MODSIM-based water allocation modeling of Awash River Basin, Ethiopia

• Water allocation modeling was conducted for Awash River Basin of Ethiopia using MODSIM.
• Expansion of the irrigation schemes in the Upper Valley is sustainable till 2018.
• Present irrigation can be sustainable up to year 2028 without having additional storage.
• The construction of Kesem Dam has no significant influence on Tendaho Reservoir.
• Gedebessa Swamp outflow contribution to Tendaho Reservoir is insignificant.

Awash River Basin is one of the twelve basins of Ethiopia which is highly utilized and the first basin to be introduced to modern agriculture. A study was conducted on water allocation modeling of Awash River Basin, Ethiopia, using MODSIM, a river basin management decision support system (DSS) designed as a computer-aided tool for developing improved basin wide planning. This study was conducted to analyze the water balance of the Awash Basin under different levels of irrigation development and also determine the water allocation in the Upper, Middle and Lower Valleys in the basin. Awash Basin includes Koka Dam and two dams under completion: Kesem and Tendaho Reservoirs. Four scenarios were set: Scenario I — present withdrawal rate in the basin; Scenario II — Scenario I plus downstream Tendaho Dam operational; Scenario III — Scenario II plus expansion of middle valley farms and Kesem Dam operational; and Scenario IV — Scenario III plus additional expansion in the middle valley. Analysis of flow records within the basin was done for a period of 1963–2003. Estimation of system losses, runoff from ungauged tributaries, and Gedebessa Swamp model parameters were considered in the flow process study. Simulation was conducted based on four scenarios. Consumptive and non-consumptive uses were considered in allocation modeling. The results of MODSIM model depict that there will be incremental release from Koka Dam from 2.8% to 5.7% in years 2018 and 2038, respectively. Due to increased diversions in Scenario III when compared to Scenario I, losses in to Gedebessa Swamp will significantly decrease by an average of 27.6%. In the year 2038, owing to less capacity of upstream reservoirs due to sedimentation, water will be lost in the swamp complex causing slight decrease of inflow to Tendaho Dam. Additional storage at or upstream of Koka Dam will be mandatory in the future. Unaccounted water diversions upstream of Koka and water losses in Gedebessa Swamp should be considered in the basin planning.