In this study, HEC-ResSim (Hydrologic Engineering Center-Reservoir System Simulation)rnmodel was used to simulate water use and operation of existing, ongoing and planned hydraulicrninfrastructures, and irrigation schemes in Omo Gibe river basin. For these purpose five differentrnwater resource scenarios were simulated viz. Scenario-T, Scenario-A, Scenario-B, Scenario-Crnand Scenario-D. Scenario-T was first set up to simulate gauged flow routing without taking intornaccount effects of development and a good agreement was observed between simulated andrnmeasured flow at Karadus station. The Nash-Sutcliffe efficiency criterion obtained for therncatchment was 0.782. Following this, the model was configured to simulate the recent andrnproposed development interventions on water resource. Scenario-A result showed that anrnaverage annual energy of 780GWh/year and 1911GWh/year are produced by Gibe I and Gibe IIrnrespectively when they are operated in tandem. Equivalent storage distribution was alsornexamined among reservoirs in the entire simulation period. The current study, HEC-ResSimrnsimulation in scenario B, has discovered that long term effects of operation of Gibe III powerrnplant would increase the dry season outflow and decrease in flood season outflow from the Omo-rnGibe river system. An average increase of 130% in mean monthly inflows from November tornJune and decrease of 25% in mean monthly inflows from July to October was observed atrnKaradus. But the mean annual outflow from the basin at Karadus will be decrease by 1.14%.rnSimulation of Gibe III power plant produced an average annual energy of 6,488GWh/year andrnpower of 758.94MW.The firm energy and power that can be guaranteed 90% of the time arern5885GWh/year, 625.0MW respectively. The average energy obtained is close with the studyrnmade by EEPCO. The firm energy obtained is also similar and greater than the energy obtainedrnduring reservoir operation studied by EEPCO.rnDue to upstream regulation i.e. intervention the proposed power plants, Gojeb and HallelernWarabesa stage I and II, in the upstream Omo-Gibe basin under scenario D can increases firmrnenergy production of Gibe III power plant to 6023GWh (2.75%) per year with same reliabilityrnand the average annual energy can also increase to 7587 GWh per year (by 15.66%). Moreover,rnduring scenario D simulation annually about 1.87BCM volume of water will be diverted intornirrigation site under full irrigation development (142,000ha) and the annual volume of waterrnfrom Omo-Gibe river system below the proposed irrigation sits will decrease by 14.15%.rnKeywords: Firm Energy, HEC-ResSIM, Reservoir Operation, Simulation scenario, Karadus,rnand Reservoir NetworkrnIVrnDedicationrnThis work is dedicated solely to my motherrnAbeba Yemataw,