Below average precipitation

Although the hydrograph above shows small to moderate peak flow events during July, the total precipitation for the month was well below average (170mm compared to 30-year average of 263mm). The full force of the rainy season never arrived in Niigata this year. Continuing dry conditions in the first half of the month allowed flows to decrease to similar levels as late June (pre-rainy season). Interestingly the ET induced diurnal fluctuations become less clear as the lowest flows are reached.

The peak flow on July 18 was due to 31mm daily precipitation at Miomote, while the multi-peak flow during July 23-24 was due to nearly 90mm total precipitation and a peak hourly rate of 19mm/h.

Low flows and peak flows

This year has seen below average precipitation during June (125mm compared to 180mm average). You can see in the hydrograph above that the period 17-26 June was completely dry, and we can even see small downward dips in the water level during day-time as evapotranspiration rises. This phenomena is only seen during mid-summer when river and ground water levels are relatively low and soil conditions are dry. A minimum stage of 0.261m is observed on 26 June.

During 27-28 June a rainstorm drops 66mm (max. 9mm/h) causing a moderate peak flow. However, soil and ground water storage is barely replenished, and flow levels quickly decrease to near 0.3m after a week or so.

Ta = 22C, Tw = 17C, Stage = 0.32m, Q = 0.357m3/s

Today we also carried out a survey of the channel long-profile, measuring by level the elevation of the channel thalweg and the water surface slope. Over the 90m reach shown in the photo above, the channel slope was 1.14%, while the channel slope over the 100m upstream of the gauge was 0.78%. The overall water surface slope for the 190m reach was 0.948%, with a drop in elevation of 1.801m.

Considering the reach between 50m upstream and 50m downstream of the gauging point, the slope was 0.84% compared to 0.90% surveyed in 2004. This suggests the slope at the gauging point has decreased slightly since 2004. We are currently undertaking an analysis of the hydraulic geometry and stage-rating curves for the gauging point, for which estimates of channel slope are essential. Measurements of channel slope during flood discharge will also be required.