Impacts of an Agricultural Water Use Restructuring Project on the Water Quality of Oxbow Lake

抄録

Oxbow lakes in the Ishikari River Basin, Japan, are used as irrigation water sources, flood control ponds, and waterside parks, etc. In this study, we monitored water quality over the course of the restructuring project at Lake Chashinainuma. In this catchment area, irrigation water was pumped from the lake until 2010, but is now drawn from a distant river. As a result, the total nitrogen load balance changed from an outflow type to a storage type due to decreased removal of water from the lake. The lake water retention time also increased significantly. However, total nitrogen concentration decreased after the pumping station was abolished. We suggest that this reduction in total nitrogen concentration after 2011 reflected decreased particulate nitrogen due to biological purification effects that are expected with increased chlorophyll concentration. Hence, the change in load balance to an accumulation type may have been influenced by changes in agricultural water management and transformation of paddy fields into upland.