First snow survey

Trace snowpack of 10-20cm depth was measured around the stream gauging location. The snow lysimeter experiment was set up for one last time before likely decommissioning next spring.

New local logging activity

The above hydrograph shows that there are frequent rainfall events during November, with two significant peaks of about 0.6m to 0.7m stage (maximum daily rainfall of 37.5mm on 21 Nov.).

Below you can see that some local logging activity has begun close to the gauging station. The impact on the ground and soil of the caterpillar tracked felling machine appears to be rather severe, and does not compare well to the traditional aerial yarding technique which I witnessed being practised here about 20 years ago.

Autumn rains kick in

Big change in the situation of the basin since late October. Over the period of a week the river stage rose from about 0.3m (low flow) to 0.8m (moderate high flow) due to continuing and unusually heavy rainfall (190mm total with maximum intensity of 16.5mm/h).

October - new water year

First field trip since the start of the new water year on 1st October. The 1st October begins with low flow conditions, but the first autumn rainfall event of the new water year arrives with a small peak flow.

Stage data was downloaded for both sensor systems - the Kadec/Druck and Hobo systems. The photos below show the Hobo logger attached to the same support pipe as the staff gauge, within a grey PVC casing.

September wetter than usual

Early September began wetter than usual and another powerful typhoon (#24) is due to arrive around 30th September. Today's discharge measurement was 0.975 m3/s at 0.425 m stage, which fit exactly on the current rating curve.

Persistent rains during late August

As you can see from the hydrograph above, the drought finally broke on 16th August with a good sized flood, followed by unusually persistent rains (similar to rainy season) during the last week of August. This season normally has flash-floods with short-lived peak discharges. This kind of persistent rain will have done much to replenish the groundwater levels which were at extremely low levels through June and July.

The updated stage-rating curve below shows that recently measured high flows seem to plot well below the previous curves, while medium to low flows are similar. As always, further measurements are needed to confirm this apparent change in the shape of the stage-rating curve.

Drought conditions and low flows

Except for short-lived rainfall events end of June and mid-July, the drought conditions have continued throughout most of June and July. These two months usually bring the rainy season and 443 mm of precipitation (1981-2010 mean), but this year only 181 mm was recorded (see monthly data given below).

Miomote Monthly Precipitation (Amedas)
Month  2018  Mean (1981-2010)
   1         319     266
   2         231     190
   3         128     160
   4         231     149
   5         319     150
   6         96       180
   7         85       263
   8         477     209
Total     1886    1567

August became extremely wet for the second half of the month, recording nearly 500 mm over just 2 weeks or so. This unusual distribution of precipitation led to extreme low flow conditions and water shortages until the rains finally arrived on August 16 (see next blog post). Despite above average precipitation overall for the period January to August, the unusual distribution led to a three month droughty period from mid-May to mid-August.

Rainy season false start

The end of June brought a false start to the rainy season. Going into July and continuing for two weeks and more there was almost no rain and conditions were unusually hot and sunny.

Rainy season begins

Rainy season weather began on 27 June with daily precipitation of 55 mm, and maximum hourly intensity of 17 mm/h (Miomote Amedas). The stage increased rapidly, reaching nearly 1 m for a moderate size flood.

A short explore upstream from the gauging station revealed the inherent stability of the reach, influenced strongly by the high level of vegetation cover and the roots of riparian cedar and walnut trees. Only one short section of the bank was exposed to erosion, where a high terrace is being undercut.

Stage = 0.47 m, Q = 1.28 m3/s

Early summer low flows

Stage = 0.313 m, Discharge = 0.48 m3/s

Early June, before the arrival of the seasonal rain front, is a period of low flows. Since the floods over 17-19 May there has been a period of almost continuous flow recession, only interrupted by a minor rainfall event on June 1st (see stage hydrograph plotted below).

Today's discharge measurement plots very close to the stage-discharge curves developed before the 19 May flood, indicating that the aggradation of the channel seems to have been temporary. The red symbols plotted below represent the three most recent measurements taken since the flood. Only the first measurement taken at stage value 0.65 m is significantly different from the established curves. Points plotting to the right of the curves indicate channel aggradation (sediment deposition), while points plotting to the left of the curves indicate channel degradation (erosion of the channel bed).

Spring hydrograph

The winter to spring hydrograph above shows that there were two significant runoff events during January, followed by two smaller runoff events during February. The main snowmelt season starts at the beginning of March and continues until mid-May. An unusually heavy rainstorm occurs on May 19th causing stage to rise above 2m and estimated discharge to reach 100m3/s.

Today's discharge measurement plotted very close to the stage-rating curve prior to the May 19th flood, indicating that the aggradation of sediment in the reach may have been temporary. Future field work this month will confirm the status of the rating curve.

Cedar tree at the gauging section scarred by flood debris

Cedar tree roots stabilizing the bank at the gauged section

The following photos show the reaches of Takiya River below the gauging section over a distance of about 1 km to the small irrigation diversion close to the confluence with the Miomote River.

May 17-19 rainstorm event (>230mm)

Localized intense thunderstorms continued over the three days of 17-19 May, culminating in a major flood peak about 3am on the 19th with a stage of about 2.1 m (discharge >100 m3/s). Over the past 18 years of record, flood peaks have exceeded 2 m on only a few occasions during the June-August mid-summer season, but this is the first time to experience such a flood in May. The maximum precipitation intensities measured are new records for the month of May.

Precipitation (17-19 May):
Station       Total        Max intensity
Miomote   216 mm      23.5 mm/h
Takane      238 mm      38 mm/h
Takiya-2   235 mm      28.5 mm/h

The discharge measurement taken on 21 May showed that significant channel aggradation has taken place. We will have to quickly establish a new stage-rating curve and monitor for changes closely over this summer season.