Rainy season arrives

Stage = 85cm

Today brings typical rainy season conditions as the seasonal front passes over Niigata Prefecture, bringing very unstable conditions with heavy rain and thunder storms. Miomote Amedas gauge records up to 13mm/h while just to the north Takane Amedas gauge records 19mm/h, with total rainfall of 76mm and 110mm respectively by 4pm.

We arrived at 10am and set about making a discharge measurement using the float method. The flow was too high to try the wading method with current meter. Using a distance along the bank of 25m, we used branches as floats to measure surface velocity with a stop watch. We used six points across the width of the channel from the bridge. We estimated the mean surface velocity was about 1.5m/s, giving a mean profile velocity of about 1.2m/s (multiply by 0.8). Multiply this by mean depth (0.63m, estimated from current stage and the most recent channel cross-section), and channel width (11.6m), gives total discharge of about 8.75m3/s. Although this estimate is very approximate, it does fit within the bounds of the current stage-rating curve.

Waiting for the rain

Stage = 33.25cm, Discharge = 0.374 m3/s

Depending on the arrival of the rainy season downpours, mid to late June can have the lowest discharge levels of the year for Takiya River. The snowmelt season has ended, and conditions can be extremely dry and hot. Levels of ET (evapotranspiration) rise and this reduces groundwater levels and the amount of flow in the river. The hydrograph below shows some rain fell on 13-14 June (45mm), but it is not enough to prevent the continuing fall of river discharge towards annual minimum levels.

A walk in the river

Discharge measurement in the tributary, Takiya 2 (basin area approx. 1km2)

A walk in the river can teach you many things. Today we walked the several hundred metres from the gauging point on Takiya River to the point where irrigation water is taken, close to the confluence with the Miomote River. Along the way we walked in the flowing water through riffles and pools, observing the changing channel form and channel bed materials, and the ecological environment.

Gauging point on Takiya River

Right bank with staff gauge and pressure transducer inside the steel pipe

Left bank with stable bedrock and moss

Dozens of small frogs were spotted in the river

Huge keyaki tree interrupts the plantation cedars

Channel becomes wide and shallow upstream of the irrigation diversion dam

Irrigation diversion dam made from gabions - wire crates filled with local rock material

Irrigation canal headworks gate

Settling pond for sediment below the headgate

Pure mountain water flows off towards the paddy fields

End of snowmelt season

The hydrograph above shows two small floods which were caused by rainfall, just as the snowmelt season is drawing to a close in the Takiya River basin. The diurnal snowmelt pattern is almost completely lost now, and the water levels and discharge are decreasing steeply. Temperatures have been fluctuating widely during the second half of May, from highs of around 12 degrees C to highs of over 30 degrees C. With mid-summer temperatures, the rates of ET rise sharply and speed the decline in river discharge.

Today I was joined in fieldwork by participants and staff from JICA, as part of a training course in irrigation and water management. We measured the discharge of Takiya River using three different current meters - the usual electromagnetic meter, a hand-held propellor meter, and a large propellor meter for use from the bridge. We also enjoyed surveying the channel bed materials using Wolman Pebble Counts, and walked the river channel about a kilometer downstream to the small irrigation diversion point.

To complete our 2-day field trip we visited the lower Arakawa and Tainai Basins, including the irrigation diversion headworks on the Arakawa River as shown in the photos below. This is an excellent example of water management, where the irrigation water for both the left and right banks of the river is taken at the same point, just as the river exits from the mountains and flows onto the coastal plain.

Fine weather melt vs rain-on-snow

The above hydrograph shows the diurnal snowmelt pattern continuing through to mid-May, even though the snowpack was less than average this year. Dry fine weather continued during the last week of April and you can see this very clearly in the hydrograph by the gradually increasing diurnal snowmelt peaks. Both the daily peaks and the daily minimums are increasing, as mean daily temperature steadily rises from 8.6C on 23 April to 18.2C on 30 April.

During the first week of May cooler temperatures and a decrease in the snow-covered area are likely to have caused the decrease in water levels. However, a rain storm from the evening of 9 May (rain-on-snow event) caused a sudden rise in water levels and a moderate flood event. You can see the pattern of the hydrograph is very different comparing the fine weather melt during the last week of April to this rain-on-snow event on 9-10 May. Rainfall intensity reached 9 mm/h with a storm total of 27 mm at Miomote Amedas station. Much more rain likely fell in the mountains.

Stage = 55cm, Ta = 13.9C

Fine weather snowmelt

Stage = 55cm, Ta = 11C

Fine weather snowmelt conditions have continued for the past two weeks. Precipitation of just 3mm on April 15 and 6.5mm on April 21 was barely enough to disturb the daily snowmelt pattern which dominates the hydrograph shown below. However, the fluctuations in stage due to snowmelt are less than 10cm and decreasing in magnitude which indicates that the snowmelt season will soon be finished for this basin.

Snowmelt season

First discharge measurement of the spring season, stage = 55cm, Ta = 7C

Snowmelt season is in full swing as you can see from the hydrograph above. First the hydrograph shows some small peaks in late January, followed by a period of steady flow recession during which time temperatures were mostly freezing.

Rain-on-snow and melt events start suddenly at the end of February, and you can see that during the second half of March the peaks are rising steadily as the basin becomes more and more saturated with snowmelt water, and groundwater levels rise. There is a moderate size rain-on-snow peak of almost 1.0m stage on 31 March (41mm of rain during 30-31 March). Recent cooler temperatures and the disappearing snow cover have led to river levels dropping. However, the daily snowmelt pattern is still clearly visible over the last three days of record.

Larch lysimeter site

Cedar lysimeter site

Young cedar lysimeter site with trace of snow remaining