Rainy season

The hydrograph above shows the arrival of the summer rainy season on June 19th, firstly with a series of minor rainfall events followed by a moderate flood peak on July 3rd, and a major flood event on July 6th. Total daily rainfall on July 6th was 85mm and 105mm at the local Miomote and Takane Amedas stations, where maximum intensities reached 29mm/h and 33mm/h respectively. These high intensities produce a flash-flood response, and river stage rose by as much as 30cm over 10 minutes.

Stage = 43cm, Q = 1.05m3/s, Ta = 21.0C, Tw = 16.6C

Today I was joined by two doctoral students, Mirlan from Kyrgyzstan and Tsyden from Lake Baikal region in Russia. We measured discharge and confirmed no major changes in the rating curve, though there was some evidence of scour in the river bed. The water colour is light grey due to fine sediment being supplied at the upstream landslide site.

Mirlan and Tsyden prepare to start the discharge measurement

Flattened vegetation is clear evidence of how high the flood waters reached

Cool water temperatures and warm humid air produce a river mist

Stage-Discharge Rating Curve

Today we took another discharge measurement in similar low flow conditions and confirmed the stage-rating curve. The new measurement (0.358 m, 0.500 m3/s) plots very close to the current rating-curve estimate (0.459 m3/s). The difference between the measured and the estimated is 0.04 m3/s or 8.8% above the expected value.

Stage = 0.358 m, Q = 0.500 m3/s

Early summer low flows

Stage = 35.5cm, Q = 0.47m3/s, Ta = 23C, Tw = 15C.

Today participants from JICA took part in surveying discharge at Takiya River. We used 4 different teams in different cross-sections and compared the results. Two teams produced a very similar result within 5% of the current stage-rating curve, while the other two teams were overestimating discharge by a very significant amount (30%). The most likely reasons are systematic error in setting the depth of the current meter (too high) and overestimating the flow depth.

The hydrograph above shows clearly that snowmelt diurnal fluctuations continue until late April, after which there is a period of unsettled rainy weather. May 3rd was dry but we can see evidence of a small snowmelt peak during the afternoon, indicating the end of the snowmelt season for this basin. The diurnal fluctuations after this date are due to the influence of high rates of evapotranspiration, which has the opposite effect to snowmelt in drawing water levels down during the daytime. This pattern is very clear during the last few days of data recorded.

The below-average snowpack from this past winter has melted off about 4 weeks earlier than last year, and so discharge is already down to about half the amount of last year for late May.

JICA participants visit Niigata University (Course: Irrigation and Drainage through Integrated Water Management)

Snowmelt season

Fine weather snowmelt conditions. Stage = 50cm, Tw = 8C.

From the hydrograph below we can see that fine weather snowmelt conditions begin over the last few days of March and into April, as indicated by the daily fluctuations or sine wave pattern. This is characteristic of streamflow hydrographs in snowmelt regions. As warm sunny days continue, the daily fluctuations rise higher and higher as groundwater levels rise. On top of this we have the influence of rainfall events, most notably on 30 March (18mm) and 7 April (36mm). However, these events do not reach the high stage of over 1m that was recorded on 14 February this year due to rain-on-snow.

Larch lysimeter site after snow has completely melted

Cedar lysimeter site after snow has completely melted

Time-lapse cameras showed that the snowpack had completely melted by 27 March at the larch site, while it remained four days longer until 31 March at the cedar site.

February rain-on-snow flood

Larch snow lysimeter site (depth = 63cm, density = 0.38, SWE = 24.0cm)

Today's snow survey showed that the snowpack is more or less the same as in the last survey in mid-February. Snow densities have increased from about 0.3 to nearly 0.4 at the larch site, indicating that the snowpack has ripened considerably and reached a melt condition. The snow water equivalent is up slightly to 24 cm at the larch site, whereas it dropped about 2 cm to 16 cm at the cedar site.

4 March: Stage = 51cm, Tw = 5C

Downloading the stage data and plotting the hydrograph as below, we can see a very prominent flood event on 14 February. This is a typical late-winter rain-on-snow event, but the size of this event has only been exceeded once in the last 16 years of stream gauging for the months of January and February. Daily snowfalls occurred during the first 10 days of February, followed by rising levels of sunshine and rapidly warming temperatures, with a Tmax of 11.8 degrees and over 9 hours of sunshine on 12 February. This was followed by warm, cloudy and windy weather when overnight temperatures remained above freezing. On 14 February heavy precipitation of around 10mm/h and 40mm/d fell in the upstream area and Tmax reached 12.3 degrees, producing perfect conditions for rain-on-snow melt to occur throughout the basin. No sunshine was recorded, but wind speeds reached 11.3 m/s.

Rain-on-snow flood event on 14 February (peak Q estimated to be 18.5 m3/s)

The three largest January/February rain-on-snow events over the past 16 years:

Date               Stage (m)    Q (m3/s)   Daily Precipitation (mm)
2004 Feb 23      0.994           17.0                  44
2009 Feb 14      1.278           32.0                  52
2016 Feb 14      1.042           18.5                  37

Snowmelt time-lapse

This time-lapse video shows the spring melt-off for the snowpack at the larch lysimeter site from 27 March to 6 April 2015. The 10 day period was captured at 5 minute intervals with a time-lapse camera. On 18 March the snowpack was measured at 25.3 cm SWE (depth 65 cm). The period starts with nighttime temperatures below freezing, but mean daily temperatures rise steadily from around 6 degrees C to around 12 degrees C, and there is frequent rainfall of up to 6.5 mm/h and 32 mm/day.

Low levels of snowpack

Snow survey in the middle of February should give us close to the maximum snow accumulation for the season, at least for the low elevations. Melt conditions become more common during the second half of February, and precipitation amounts decrease. However, current snowpack amounts remain low compared to average conditions.

The chart above shows that the snow water equivalent for the 60 m and 140 m sites is the same at about 22.5 cm. The snow density at the 60 m site is slightly higher at 0.30 for a snow depth of 74.5 cm, whereas the values for the 140 m site are 0.28 and 79.5 cm respectively. These snowpack amounts are some of the lowest recorded since 2002, with only 2007 having snowpack significantly less than the current year.