Many regions has been facing formidable freshwater management and planning challenges. Concerns about limited water allocations, conservation of environmental and water qualities and policies for sustainable water use have been increased because rising water demand would cause water shortage in the near future. Therefore, it is necessary to look into possible alternative water resources management plans to mitigate the potential water shortage. However, it is not straightforward to predict and analyze the various situations likely to be occurred in the future. Also, finding an optimal solution among many alternatives to mitigate the water shortage is a complex task. In this study, a methodology of predicting and analyzing the water resources situations in the future using the K-WEAP (Korea Water Evaluation and Planning system) is presented and an optimal alternative is determined using the MCDA (Multi-Criteria Decision Analysis) that takes into account the economic, environmental, and social sectors. The proposed methodology is applied to the Nakdong River basin in South Korea to calculate water budget and possible water shortage. An optimal water shortage mitigation policy for the study basin is also suggested to help decision maker develop long-term water resources management strategies.