Study on the Water Environment and Material Circulation in the Eastern Coast of the Lake Biwa

[Synopsis] With the inflow of pollutants or murky water, water environment and bio-environment of the lake Biwa have been deteriorating. Conservation measures including the construction of sewerage system, local initiative of reducing waste water or using of non-phosphate detergent are being adopted, but farming practices and reduction of agricultural chemicals have not been very effective due to the fact that wide area is covered, large volume of water is used, and the application of chemicals coincide with the particular period of the year. To establish effective measures for improving the water and bioenvironments, the origin and the circulation mechanism of pollutants need to be identified. In this study, the circulation of nitrogen in the paddy field or upland field was analyzed using DNDC (de-nitrification and decomposition model) and the potential of reducing the nitrogen leaching by changing farming practices was calculated. In addition, the movement of nitrogen or phosphate among different land uses and human/commercial activities was simulated by applying biomass circulation model. Major sources of pollutant, particularly nitrogen and phosphate are from livestock waste, fertilizer, and human waste. Purification capacity of paddy fields or water body can be estimated and measures to strengthening the capacity can be identified by improving the accuracy of these models, which will lead to the establishment of effective conservation measures