DEVELOPING OPTIMAL RESERVOIR OPERATION (CASE OF KOKA MULTIPURPOSE RESERVOIR, ETHIOPIA

Abstract

Reservoir operation is the most challenging task in the management of water resources systems. Hence, water resources models are needed to minimize these challenges, as much as possible if they are supplemented with sound engineering judgments. In this study, the rainfall runoff simulation HEC-Hydrologic Model System (HEC-HMS) and reservoir simulation model (HEC-ResSim) were applied to the koka multipurpose reservoir to optimize the power fluctuation observed and water scarcity due to reduced water release from the reservoir. Inflow was generated by the HEC-HMS model by using twenty years of daily meteorological data collected from NMA. The generated inflow was calibrated and validated with 16 years of observed flow data from Melka Kunture, Hombole, and Mojo gauging stations. The performance of the model was evaluated by NSE, R2, RMSE, and PBIAS performance indices criteria reviewed in different literature. For example, the NSE value of Melka Kunture, Hombole, Mojo, and Koka Inlet is 0.75, 0.78, 0.63, and 0.8 for calibration, and 0.7, 0.72, 0.53, and 0.75 for validation, respectively. The value of R2 for Melka Kunture, Hombole, Mojo and Koka Inlet is also 0.74, 0.77, 0.64 and 0.77 for calibration and 0.74, 0.75, 0.52 and 0.76 for validation respectively. Although the model slightly underestimates the flow, for both calibration and validation, the model shows acceptable performance to generate an inflow of the upper awash watershed. To simulate the reservoir the inflow generated by the HEC-HMS model, reservoir physical, and operational data were collected from governmental organizations and provided to the model. Since the HEC-ResSim model cannot optimize the constraints directly, trial and error have been applied through a prioritization rule between three main demands; i.e., Hydropower, Irrigation, and Domestic, Municipal and Industrial demands. The best alternative was selected based on the power target, release target, and pool elevation target. Accordingly, from the three alternatives applied in the simulation, ALT-1 gives maximum power and maximum release that supports the downstream water needs. When power demand was given the highest rule priority, the reservoir generate an average energy of 504.76MWh per day or 55GWh per year which is greater than the power generated in ALT- 2 and ALT-3 by 32% and 64%, respectively. The reservoir reaches its minimum elevation of 103.93m in June (except in the drought year of 2003 and 2016) and its maximum elevation of 110.39m in August. Generally, the reservoir can support the downstream water needs safely if the operation will be conducted by the power demand priority rule.