Analysis of Factors Contributing to Change in Water Temperature in Lake Biwa, Japan

Changes of water temperature in Lake Biwa and the atmospheric factors that can affect the water temperature were analyzed. The water temperatures of effluent from sewage plants were found to increase, but it did not affect the annual mean temperatures of the lake surface water. The change pattern of surface water temperatures in summer resembles the change pattern of the number of hot days with temperatures exceeding 25°C. On the other hand, the change pattern of the water temperatures in winter resembles the change pattern of the number of cold days with temperatures dropping below 0°C. These results show that the water temperatures of Lake Biwa conspicuously increased in the past 30 years, and it was caused mainly by increasing atmospheric temperatures. It is suggested that the increase in the annual mean temperature of the deep water is affected by the absence of a decrease in the water temperatures in winter, and the increase in the annual mean temperature of surface water is affected by the increase in the water temperatures in summer.