This research is intended to assure the equilibrium between energy production andrnconsumption from hydroelectric energy. Hydroelectric power plant development on GenaleDawarnRiverrnbasinrnis,rnarnpartrnofrndevelopmentrnplanrnforrnEthiopianrnenergyrnproduction.rnModelingrnrnofrnWaterrnbalancernandrnReservoirrnOperationrnisrnfrequentlyrnrequiredrninrnHydraulicrnengineeringrnforrnrnreservoirrnmanagementrnand torncontrolrnfluctuationrnofrnwater.rnThisrnstudyrnmainlyrnfocuses onrnwaterrnrnbalancernandrnreservoirrnoperationrnanalysisrnforrncascadedrnhydropowerrnplantsrninrnthernGenale-Dawarnrnriverrnrnbasin by using HEC-ResSim and HEC-HMS model. The stream flow generation werernestimated by using HEC-HMS model to enhance the water balance components at Genalerncascade reservoirs in the semi-ungauged parts of upper river basins. A Digital Elevation Modelrn(DEM) with 30 Ã— 30 resolution of the study area was used to extract the physicalrncharacteristics of watersheds using Arc-Hydro and the Geospatial Hydrologic ModelrnExtension HEC-GeoHMS. Then the HEC-HMS program was selected for this study, to attainrnthe water balance components of the basin due to its versatility, capability for flow generation,rnautomatic parameter optimization and its connection with GIS through HEC-GeoHMS. rnHEC-ResSim (Hydrologic Engineering Center-Reservoir System Simulation) was used tornsimulate the water allocation and operation of ongoing and planned reservoirs system inrnGenale-Dawa River basin. It is capable of modeling any reservoir system and designed tornperform reservoir operation modeling at single or multi reservoir system. The performance ofrnthe model has been evaluated through calibration and validation process. The HEC-HMSrnmodel was calibrated and validated on a monthly time scale, with reference to Chena-Mansarngauging station to estimate the flow from Genale-Dawa river basin using a time series datasetrnof 16 years from 1990-2005. For calibration the model results in the performance criteria ofrnRrn2 rn= 0.903, and NSE = 0.8926, and D = -0.0002%, and for validation performance criteria of rn2 rnRrn= 0.866, and NSE = 0.8447. Thus, HEC-HMS model has the ability to predict the water rnpotential of the basin. rnFollowing the configuration, and application of HEC-ResSim model, the position of guiderncurve was fixed at a minimum operative level for GD-3 and assigned at top of conservationrnzone for GD-5, and GD-6 along with explicit system storage balance which is used to force thernupstream reservoir, GD-3, to fill first, allowing the downstream reservoirs, GD-5, and GD-6rnto stay empty as long as possible when operating for flood control. The results of annualrnaverage hydropower energy generated by the model from joint operation of the reservoirsrnsystem is 4,311.25 GWh/year. The whole cascade power generation increment is 381.25rnGWh/year and it is a 9.701% improvement over the current design which is 3,930 GWh/year.rnAlso, Combined reservoir system operation model is capable to store 517.13 million cubicrnmeter flood water resources annually which shows 16.75% total reduction of spill release overrnthe current design.