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.

Two major summer floods within 3 days

Stage = 0.437 m, Q = 0.615 m3/s

After an enforced break of 2 months, we finally managed to visit Takiya River to carry out a routine discharge measurement, and download both the stage recorders. The river channel seemed to be filled with an abundance of fine gravel, and indeed we found that the stage-rating curve had shifted to reflect this channel aggradation (higher stage level for a given rate of discharge).

Our previous visit (30 July) came just after a major flood on 28 July of stage 1.8 m, while a second major flood occurred soon after during 31 July to 1 August. Although both floods were of similar magnitude, the first flood was due to prolonged rainfall, while the second was due to very intense rainfall of short duration. At Takane Amedas gauge peak rainfall intensity reached 56 mm/h, and 71 mm fell over two hours. At Miomote Amedas gauge peak rainfall intensity reached "only" 19 mm/h, indicating the localized nature of the most intense rainfall.

These back-to-back floods appear to have introduced a larger amount of fine sediment to the channel than it can transport downstream, and hence the channel aggradation and shift in the rating curve.

Wettest month on record at >700mm

Stage = 0.705m, Q = 4.91m3/s (grey coloured suspended sediment is common after a large flood)

Much of northern Niigata Prefecture has recorded the wettest July and the wettest month ever recorded. So far the Miomote gauge has recorded 710mm and the Takane gauge 743mm this July. The previous wettest month for Miomote was 639mm in July 2013.

The wet weather culminated in a prolonged downpour over a 48-hour period during 26-28 July, when Miomote and Takane recorded 185mm and 206mm respectively. Maximum hourly precipitation was not so extreme, at 18mm/h and 26mm/h respectively. This resulted in a peak stage of 1.816m for Takiya River at 14:20 on July 28.

In measuring the discharge, we found a large accumulation of fine gravel in the channel with active bedload transport for this pea-sized gravel. The flow was especially shallow and with high velocity compared to our previous measurement on July 9 at similar stage.

Rainy season in full swing

Stage = 0.67 m, Discharge = 4.03 m3/s

Start of the rainy season

Stage = 0.345m, Discharge = 0.422 m3/s

The stage hydrographs above show the past two weeks (upper) and the past six months (lower). We have already had some heavy rain marking the start of the rainy season. The lower graph shows the high flows during winter compared to the low flows during April and May and into June. We cannot see the usual pattern of snowmelt runoff during March to May, due to the almost total absence of snowpack this year.

Record low runoff for May

Stage = 0.336 m, Discharge = 0.352 m3/s

At a first glance today's discharge appears typical of early summer low flow conditions. However, May was much drier than normal with only 96 mm of precipitation (Miomote Amedas) compared to the 30-year average of 150 mm. In addition, we have just experienced record low snowpack for this past winter. The lack of precipitation during May, combined with the lack of snowmelt runoff, have produced the lowest recorded basin runoff for the month of May at just 111 mm (20-year average is 287 mm).

This follows directly after the record low runoff for April, and both these records can be attributed to the record warm temperatures and record low snowfalls this past winter.

Depending on the timing of the rainy season later this month, and the amount of rain it brings, we could be expecting record low flow conditions for many basins in Niigata region and other parts of Japan.

Record low runoff for April

The provisional estimate for April runoff is just 240 mm, which is a record low by a large margin.

April runoff is very important as in almost every year April runoff is the greatest of any month, and it coincides with the planting of the rice paddies at the end of April to early May when demand for agricultural water resources is high.

Year    April runoff (mm)      Comment
2020          240                       Lowest on record (since 2001)
2019          348                       2nd lowest on record
2018          617                       4th highest on record
Note: Average April runoff (2001-2019) is 500 mm.

The table above shows April runoff for the last three years. The two lowest April runoff values have occurred in the last two years, and this year's record low is a full 108 mm lower than the previous record low, and less than half the average value of 500 mm.

Lack of seasonal snowpack is the main reason for these low April flows. April precipitation was actually a little above average at both Miomote and Takane (168mm/198mm compared to averages of 149mm/171mm respectively) and so cannot explain these low flows.