Moderate autumn rains

November can be a time of prolonged and quite intense rainfall, but this year there are only three moderate events in the first half of the month. The event of 8-9th November included about 40mm total precipitation with hourly intensities up to about 7mm/h.

Autumn dry season

Stage = 0.370m, Q = 0.538m3/s

October can be very fine and dry. This year we experienced very little rain during the two weeks 10-24 October, and stage dropped to levels similar to summer minimum flows.

Again overall the match is good between the two stage recording systems, although the Hobo logger memory became full as I had not been resetting the logger since August. Resetting the logger to a 10 minute interval means that it will be able to log for 5 months until next April. This will avoid the need to remove the logger from the water column during the winter in snowy conditions. The Kadec system definitely has the advantage of more easily downloading the data, although even then it is not really possible during heavy rain or snow.

The importance of stage measurement

Stage = 0.378m, Q = 0.521m3/s

In monitoring river discharge, the importance of stage measurement cannot be over-stressed. Accurate and reliable stage measurement in addition to regular maintenance of a site stage-rating curve will be fundamental in obtaining continuous discharge estimation.

For 16 years I've used the same system to monitor stage (pressure transducer and Kadec data logger) with no reliability issues at all. I've just had to change the batteries once per year, and check the offset on each field visit to make sure the visual stage reading matches the data logger reading.

However, recently some issues with the offset adjustment within the data logger have led me to acquire a second back-up system to monitor stage. I chose the inexpensive and widely used Hobo loggers, and have been comparing the Kadec and Hobo loggers since the summer. The Hobo logger system is unvented and so we need to install two sensor/loggers - one to monitor pressure in the water column and another to monitor atmospheric pressure.

The other major difference between the two systems is that the Kadec logger can be easily downloaded on the river bank, while the Hobo logger must first be retrieved from the river and then re-installed after download, making careful note of stage levels during this process.

The two stage hydrographs above show that overall the agreement is good between the two logger systems. However, there are some significant differences, especially during periods of moderate to low flows, in particular the period in early September. The Kadec logger seems to show a more unsteady stage curve during lower flows, and this needs to be further investigated. Perhaps the atmospheric pressure venting or the temperature compensation is not working as effectively as it should be.

Low flows and typhoons

During the month from mid-August to mid-September river stage has varied widely from low flows of 0.25 m stage to peaks of up to 1.3 m. Typhoon No.10 passed over northern Japan during August 30-31 and brought moderately heavy rain to the Takiya River basin. Total storm precipitation of 92 mm and an hourly maximum of 24 mm/h fell at Mt. Ishiguro (elevation 900 m), while low elevation gauges at Miomote and Takane (Amedas) recorded lower intensities and about 60 mm for the storm total.

Stage = 0.343m, Q = 0.380 m3/s

Very large accumulations of fine gravel have occurred at the gauging cross-section (likely due to the typhoon event), although confirmation of the stage-rating curve showed that there had not yet been a significant change. The newly installed Hobo logger was almost buried under the loose and unconsolidated gravel.

Summer low flows

The end of July and the first half of August have been mostly dry, and you can see the stage following a decreasing trend interrupted by a couple of rainfall events. Conditions are hot with highs in the 30s Celsius, and so ET is high during the daytime and this shows up on the stage hydrograph as a series of dips.

Today I measured the discharge at a stage of 30cm, and then installed a new stage recorder (HOBO) attaching it to the steel pipe of the staff gauge within a housing of PVC pipe. The pressure transducer I've been using since 2000 has started to become rather erratic and I suspect it is no longer venting properly with changes in atmospheric pressure. It shows drift from the observed staff gauge reading of up to 25mm after a couple of weeks. Today it was 10mm too low before I reset the offset.

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