Water is a vital resource for humanity and nature, and the lack of water affects life in all areas. Today, problems such as the inability to protect the status of existing water resources and excessive water withdrawal cause the amount of water to decrease day by day. In addition, conditions such as urbanization and industrialization and the resulting population increase, deterioration of water quality due to chemicals used in agricultural activities, and climate change affect the availability of water resources negatively. In this study, a basin-based water management study was carried out by applying the "Integrated Water Resources Management" approach to the Tahtalı-Seferihisar SubBasin located in Turkey, where it is expected to experience water stress in the future. The hydrological (precipitation, flow, evaporation) data of the water resources that are important for the basin and İzmir (Tahtalı, Seferihisar, Ürkmez, and Kavakdere Dams) were used to predict the availability of water resources in the future using the WEAP (Water Evaluation and Planning System) program, and several possible scenarios for water demands/supplies were analyzed. Under these situations, the water budget balances expected to occur 2050 have been estimated. Basically, seven different scenarios were created to transfer possible future possibilities to the program: Reference Scenario, Best Case Scenario, Worst Case Scenario, Report Consumption Scenario, Return Flow Scenario, Population Projection Scenario and Various Forecast Scenario. The water balances that can be obtained under different conditions in each scenario were calculated and compared with each other.