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

本研究的目的是评价农业用水结构调整工程对日本北海道牛轭湖水理化性质的影响。我们预测，取消抽水站后，去水量的减少会增加湖水中TN的浓度。虽然抽水取水倾向于将污染从湖中带走，但废除泵站后TN浓度下降。全氮浓度下降的主要原因是有机氮浓度下降。2011年和2013年，在输入负荷减少的情况下，全氮负荷平衡由流出型向储存量型转变。我们认为，2011年后TN浓度的下降反映了生物净化作用导致有机氮浓度的下降。这是因为滞留时间的增加导致了湖中负荷的移除，并且该湖中的水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.