Abstract
A vertical one-dimensional diffusion model was applied to study formation and destruction processes in density stratification in Lake Koyama, Tottori Prefecture. In this model, a vertical eddy diffusion coefficient was dealt as the function for the local Richardson number, and for the parameters contained in this model, the optimum value for the water region was determined using a genetic algorithm. Using this model, we were able to effectively reproduce the observed results for temporal and spatial variations of water temperature and salinity. These results of numerical simulations indicated that thermal stratification occurred at wind speeds of 5 m/s or less, and that depending on the wind speeds, the position of the density interface changed within a scope of 1.5 to 3 meters. The wind drift time required for destruction of the strata was determined using an exponential function of wind speed. The factors resulting in progression of anoxia at a depth of 3 meters or more during the warming period in the summer where the maximum water depth was about 6 meters were examined based on the results of numerical simulations. Because the density interface is located at a depth of less than 3m at wind speeds of 4m/s or less, and because the compensation depth is about 2.4m, we can assume that progression of anoxia occurs at depths of 3m or more, because the oxygen consumption rate exceeds the oxygen supply rate in a day.
