This study aims to propose a water management scheme that will balance two objectives, irrigation and hydropower, assuming a hydropower plant is annexed at an existing irrigation dam. A case study was conducted at a dam in northeast Japan that has operated solely for supplying irrigation water since construction. We simplify and focus the analyses by setting the current water management scheme as a benchmark. We then propose two alternative Scenarios designed to increase efficiency of hydropower generation, and investigate their energy potentials and economic performances. Scenario A adheres to maintaining a dependable discharge throughout the year. Consequently, it offers distinctively larger net benefits than the current scheme, with a benefit-cost ratio (B/C) of 1.851 as compared to 1.497 for the current scheme. However, Scenario A has a side effect of insufficient recovery of reservoir storage at the start of the irrigation period in drought years. Therefore, Scenario B tries to mitigate this effect by occasionally cutting the dependable discharge to one half during the winter months. Scenario B, however, exhibits a smaller net benefit from hydropower generation (with a B/C of 1.784) than Scenario A. Thus, there is a trade-off between the competing objectives of hydropower and irrigation. A desirable water management scheme depends on which objective of water use farmers and other stakeholders regard as most important.