In this study, HEC-ResSim model was used to simulate operation of existing hydraulicrninfrastructure, and major irrigation schemes in the Eastern Nile river basin and the new hydraulicrninfrastructure (GERDP).The main objective of this research is to find out the extents of thernimpact to which the introduction of new upstream reservoir, GERDP, on downstream users ofrnthe Nile waters by using the HEC-ResSim reservoir simulation model. Repeated runs of thernHEC-ResSim model were made using different filling and after filling of GERDP scenarios torncompare the effect of GERDP under alternative operating policies.rnThe computational algorithm in HEC-ResSim is divided in to three major components: (1)rnwatershed setup, (2) reservoir network, and (3) simulation for Calibration, Baseline andrnAlternative (Dry, Wet and Average filling and after filling) phase scenarios. The methodologyrncomputes the required releases to limit storage to the capacity available based on thernprobabilistic properties of future flows, conditional to current stream flow conditions.rnThe first setup was to simulate gaged flow routing without taking into account effects ofrndevelopment using the flow data of 1956 to 2003 and a good agreement was observed betweenrnsimulated and gaged data at El-Deim, Khartoum, Dongola, Hassanab and Tamaniat stations. Therncorrelation coefficient R2 values for those stations were found to be in good agreement and foundrnto be more than 0.925 for each station. Following this, the model was configured to simulate thernexisting and proposed development intervention.rnThe baseline development Scenario is considering only existing major water infrastructures (i.e.rnreservoirs) and major irrigation schemes in Sudan and Egypt for a period of 1956 to 2003. Thernsimulation result showed that an average annual energy of 14,810.83GWh/year is produced byrnRoseires, Sennar, Merowe and HAD.rnThe Alternative scenario development which includes during filling and after filling of GERDP,rnHEC-ResSim simulation, has discovered that long term effects of new upstream reservoir on thernoperation of downstream reservoirs. After filling of GERDP phase scenario simulation usingrnflow data of 1956 to 2003, the simulation result showed that an average annual energy ofrn31,363.63GWh/year will be produced by GERD, Roseries, Sennar, Merowe, and HAD (111.76%rnincrease than without GERD), the reduction of average annual energy by 395.41 GWh/year fromrnHAD will be compensated by the energy produced by GERDP and Sudan power plants, whichrnwill increase the total energy produced by the whole Eastern Nile system. During the fillingrnVrnphase scenario simulation results showed that an average annual energy of 27,041.96 GWh/year,rn25,695.48 GWh/year, and 31,213.61 GWh/year will be produced by the 5 power plants duringrnAverage, Dry and Wet filling Scenarios respectively and average annual energy production fromrnGERDP will be 11,314.38, 8,974.19, and 11,925.61GWh/year for the respective sequence ofrnfilling years. An average monthly power of 1,625.46MW and average annual energy ofrn14,238.99GWh/year will be produced during normal operation of GERDP (i.e. after filling fromrn2019-2066 for 48 years).rnDue to upstream regulation (i.e. intervention of GERDP, in the upstream of the Eastern Nile riverrnbasin) there will be an increase in average monthly inflow downstream in the driest month of thernyear and the annual average reservoir pool level increase which result in increase of power head.rnKeywords: HEC-ResSim, Reservoir Operation, Simulation scenario, GERDP, Roseries, Sennar,rnMerowe and HAD