means that the rating curve is constantly shifting, so there are large errors in estimating discharge from river stage.

Intensive snow survey no. 4

Active forest thinning of cedar stands at Takiya River

This winter we are undertaking intensive snow surveys from the season of peak snow accumulation to the spring melt-off. Our objective is to investigate the influence of forest canopy characteristics on snow accumulation and melt, and in particular to compare varying levels of thinning in cedar stands.

Below you can see a summary of the results from the first four surveys. Melt season is now well underway in most study plots, although surprisingly it seems the forest opening plot is still maintaining the peak level for snow water equivalent (SWE). The larch plot has been surveyed during more than 20 years, and this year shows a peak value of SWE which is the second highest on record.

The fall of a freshly cut cedar tree is cushioned by the deep snowpack

Rain-on-snow conditions bring the first high flows of this winter-spring season