Ethiopia has long since been an area strongly affected by sedimentation. Sediment accumulation modeling in the constructed dam is an obstacle by the lack of historic sediment concentration data in developing countries. Nevertheless, the purpose of dams is affected by sedimentation. Water balance models then used to simulate sediment distribution in the reservoir. This often results in the estimation of sediment variability that is not identically distributed throughout the time. To show the temporal distribution of sediment in the Koga reservoir water balance model was used. It is likely that the volume of the Koga reservoir would reduce over time due to reservoir sedimentation. This reservoir sedimentation could mean a decrease in water supply for the irrigation project in the future. In addition, the method applies existing and future knowledge of sedimentation and annual climate variability relative to the Koga reservoir. Data collection of hydrology, meteorology, and irrigation water supply for the project site have been statistically compared and arranged as an input data source to fit the model. The volume change was incorporated into the water balance model. According to the water balance model result a reservoir volume reduction of 0, 5, 10, and 15Mm3 leads to minimum water storage of 6.7, 4.8, 2.9, and 0 Mm3 respectively in May. Because the minimum result appears just the end of the winter season as this is the time of year when the reservoir has been in the largest use. The thesis result shows sediment accumulation increase reservoir storage volume decrease. From this study, it is possible to be concluded that the dam is capable of providing enough irrigation water around 2027 E.C.