2015 Volume 83 Issue 5 Pages II_89-II_93
About one million people in Kumamoto uses groundwater, which is recharged from paddy fields in the middle area of Shirakawa river watershed. This study clarified water management practices and water budget in a paddy field in the area by hydrological measurement during the irrigation period. In the study field, which had infiltration capacity of about 30 mm/d, the total amount of irrigation water during the irrigation period after the puddling season was approximately 4,000 mm. Irrigation was conducted as spill-over irrigation and 30% of water intake resulted in drainage water. It was concluded that irrigation, drainage and infiltration water accounted for large portion of the water budget in the paddy fields in the area with high infiltration capacity. In addition, it was confirmed the paddy field could reduce nitrogen because nitrate nitrogen was not detected in the paddy field although the concentration in the irrigation water was about 0.5 mg/L.