Saturday 31 May 2014

End of snowmelt season


The hydrograph above shows two small floods which were caused by rainfall, just as the snowmelt season is drawing to a close in the Takiya River basin. The diurnal snowmelt pattern is almost completely lost now, and the water levels and discharge are decreasing steeply. Temperatures have been fluctuating widely during the second half of May, from highs of around 12 degrees C to highs of over 30 degrees C. With mid-summer temperatures, the rates of ET rise sharply and speed the decline in river discharge.

Today I was joined in fieldwork by participants and staff from JICA, as part of a training course in irrigation and water management. We measured the discharge of Takiya River using three different current meters - the usual electromagnetic meter, a hand-held propellor meter, and a large propellor meter for use from the bridge. We also enjoyed surveying the channel bed materials using Wolman Pebble Counts, and walked the river channel about a kilometer downstream to the small irrigation diversion point.





To complete our 2-day field trip we visited the lower Arakawa and Tainai Basins, including the irrigation diversion headworks on the Arakawa River as shown in the photos below. This is an excellent example of water management, where the irrigation water for both the left and right banks of the river is taken at the same point, just as the river exits from the mountains and flows onto the coastal plain.





Friday 16 May 2014

Fine weather melt vs rain-on-snow


The above hydrograph shows the diurnal snowmelt pattern continuing through to mid-May, even though the snowpack was less than average this year. Dry fine weather continued during the last week of April and you can see this very clearly in the hydrograph by the gradually increasing diurnal snowmelt peaks. Both the daily peaks and the daily minimums are increasing, as mean daily temperature steadily rises from 8.6C on 23 April to 18.2C on 30 April.

During the first week of May cooler temperatures and a decrease in the snow-covered area are likely to have caused the decrease in water levels. However, a rain storm from the evening of 9 May (rain-on-snow event) caused a sudden rise in water levels and a moderate flood event. You can see the pattern of the hydrograph is very different comparing the fine weather melt during the last week of April to this rain-on-snow event on 9-10 May. Rainfall intensity reached 9 mm/h with a storm total of 27 mm at Miomote Amedas station. Much more rain likely fell in the mountains.

Stage = 55cm, Ta = 13.9C