The eutrophication of Lake Biwa, which is the largest lake in Japan and one of the most important fresh water resources, has been warned since 1960s. In order to improve the water quality of the lake, it is inevitable to reduce the pollutant loads from the river basins. The Hino river and the Soma river basins are considered in the present study and models which predict the influence of pollutant load generation on the discharged pollutant loads through the rivers. These basins mainly consist of forests and rice paddy fields. The identified models reproduce the observed water quality relatively well. Evaluation of loads from point-sources and non-point sources indicates that the loads from non-point sources occupy the most of pollutant loads from the basins. Especially, the loads from rice paddy fields occupy most of the pollutant loads from the basisn. Therefore, controlling non-point source pollutant loads in the Hino and Soma river basins is an important step in checking the eutrophication of Lake Biwa.
Chemical and ecological studies for water quality of Lake Biwa
