587 mm of rainfall in July

This July has had more than twice the usual rainfall, with 587 mm at the Miomote Amedas Station compared to an average of 263 mm (1981-2010). Maximum daily precipitation was 119 mm on July 24, and maximum hourly precipitation was 48 mm on July 18th. While the above hydrograph only shows data until July 21, it shows that the rainfall did not produce any particularly large floods. Instead there were multiple floods and peaks with a moderate stage of 0.8 m to 1.1 m.

The above two photographs show the water to be running very clear. However, the intense and localized storm of July 18 (max. 48 mm/h) seems to have been centered in the tributary basin which joins the main stem of Takiya River by the bridge abutment shown in the photographs below. There is a large accumulation of bedload sediment at the confluence below the bridge as the main stem did not appear to have the flow required to transport the sediment downstream.

Waiting for the rainy season

Stage = 31.3cm, Discharge = 0.38m3/s

No significant rainfall has occurred since the rainy season was officially declared, and discharge levels are close to annual minimum values. Early June saw a moderate rainfall event over 2-3 June with 56mm of precipitation. Visiting the upper part of the neighbouring Miomote River basin we could see that the high peaks (>1500m elevation) still had some snow cover. However, lack of rainy season precipitation could lead to low flow conditions in the coming weeks.

Asahi Mountains from near Okumiomote Dam

End of snowmelt season

The hydrograph above shows the recession of snowmelt runoff during the second half of May. The diurnal melt pattern has mostly gone by 24 May, and the subsequent peaks are due to rainfall (51mm rainfall over 26-27 May at Miomote Amedas station). A discharge measurement confirmed no change in the stage-rating curve, which has remained stable for nearly three years now.

Stage = 0.45m, Discharge = 1.18m3/s, Specific discharge = 0.0607 m3/s/km2

JICA field training

JICA (Japan International Cooperation Agency) participants on the course "Irrigation and Drainage Technology Based on Integrated Water Management" joined me at Takiya River for basic hydrological field survey. We measured discharge in three teams using different equipment and compared the results. Sampling of the channel bed materials using the Wolman Pebble Count was also undertaken. Participants came from various countries in Africa and Asia (Cambodia, Malawi, Mozambique, Myanmar, Philippines, Rwanda, South Sudan, Tanzania and Uganda) to study for six months in Japan.

Arakawa from Maruyama Bridge (Sekikawa Village)

Snowmelt season

Today I was joined by participants from my graduate class in Snow Hydrology. We observed the snowmelt runoff condition, and measured the discharge, confirming no changes have occurred in the stage-rating curve.

Above you can see the typical snowmelt runoff pattern in the stage hydrograph. The larger peaks above 0.8m are all connected to rainfall events, while the regular sine-wave fluctuations are due to fine weather snowmelt (e.g. early May). These fluctuations are closely related to the energy balance of the snowpack and the diurnal change in air temperature.

High winter runoff

Takiya River stage during winter 2016-2017 (good agreement between the two logger-sensor systems)

The hydrograph above shows the period from late November to early April, and from this we can see that winter runoff was relatively high due to mild temperatures and a greater proportion of precipitation falling as rain as opposed to snow. This is what we expect in a warmer climate.

There is only a relatively short cold snowy period with decreasing stage from 10-29 January. There are multiple peaks from late November to early January as precipitation was mainly rain, and the period late January to February shows five peaks which are all connected to rainfall events. Snow survey shown in the figure below shows that seasonal snowpack did not accumulate at the basin mouth until 10 January, about 2-3 weeks later than average. Peak accumulations measured on 20 February were also well below average.

Today in very cold but dry conditions we measured the river discharge to check the stage-rating curve. For a stage of 49cm the discharge was 1.66m3/s, or about 6% above the stage-rating curve estimate. The depth and cross-section data showed that the greater than average winter flows may have caused some scour (deepening) of the streambed.

Photos: T. Kobayashi

Young hydrologists

Now into the melt season, today I introduced a young hydrologist to the joys of snow survey. You need to have strong arms to hold up and weigh the snow sample, and stamina to deal with the cold conditions, but today we could enjoy fairly mild spring temperatures. Nearly 50cm snow depth (19cm SWE) remained at the larch site.

Snow survey result
Depth:    larch 48cm / cedar 33cm
Density: larch 0.39 / cedar 0.33
SWE:     larch 18.5 / cedar 10.5
(SWE = snow water equivalent)