Abstract
The nitrogen outflow model in agricultural area has been constructed. This model is composed by three land use elements ; field, forest and paddy field, and the livestock. Field and forest area, and livestock exist in upland part. Paddy fields exist in lowland part. Water flows naturally from upland to lowland. Then nitrogen caused from the field, forest and livestock in upland area flows into lowland. If the water is irrigated to paddy fields in lowland, nitrogen will be removed in this process by denitrification under anaerobic condition and uptake by rice plants. This kind of nitrogen removal function by paddy fields is investigated by many researchers. The author represented this function by the equation (1)and attached this function to the model of nitrogen outflow in agricultural area. Calculated results by this model are shown in Table 1,Fig. 3 and Fig. 4. In the case in which 100 kg/ha nitrogen outflows from the field, the concentration of outflow water from this watershed will attain to 10.3 mg/l when the percentage of the field area is 70%. However concentration will be decreased to 5.7 mg/l, if 50% of water is irrigated to paddy. Nitrogen removal by paddy fields has a large effect on the concentration of outflow water. Calculated values will be examined by the measured field data in next paper.
