<p>As claimed by the Ethiopian, GERD main purpose is to generate electricity. Technically, hydropower plant operates to maximise the energy generated regardless of its reliability; or secure targeted amount of electricity that the plant can provide over a time period with specific reliability level. Even though, GERD’s operation rules can be derived from the different negotiation perspectives: absolute maximum, rational, and downstream perspective. Both of the first two perspectives are conflicting with each other; moreover, with any downstream requirement. The objective of this manuscript is to support the trilateral negotiation with scientifically based information on the trade-offs among the contradictory operation policies. Rules of the main operation scenarios: Absolute Maximum Energy (AME), Firm Energy (FE) and Minimum Annual Release (MAR), are formulated. The operation scenarios and sub scenarios are assessed stochastically using hundred series of 100-year monthly Deim station flow. MATLAB is used to develop a simulation model that combines GERD reservoir hydrology and hydropower plant design. It is developed to test accurately the impact of the different operation rules and compare them in terms of the generated energy and its reliability, total release and reservoir elevation. Presenting trade-offs and comparison results, of the different operation scenarios, in probabilistic terms provides negotiators with a more flexible framework, helping facilitate compromises without rigid adherence to specific values. The AME scenario is neither rational (expressing Ethiopian interests) nor considering the upstream hydrologic conditions and downstream needs. Results indicate that in a low-flow year, adopting a minimum annual release rule of 37 BCM would result in only a 10% reduction in Ethiopia’s targeted annual firm energy. However, prioritizing maximum firm energy generation during such a year could lead to 50% of the requested minimum annual release shortages downstream.</p>