Snowmelt season draws to a close

The above hydrograph shows how water levels have receded over the past three weeks as snow cover has receded during mostly fine and dry weather. The most recent four days still show a slight diurnal snowmelt response, but it is much smaller in size than at the start of May, indicating snow cover has almost disappeared in the basin. On 1 May discharge was around 2.8 m3/s, but it has fallen to just 0.56 m3/s by 26 May.

There are two noticeable peaks due to rain-on-snow. The first event on May 4-5 occurred after 39 mm of rainfall, while the second event on May 15-16 occurred after 47 mm of rainfall.

Stage = 38 cm, Q = 0.56 m3/s, Ta = 22C, Tw = 14C

JICA study tour

Today I introduced participants from a JICA irrigation and drainage course to the Takiya River field site. Our main exercise was to split into 4 teams and measure the discharge by different methods; (1) electromagnetic current meter with wading rod, (2) hand-held propellor meter, (3) bridge-suspended propellor meter, and (4) float method. The stage was 0.47 m, which is typical for the tail-end of the snowmelt season. Methods 1 and 4 gave a similar result of close to 1.32 m3/s, whereas the propellor meter methods gave somewhat higher estimates. Reasons for the differences are being investigated.

Hand-held propellor method

Bridge-suspended propellor meter method

Electromagnetic current meter method

Float method

Diurnal snowmelt pattern

The last 9 days of April show the typical diurnal snowmelt pattern where water levels rise and fall at a daily frequency due to fluctuations in the snowmelt energy balance between daytime and nighttime. The pattern is especially clear due to the fact that no precipitation at all fell during this time.

The moderate flood peak on April 20-21 was due to rainfall of up to 10 mm/h and an event total of nearly 60 mm (rain-on-snow event).

Stage = 58cm, Discharge = 2.84m3/s

Although levels of suspended sediment were very low, we could observe fine gravel-sized bedload moving on the streambed in the central part of the channel where depths were about 0.45m and velocities about 0.85m/s.