Hot dry summer

Over a month has passed since our last fieldwork at Takiya River. This summer has turned out to be very hot and dry, and river levels are very low throughout Niigata. The hydrograph above shows there has only been two or three rainfall events in the past 5 weeks! There are two very long periods of flow recession without any rainfall. The influence of the forest vegetation evapotranspiration on the low flows can be seen by the daily fluctuations over the past two weeks.

Although this is high season for leeches and horseflies, we were only hassled by a few of the latter. The very dry conditions this year have kept the number of insects to a minimum.

Of particular note is that the authorities have removed all the fallen trees from the river below the gauging point. About 10 fallen cedar trees in total were removed. Apart from the caterpillar tracks on the river bank, the negative impacts on the river environment seem to be minimal.



Taking a discharge measurement (stage = 42cm, water temperature = 21 C)

All the fallen trees have been removed from the river below the gauging point

Uonogawa field trip

Gauging station on the Uonogawa at Horinouchi (A = 1400 km2)

Currently Team Hydrology are working with data from the Uonogawa River in the upper reaches of the Shinano River. This river basin has especially heavy snowpack in winter and very rich water resources which are used downstream throughout the Niigata Plain for rice paddy agriculture.

Today we visited several hydrometric monitoring stations that measure streamflow, precipitation, and snowpack in the basin, as well as the major dam site within the basin at Kuromatagawa Dam. When using data for hydrological simulation, it is important to see first-hand where and how the data has been collected!

Amedas weather station at Irihirose where snowpack exceeds 3m in winter

 

Snow cat machine at Irihirose

Kuromatagawa Dam

Shinano River gauging station at Ojiya (largest river discharge in Japan!)

Not much of a rainy season

This year, we did not have much of a rainy season here in Niigata. Southern Japan had torrential rains and flooding, but in Niigata we only had relatively light rains. The hydrograph above for Takiya River shows only minor floods, and by the end of July water levels are already quite low.

If we don't get more rain throughout the summer then we may be having water shortages. However, the spring and early summer snowmelt has topped up the groundwater levels which are so vital in maintaining river flows throughout the summer season.

As usual we measured the discharge to add further data to the stage-discharge relationship (H-Q curve). While doing this work, we could enjoy watching the "akatonbo" colourful dragonflies dancing above the river.

Stage = 48.5 cm, Tw = 17.7 C

Waiting for the rain

The above hydrograph shows a small runoff event on June 20, when about 20-30mm of rain fell over the catchment (maximum intensity 8mm/h at both Miomote and Takane Amedas stations). The big rains of the season are yet to arrive. Sometimes they come right at the end of June, or during the first two weeks of July.

27 June, stage = 47.5cm (exactly the same as on 13 June)

Amedas precipitation gauge at Miomote. This gauge has a heated jacket system to measure both winter snow and rain. It is also set high at 3.5m above ground level to be clear of the winter snowpack.

Miomote River just below Miomote Dam

Early summer low flows

The hydrograph above shows falling low flows. The last diurnal snowmelt pattern is just visible initially, but this disappears as the snowpack finally melts away. We enter a period of dry sunny weather when river levels become low before the arrival of the rainy season storms.

13 June, stage = 47.5cm, snowmelt season is over

Snowmelt tails away

The hydrograph above shows one rain-on-snow peak on 16 May, followed by mostly dry fine weather when you can clearly see the daily snowmelt pattern. You will also see that the water level is on a downward trend through the second half of May, with the daily melt wave-pattern becoming smaller and smaller. This tells us that the snowpack is disappearing quickly.

30 May, stage = 54cm, snowmelt season nearly over

Peak snowmelt

11 May, stage = 71cm. Is peak snowmelt over?

End of April to early May usually brings the peak snowmelt. This year is no exception. See the clear sine-wave pattern in the hydrograph that reflects the same pattern in daily air temperatures which are driving the snowmelt. April 28 to May 2 is a period of fine dry weather with only snowmelt driving the streamflow, but the next three days are influenced by rainfall which causes the largest peak on May 5, a typical rain-on-snow flood event.

Snow has now melted off from the lower elevations, and so the fine weather snowmelt pattern is likely to slowly fade out over the next couple of weeks as the snowline retreats up into the mountains.