© 2021, Saudi Society for Geosciences.Due to the increase in water demand, it is getting more critical to operate reservoirs in an efficient manner in which multiple demands should be sustained. In Turkey, the Seyhan Plain irrigation project has great importance in terms of increasing agricultural production and development. To evaluate the water storage changes considering alternatives, a daily reservoir operation model was developed using the Hydrologic Engineering Center’s Reservoir System Simulation (HEC-ResSim) watershed model with 33 years of data. Four alternatives were created by considering current and projected water usage in the upstream areas and two different flow types, pipe flow and open channel flow. Findings show that Seyhan Reservoir could not meet the irrigation demand considering the worst-case scenario and the minimum water storage amount was found as 240×106 m3. However, with the modernization in the irrigation transmission system which significantly reduces water loss and prevents water ponding on the soil surface, the simulated minimum water storage amount for the same time was found as 380×106 m3. With this study, it was highlighted that the water supply problems for existing reservoir systems can be mitigated by the modernization of the transmission lines via tandem reservoir operation rules.