Flash flood on August 10

 Intense rainfall in the early hours of August 10th caused a moderately sized flash flood, peaking at nearly 1.5 m stage. During one 10-minute interval, the stage rose by nearly 0.5 m! Rainfall intensities measured at Miomote Amedas gauge were up to 29 mm/h, with a daily total of 104 mm.

Today we measured discharge and we observed that there had been much new scouring of the streambed towards the left bank (looking upstream), and in the pool beneath the road bridge. However, the new measurement plots within 4% of the rating curve value, so no change in the H-Q curve detected.

I was glad to see that a developing potential log-jam at the lower end of the riffle below the gauging section had been shifted by the flash flood. The fallen cedar tree was rotated into a new position parallel with the bank, where it is less likely to cause a blockage.

Stage = 0.468 m, Q = 1.17 m3/s

Developing log-jam cleared from the main channel

Short rainy season

 

The hydrograph above shows several small peaks of around 0.8-0.9 m stage during the unusually short rainy season this year. The rains normally continue until late July, but this year you can see the rains stop before the middle of July. We can see a long period of flow recession from 12-27 July, during which only 4 mm or precipitation was recorded at Miomote Amedas gauge, and temperatures were consistently in the mid-30s Celsius.

Stage = 0.348 m, Q = 0.348 m3/s

Intense rainfall on 14 June (76.5 mm/h)

 

 Early rainy season and early summer conditions today with high humidity and strong sunshine, but not such high temperatures yet (just below 30 degrees). The hydrograph above shows four sizable rainfall events, with the largest on 4 June producing a stage of 0.8m and followed by a long flow recession to below 0.4m. On the evening of 14 June there is a very short and intense burst of rainfall (76.5 mm/h at Miomote Amedas gauge) which gives a very sharp peak and rapid decline back to baseflow conditions.

Today the river stage had dropped to 0.372m (discharge measured at 0.458 m3/s) which is one of the lowest levels for the season so far. Depending on the timing of the heavy rains expected during rainy season (late June to late July), flows may fall much lower still.

Stage = 0.372m, Discharge = 0.458 m3/s

Snowmelt season runoff

 Snowmelt pattern with daily fluctuations in the stage level during fine weather is visible in the hydrograph above during the long flow recession in mid-May. But the signal is not very strong due to the below average snowpack this year, which has largely melted off already by mid-May. May is wetter than average with several rainfall events in early and late May.

We measured the discharge soon after a medium-sized flood (about 0.8m stage), when the stage had fallen to 0.64m with a discharge of 3.35 m3/s. The flow was swift with a maximum velocity of 0.9m/s.

In summary, much of the runoff over the past month was due to rainfall runoff rather than snowmelt runoff, compared to an average year.

Stage = 0.64m, Discharge = 3.35 m3/s

18 April Rain-on-snow Event

 

In the stage hydrograph above you can see the moderate size flood caused by a rain-on-snow event during 17-19 April. You can also see the typical diurnal snowmelt runoff pattern which looks like a sine-wave with a steep rise and more gradual fall in water level each day. For Takane Amedas gauge, daily precipitation was 79 mm on 18 April with maximum intensity of 12.5 mm/h. Warm temperatures up to 13.4 degrees Celsius were measured in Murakami, with peak wind speeds of up to 21 m/s causing significant sensible heat transfer to the mountain snowpack. Total event precipitation was 105.5 mm at Takane.

However, today we visited Takiya River under perfect blue sky conditions for our survey and discharge measurement. The following photos show the discharge measurement and channel features around the gauging reach.

Winter season runoff

Stage = 0.545m, Q = 2.02m3/s

 Today we downloaded the stage data since December 2020. We can see there are many peaks during the winter and early spring season due to rain and rain-on-snow events which occur with warmer temperatures. Similar to the previous winter, we had some record-breaking warm temperatures, and snowpack was below average. January 2021 received snowfall, but February was much warmer than average (7th warmest in record), and March was the warmest on record. We can see that snowmelt has really progressed during March. Precipitation was well above average during January to March.

River stage during winter of 2021

River stage during winter of 2020

The table below highlights the recent record warm temperatures recorded in March for Murakami Amedas station. The last three years are all in the top 4.

Rank    March mean temp.    Year

                    (Celsius)

  1                    7.3                    2021

  2                    7.0                    2020

  3                    6.3                    2002

  4                    6.0                    2019

  5                    5.9                    2015

(Data since 1979)

March snow survey

 The table above summarizes the snow survey data for this winter season. The snowpack was below-average, with a peak of 28cm of snow water equivalent (SWE) in the larch stand, or 21cm of SWE in the cedar stand. The ratio of SWE for larch/cedar is 1.33 which is quite typical. There was 33% more snow accumulation in the larch stand due to lower interception loss of snow in the deciduous canopy.

Record warm temperatures during March meant that the snow accumulation peaked very early. Between 3 March and 19 March the snow melted at twice the rate in the larch stand compared to the cedar stand due to differences in the energy balance. On 19 March we managed to make the final snow survey and the first discharge measurement of the 2021 season.

Photos from 3 March:

Larch stand

Cedar stand

Photos from 19 March:

Snowmelt season now in full swing

Cedar stand

Mid-winter Snow Survey

Our first snow survey this winter season. And our first in two years, due to the lack of snow last winter. This year is perhaps close to average conditions. Tough weather for our survey, with first rain and wind, and then wet snow. The rain-on-snow conditions caused the river to be high (about 0.7m stage), and muddy with sediment.

After much rain over the past couple of days the snowpack was very soft and wet in the larch stand. It makes it difficult to take a snow core sample as the lower end will easily break off. The cedar stand was still a little hard and icy in certain layers.

Snow densities were high for this time of year! About 0.35 for both sites. As we measure every year, the larch stand has deeper accumulation and higher snow water equivalent (about 25% higher than cedar) due to the reduced snow interception in the deciduous larch stand.

Larch stand

Cedar stand

Rain-on-snow conditions