Different projects in Tana-Beles basins were identified as water resource development project and implemented to attain the country s development goals. One of these projects is Tana-Beles multipurpose hydropower project which has been under operation since May 2010. Water is diverted from Lake Tana through tunnel to power plants and then joins Beles Basin River towards irrigation area. Seasonally varied flow influence both products. That means at dry season water demand in the irrigation area is very high with low natural flows whereas during wet season the demand is low with high natural flow. Both upper and lower Beles irrigation areas are not fully covered. But, power production is high at wet than dry season in the project. Therefore, optimization of the project is necessary to obtain good combination and coverages of both purposes. In addition, the study has also assessed the feasibility of damming water at Dangura site.
The study found nMAG model feasible and applicable to run the given data specification. This study has analyzed hydrological data of the basin which is used for model setting. Optimization is undertaken through the development of two phase scenarios. The first phase involved regulation of Lake Tana within four scenario while the second phase involved optimization of Tana-Beles MPP with six scenarios. The second phase of optimization is followed by the identification of the best scenario in first phase. Scenario D which is release of seasonal varied flow to downstream, is selected on the basis of environmental flow, the amount of water released, power production, and energy equivalent in first phase. This scenario proceeded to the second phase and developed other six scenarios. In the second phase, scenario D2R is selected on the basis of environmental flow, the amount of water released, power production, irrigation coverages and water requirement, water consumption and energy equivalent basis which show better result.
This study found that the mean annual water release to downstream is 978 Mm3 higher than 862 Mm3 value suggested by Bellier et al. (1997) and lower than 995 Mm3 value recommended by McCartney et al. (2009). The mean annual energy production is 2,356 and 1,278 GWh in Tana-Beles and Dangura power plant respectively. Damming of water improve irrigation product by 1% and add 1,278.1 GWh annual power to the national grid. Mean annual water of 2,043 Mm3 is lost due to artificial storage in lower Beles. However, this artificial reservoir can be the site for tourism, fishing, and navigation. The available water in the basin is about 5,226 Mm3 annual. To cover all irrigation area, minimum of 2,806 Mm3 mean annual water is needed. But, this study indicated about 36.1 % of upper Beles and 100 % lower Beles irrigation area can be covered with the available water. Overall 71.2 % of the total irrigation area (11,312 ha) in Beles basin can get enough water.
Thus, the study concluded to prioritize lower Beles irrigation area with damming of water on Dangura site in order to get optimum productions of both combination, and release seasonal variable water to downstream. Upgrading Chara-Chara weir max regulation level by 0.2 m is essential for extra storage and production. The study suggest the use of night storage, highly efficient irrigation system, weir in the river, the use of special boat design, change water rout, and dredging shallow lake to maximize productions and overcome navigation problem. The study recommends a dam to be constructed at Dangura site.