In this study it has been intended to develop operation modeling for Tendaho reservoir usingrnHEC-ResSim software. The input data considered for the model are daily inflow, monthlyrnirrigation and environmental diversion flow requirements, monthly reservoir surface evaporationrnand Tendaho dam and reservoir physical data.rnThe reservoir operation modeling has been done for two inflow source options. These arernupstream and Tendaho station inflow source options. In the upstream inflow source option thernten years (1992-2001) daily flow records of Awash River at Adaitu and Mille River at MillernBridge have been routed to the reservoir. Moreover, for the ungauged catchments downstream ofrnthe hydrological gauging stations considered, daily flows were generated using SCS-CN method.rnHowever, in the Tendaho gauging station inflow source option, the ten years daily flows ofrnTendaho gauging station has been used as source of inflow to the reservoir.rnThe SCS-CN method was calibrated for the ungauged catchments using five years (1992-1996)rnflow data transferred from the Logiya catchment and areal rainfalls computed from Adaitu,rnMille, Bati and Dubti stations. The method was then validated using another five years (1997-rn2001) data. The parameter considered was the CN.rnFor the two inflow source options considered, operation modeling has been made for threernspillway gate setting alternatives: Low level (1m), Average level (4m) and Maximum levelrn(6.9m) gate openings. Incase of the upstream inflow option the maximum reservoir releases arern282.35m3/s, 937.98m3/s and 1015.40m3/s for low level, average level and maximum level gaternsettings respectively. Incase of Tendaho inflow the maximum reservoir releases are 259.82m3/s,rn267.73m3/s and 349.11m3/s for low level, average level and maximum level gate settingsrnrespectively. The irrigation outlet releases are similar for the three gate setting alternatives inrnboth inflow options.rnThe wet season reservoir operation rule curves have been developed for the minimum gaternopening alternative of both the inflow options. Following the ten years wet season pool levelrncurves, three Operation Guide Curves have been developed. These are Upper Bound GuidernCurve, Lower Bound Guide Curve and Average Guide Curve.