Accurate determination of reaeration coefficient in lakes is a key factor for accurate modeling of dissolved oxygen. This study developed a new numerical model to determine the lake reaeration coefficient of Sawa lake, Iraq based on solving the transform mechanisms of dissolved oxygen without the using the empirical and laboratory methods. The model solved the gas transport at the water-air surface numerically to predict the dissolved oxygen and the reaeration coefficient simultaneously. Results showed that the lake reaeration coefficient was 0.001 Day-1 at 20 °C with low RMSEs (0.138, 0.137, and 0.168 mg/L for the three years of simulation 2007, 2012, and 2017, respectively) and AMEs (0.121, 0.114, and 0.145 mg/L in 2007, 2012, and 2017, respectively). It was noticed the lake habitat was impacted causing unhealthy conditions (low reaeration rate), and there is upward dissolved oxygen movement due to the presence of positive trend in the yearly cycle of temperature. In addition, including this numerical approach along with other water quality models can improve the predictions of other water quality parameters.