Abstract
We describe the hourly changes of discharge for one day on the Nakadori River, which functions as a main drain for an irrigated paddy field of the Fukuoka-zeki Irrigation Project, Ibaraki Prefecture, Japan, and analyze the reasons for the changes. The catchment area of the river is 2,267 ha, of which 1,221 ha is a well-consolidated paddy field that has independent access to irrigation and drainage ditches at the plot level. We observed the water level in the river every 30 minutes using a pressure-sensor type water-level recorder from 2004 to 2006, and found a stable rate of hourly change each day, with the maximum around at 6:00 and the minimum around 18:00 during dry spells. We have also measured water levels at different levels of drainage, including a lateral drainage canal and a farm drain canal in the catchment area. Using evapotranspiration data obtained in a nearby paddy field, we performed a quantitative analysis of discharge flow and estimated evapotranspiration, obtaining the following results: i) The same pattern of changing water level or discharge in the Nakadori River is seen at all canal levels in the drainage system. ii) The increase in the evapotranspiration temporally coincides with that in the discharge, and the quantity of the former is accountable to the latter, as well. iii) The hourly change of discharge in the drain is caused by evapotranspiration through its influence on water level in the plots under a constant water supply to the paddy plots.
