The Ethiopian government builds the Grand Ethiopian Renaissance Dam (GERD) that is a 6,000 MW hydropower project on the Blue Nile to fulfill the country’s energy needs. This dam will have many impacts on the water supply of downstream countries like Egypt and Sudan. The objective of this chapter is to model the impacts of GERD on AHD. Illustration of the model constructed to simulate impacts of filling options for the GERD reservoir is provided. Many scenarios were conducted using the developed model to predict the optimum filling scenario to minimize these impacts on downstream countries. Simulating results show that the live water storage in AHD will reach its minimum with the minimum water level of 147 m by the end of 5 years filling period of the full storage capacity of GERD (74 BCM). Scenarios of changing GERD’s filling period and GERD filling storage capacity have been conducted, and their effect on agriculture has been sesed. Scenario results show that decreasing the water supply required for agriculture will cause a high loss in income especially in case of 5 years filling period of GERD. Therefore, increasing filling period is a solution, or the other solution is the changing of the GERD storage capacity to minimize downstream impacts on AHD. However, this will not eliminate the impacts but will just relief them. The net loss in the return of each crop was computed for all scenarios to enable the decision takers to plan for the future for the different scenarios.24