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Abstract:Water temperature near the surface of a lake increases with increasing air temperature, which results in stratification. The strength of stratification substantially influences the transport of water parcels from the surface to the bottom of a lake. In recent years, the stratification in Lake Biwa-the largest freshwater lake in Japan-has been stronger. However, it is difficult to reproduce the stratification well in the simulations. In the present study, we built a hydrodynamic model for the purpose of analyzing the structure of the stratification in detail. Using the model, we evaluated the reproducibility of the seasonal and annual changes of vertical water distribution and flow field in Lake Biwa from 2007 to 2011. The hydrodynamic model results show that the vertical water distribution approximately agrees with the field observations based on the statistical analysis. The seasonal change of thermal stratification is reasonably reproduced by the hydrodynamic model simulations. In the simulation, there are mainly two circulation flows at the surface layer of the lake. The first flows anticlockwise and the second flows clockwise in the northern part of Lake Biwa. In order to compensate for the surface water flow, the water under the thermocline sometimes flows in the opposite direction under each circulation flow.

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