In this study, a model for computing water temperature in the Lake Yanaka is established. The main feature of the model is the use of secchi disk depth to better estimate the heat budget. Secchi disk depth observation is incorporated with empirical relationship to calculate more accurately the solar radiation fraction coefficient and the extinction coefficient. Besides, the eddy diffusion coefficient, under the assumption of complete mixing, is calculated from the wind shear, density stratification, and inflow and outflow rate. Model results are compared with the observation data.