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.

Diurnal snowmelt pattern absent

Stage 0.404m, discharge 0.801m3/s within 2% of the rating curve estimate

With the coronavirus pandemic the university has severely restricted student research activity and fieldwork. Today I visited the field with family support to allow us to continue to check up on the monitoring and collect a new discharge measurement. We confirmed there has been no change in the rating curve.

The hydrograph above shows little if any diurnal snowmelt pattern, due to the extremely low snowpack conditions this winter and spring. Two significant rainstorms produced modest runoff peaks which would have been much larger with additional snowmelt contributions had there been snowpack.

Date               Event precip. (mm)    Max intensity (mm/h)
1-2     April               40.5                              5
18-19 April               52.5                              6

Overall, runoff for the month of April appears much less than average (provisional data under preparation).

Takiya below gauging point - right bank disturbed by recent logging activity

North of the Takiya basin, the Asahi Mountains (1870m) still snow-covered as viewed from Washigasu Maenodake (825m)