Tuesday, 27 April 2021

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.

Wednesday, 7 April 2021

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
  1                    7.3                    2021
  2                    7.0                    2020
  3                    6.3                    2002
  4                    6.0                    2019
  5                    5.9                    2015
(Data since 1979)

Friday, 19 March 2021

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

Tuesday, 2 February 2021

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

Wednesday, 2 December 2020

Last survey of 2020

Before the winter season we always download the stage data from both the Kadec logger and the Hobo logger, and of course take a discharge measurement. Recent stage data from the Kadec logger plotted below shows two moderate peaks during November. The lower stage hydrograph compares the two logger systems for the second half of 2020. It seems there is greater discrepancy between them since October, with the Kadec data showing more noise during flow recession periods.

Recently thinned stand of cedar located very close to the gauged cross-section


Thursday, 12 November 2020

Autumn rains and colours

Autumn rains sustained moderately high discharge through early November. It seems to have been enough to move the sediment pulse through the reach so that we are returning to the previous stage-discharge curve (before flood of August 1). Channel aggradation with fine gravel (due to a sediment pulse) often occurs in Takiya River after major floods, but this year the aggradation seems to have been quite short-lived.

We walked the channel up to a large landslip area about a kilometre upstream. This landslip is a major source of sediment for these sediment pulses which move downstream.


Thursday, 29 October 2020

Establishing the new H-Q curve

Stage = 0.47m, Q = 0.90m3/s

Our second discharge measurement since the H-Q curve changed in early August due to channel aggradation. We need more measurements this autumn to be able to plot the new H-Q curve before winter arrives. The photos below show how the selective thinning of the cedar forest in the lower reaches of the basin has really progressed during this year.