Evaluation of Loading Rate of Nitrogen from Rice-Paddies by Small Watershed Method

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Nitrogen concentrations of drainage, percolation and irrigation were monitored periodically once a week at the experimental small rice-paddy watershed (EPW, 6.96 ha) for three years. Water level of the draining ditch was recorded continuously at the end of EPW. During the cropping period, load L of nitrogen through the drainage were evaluated from the concentrations C and the flow rates Q by integration interval method. During the non-cropping period, L was calculated from L(Q)-equations and Q. The L(Q)-equations were derived from the data measured during storm-runoff events. Concentrations of total nitrogen (TN) of irrigation, drainage and percolation were 0.403, 1.39 and 1.11 mg l-1, respectively, on average for three years. L of the drainage changed in wide range in response to agricultural practices and rainfalls; e.g. puddling, transplanting, fertilizing and plowing. The annual net-load of TN (unit load Ln) was 35.8±4.05 kg ha-1 y-1 on average for three years, of which the non-cropping period occupied 37 %. L for the puddling and transplanting period was discharged 27% of Ln, and strongly affected by the applied volume of irrigation. These results suggest that observations for only cropping periods or for only one year are insufficient for evaluating the precise Ln to assess the effect of the nitrogen discharged from paddy fields on lakes.