|Budget Amount *help
¥2,400,000 (Direct Cost: ¥2,400,000)
Fiscal Year 2002: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2001: ¥1,400,000 (Direct Cost: ¥1,400,000)
Seasonal variation of dissolved and suspended material coming into Lake Biwa through main 12 rivers are studied by the successive hydrological and chemical observations. On the basis of these observed data, water quality formation and residence time of lake water are analyzed by the numerical model.
River water is characterized by the geological background and land use, and shows a large amplitude of seasonal variation in its water quality. We have an attention on some event such as waste water from paddy field, heavy rainfall, and water level depletion in summer. We also made a successive observation of water quality along the River Yasu, which has a maximum catchment area among river in Lake Biwa basin.
After heavy rainfall, electric conductivity is rapidly increased by "washout effect", but is soon decreased by dilution. In this case, river water is flowing into the thermocline depth of the lake because of its lower water temperature than usual.
Highly turbid water containing high concentration of phosphorus and nitrogen, which is originated from paddy fields just before rice-planting, flows into the lake. This phenomenon has very bad influence on the lake water quality.
Residence time of lake water is estimated by numerical model using chloride concentration as a tracer : As a result, residence time of lake water is calculated little longer than that estimated former because river water flows into the surface layer and goes out fro the lake in spring time. Concentration of chloride also is calculated to be increased about 1 mg/L one year later if the qualities of river water are not improved.