A model analysis for nonpoint pollutant reduction in paddy field watersheds

对某水田和森林流域的水质和水文进行了调查，提取了可用于建立净水模型的要素。森林流域非雨天径流量约为1 mm/d，枯水期径流量下降至0.5 mm/d。雨天径流增加幅度较大，并迅速减少到1 mm/d左右。径流量与降雨量之比约为30%，约70%的降雨量为蒸散量。并对这种现象的可靠性进行了讨论。此外，还对调查地点的污染负荷进行了量化。在采用循环水灌溉系统的水田流域中，Hii河的氮、磷和COD浓度低于其他排涝河的水田流域。磷和COD的浓度随河道的流出呈下降趋势，而氮的浓度没有表现出下降的趋势。另一方面，Hii河的DO浓度最高，河流中的氧气有助于有机物的分解。总氮、总磷和COD的降雨量与总氮、总磷和COD的降雨量之间没有明显的相关性。浓度下降是由于排水河和蓄水池中的降水和好氧分解引起的。除少数情况外，除个别情况外，除个别情况外，污染物的去除效果随水力停留时间的增加而增加。

Investigations on water quality and hydrology were conducted in a paddy field and forest watersheds and essential factors, which can be useful for establishing a water purification model, were extracted. In the forest watershed, runoff volume was about 1 mm/d during non rainy days, whereas it declined to be 0.5 mm/d during draught seasons. However, runoff showed relatively sharp increase during rainy days and decreased to be about 1 mm/d rapidly. The ration of runoff to the amount of rainfall was about 30 %, indicating that about 70 % of rainfall amount was evapotranspiration. Then the reliability of this phenomenon was discussed. Furthermore, pollution loads from the investigation sites were quantified. In the paddy field watershed with a circular irrigation system, the concentrations of nitrogen, phosphorus and COD in the Hii River were lower than those in other sites of drainage rivers. The concentrations of phosphorus and COD tended to decrease with the river flowing whereas that of nitrogen did not show such decreasing trend. On the other hand, the concentration of DO in the Hii River was the highest and oxygen in the river can contribute decomposition of organic matters. There was not clear relationship between concentration decrease with the river flowing and amounts of rainfall of total nitrogen, total phosphorus and COD. The concentration decrease was caused by precipitation in drainage rivers and retention ponds and aerobic decomposition. The amounts of removed pollutants tended to increase with increasing hydraulic retention time with a few exceptions.