Abstract
The aim of this study was to estimate the nitrate removal rate in paddy fields, which were irrigated from a reservoir. The reservoir stores the water that inflows from four small rivers to which spring water from tea fields drains. The study area was Makinohara, in Shizuoka, Japan, which is a tea field zone. The water quality and discharge were measured from June 2001 to May 2003. The results are as follows: The annual average concentrations of NO3-N were 25.5 mg·l-1 for inflow water, 21.3 mg·l-1 for irrigated water from reservoirs, and 4.7 mg·l-1 for outflow water from the paddy fields. The annual runoff load of NO3-N from the tea field zone was estimated to be 260 kg·ha-1·y-1. The average NO3-N removal rate during the irrigation period was 0.39 g·m-2·d-1 in the paddy fields. The NO3-N removal load in the paddy fields was about 3 times the runoff load from the tea field zones. It was estimated that the removed nitrate load, which ran off from the tea field, could be greatly decreased by the irrigation of the paddy fields all season.
