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A Simplified Model for Long Term Prediction on Vertical Distributions of Water Qualities in Lake Biwa Hosoda, T.(1) and Hosomi, T.(2) (1) Dept. of Civil Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan; [email protected] (2) Technical Official, Osaka Municipal Office, 1-3-20, Nakanoshima, Kita-ku, Osaka, 530-8201, Japan Abstract This paper describes a one-dimensional simplified model to predict the vertical temperature and water quality distributions through a year in Lake Biwa, which is the largest lake in Japan, using the monthly averaged meteorological data. The temperature and flow velocity equations the thermodynamic exchange at a water surface were firstly tested to calculate seasonal variations of temperature distributions in the vertical direction. It is shown that the typical patterns of temperature distribution with stratification and de-stratification processes can be reproduced numerically through a year, if thermal convection during the period of heat radiation is considered in a 1-D simplified model. Then the water quality parameters such as Phytoplankton (Chlorophyll a), Zooplankton, Organic and Inorganic nutrients, DO are combined with the water temperature prediction model, using a standard modeling of ecological processes. The long term calculations were carried out for ten years, and the results show the periodic year-by-year changes, because the inorganic nutrients are supplied due to the release from sediments. The limitation on Inorganic Phosphorus during a summer can be simulated in a surface layer in calculated results, and Inorganic nutrients in a epilimnion are supplied from a hypolimnion by both the thermal convection in an autumn and the large eddy diffusivity in a winter because of no-stratification. (A part of calculated results are shown in Figures.) Since it was shown that the calculated water quality distributions agree with the observed ones qualitatively, the model was applied to simulate the response of water quality parameters to the global warming. It was pointed out that when air temperature increases 3 C, DO concentration will be almost 0 at a bottom layer in the lake. -----------------------------------Takashi Hosoda, Dept. of Civil Eng., Kyoto University e-mail: [email protected]