Changes of water quality under an agricultural water use restructuring project and possibility of introduction of an environmental water

本研究的目的是评估农业用水结构调整项目对日本北海道牛弓湖水质的影响。我们预测，在取消泵站后，取水减少会增加湖水中的TN浓度。尽管通过抽水排出的水往往会从湖泊中带走污染，但在取消抽水站后，TN浓度有所下降。TN浓度下降的原因主要是有机氮浓度下降。2011年和2013年，尽管入湖负荷有所下降，但TN负荷平衡仍由流出型转为储存型。我们认为，2011年后TN浓度的下降反映了生物净化效应导致的有机氮浓度的下降。这是因为滞留时间的增加导致了湖泊的负荷去除，而该湖泊的水体滞留时间也显著增加。

The objective of this study is to evaluate the impacts of the agricultural water use restructuring project on the physical and chemical water properties of the oxbow lake in Hokkaido, Japan. We predicted that decreased water removal after the pumping station was abolished would increase TN concentrations in the lake water. Though removal of water by pumping tends to take pollution from the lake, TN concentrations fell after pumping station was abolished. The cause of decreased TN concentration was mainly decreased organic nitrogen concentration. Moreover, the TN load balance changed from an outflow type to a storage type in 2011 and 2013 despite decreased input load into the lake.We suggest that this reduction in TN concentration after 2011 reflects the decreased organic nitrogen concentration due to biological purification effects. This is because increasing retention time caused load removal from the lake, and the water retention time also increased significantly in this lake.