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).