A model analysis for nonpoint pollutant reduction in paddy field watersheds

通过对稻田和森林小流域的水质和水文特征的调查，提取了可用于建立水质净化模型的要素。在森林流域，径流量在非雨天约为1 mm/d，而在旱季则下降到0.5mm/d。降雨期间，径流量增加较快，并迅速减少，最大径流量为1 mm/d左右。径流量占降雨量的比例约为30%，说明蒸散量约占降雨量的70%。并对这一现象的可靠性进行了讨论。此外，污染负荷的调查地点进行了量化。在稻田流域与循环灌溉系统，氮，磷和COD的浓度在熙河低于其他网站的排水河流。磷和COD的浓度随河流的流动有降低的趋势，而氮的浓度则没有随河流的流动而降低的趋势。另一方面，在Hii河的溶解氧浓度是最高的，在河流中的氧气可以促进有机物的分解。总氮、总磷和COD浓度随河流流量的降低与降雨量的关系不明显。浓度降低主要是由于排水河流和蓄水池的降水以及好氧分解造成的。去除污染物的量往往增加与水力停留时间的增加，少数例外。

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.