14 years of stream gauging

Stream gauging at Takiya River has been undertaken since 20th September 2000, so we have now accumulated over 14 years of invaluable data. I'm currently in the process of reconfirming the stage-discharge curves over this period, and I can now present the provisional hydrographs as below. Please enjoy!

Autumn rains arrive

Today we worked in the rain, measuring the discharge at a moderately high stage of 56 cm. A maximum velocity of 0.91 m/s was recorded, and the total discharge came to 2.71 m3/s. We can see in the stage hydrograph below that a series of rainfall events has raised the baseflow water level as the catchment soil moisture and groundwater levels increase through the autumn season. Rainfall is becoming more frequent, and evapotranspiration is decreasing with falling temperatures.

Annual minimum flow

The annual minimum flow to date occurred on 24 September after a dry period of 5 days. Stage dropped to about 31 cm which is approximately 0.4 m3/s (0.0206 m3/s/km2) on the current stage-rating curve. A rainfall event of 37 mm around 25 September was enough to raise the baseflow level of the river by about 4 cm. This year's precipitation has been more evenly distributed than average which means that the annual minimum flow is also relatively high.

September low flows

The first 9 days of September were very dry (just 3 mm of rainfall on the 5th), and we can see the water level falling to only about 32 cm. This might be the lowest water level for the water year, also known as the minimum annual flow. Today we measured the discharge to be 0.45 m3/s at a stage of 34.5 cm. With a basin area of 19.45 km2, the specific discharge becomes 0.0231 m3/s/km2.

August rain

The hydrograph above shows that rains were quite frequent during August this year. Total precipitation for the month was 224mm at Miomote Amedas station, which is slightly above the average value of 209mm. The most notable storm event occurred on 11 August with a daily precipitation of 68mm (maximum intensity 15mm/h). Hot and dry weather around the week of 25 August saw the river level drop to near season-lows, and the daily fluctuations appear to show the influence of strong evapotranspiration during daytime. However, the dry first week of August does not show this diurnal pattern.

End of rainy season

The flood that we witnessed on July 9th turned out to be the largest flood of the rainy season this year. The hydrograph above shows that flood stage reached about 1m on the first peak, but then surged up to nearly 1.6m on the evening of July 9th. On that day 114mm (max. 20.5mm/h) of rain fell at Miomote, while 171mm (max. 26.5mm/h) fell at Takane Amedas station. The flood peak is very sharp and short-lived, with flood stage only remaining above 1m for about 3 hours.

Today we visited the field in typical mid-summer hot and humid conditions, and we took the opportunity to replace the base for the rain gauge located in the tributary basin. We encountered problems with rusty bolts for the rain gauge in the paddy field location and had to bring this one back to the lab for fixing. Trees are starting to encroach on both locations and are likely already having an influence on reducing the precipitation catch compared to a true open site.

Rainy season arrives

Stage = 85cm

Today brings typical rainy season conditions as the seasonal front passes over Niigata Prefecture, bringing very unstable conditions with heavy rain and thunder storms. Miomote Amedas gauge records up to 13mm/h while just to the north Takane Amedas gauge records 19mm/h, with total rainfall of 76mm and 110mm respectively by 4pm.

We arrived at 10am and set about making a discharge measurement using the float method. The flow was too high to try the wading method with current meter. Using a distance along the bank of 25m, we used branches as floats to measure surface velocity with a stop watch. We used six points across the width of the channel from the bridge. We estimated the mean surface velocity was about 1.5m/s, giving a mean profile velocity of about 1.2m/s (multiply by 0.8). Multiply this by mean depth (0.63m, estimated from current stage and the most recent channel cross-section), and channel width (11.6m), gives total discharge of about 8.75m3/s. Although this estimate is very approximate, it does fit within the bounds of the current stage-rating curve.