Impact on Sudan and Egypt[edit]
The precise impact of the dam on the downstream countries is not known. Egypt fears a temporary reduction of water availability due to the filling of the dam and a permanent reduction because of evaporation from the reservoir. The reservoir volume is about equivalent to the annual flow of the Nile at the Sudanese-Egyptian border (65.5 billion cubic meter). This loss to downstream countries would most likely be spread over several years. Reportedly during the filling of the reservoir 11 to 19bn cubic meters of water per year could be lost, which would cause two million farmers to lose their income during the period of filling the reservoir. Allegedly, it would also "affect Egypt's electricity supply by 25 to 40 percent, while the dam is being built".[34] However, hydropower accounts for less than 12 percent of total electricity production in Egypt in 2010 (14 out of 121 billion kWh),[35] so that a temporary reduction of 25 percent in hydropower production translates into an overall temporary reduction in Egyptian electricity production of less than 3 percent. The Grand Ethiopian Renaissance Dam could also lead to a permanent lowering of the water level in Lake Nasser, if floods are stored instead in Ethiopia. This would reduce the current evaporation of more than 10 billion cubic meter per year, but it would also reduce the ability of the Aswan High Dam to produce hydropower to the tune of a 100 MW loss of generating capacity for a 3 m reduction of the water level.
The dam will retain silt. It will thus increase the useful lifetime of dams in Sudan – such as the Roseires Dam, the Sennar Dam and the Merowe Dam – and of the Aswan High Dam in Egypt. The beneficial and harmful effects of flood control would affect the Sudanese portion of the Blue Nile, just as it would affect the Ethiopian part of the Blue Nile valley downstream of the dam.[36]Specifically, the GERD would reduce seasonal flooding of the plains surrounding the reservoir of the Roseires Dam located at Ad-Damazin, just as the Tekeze Dam, by retaining a reservoir in the deep gorges of the northern Ethiopian Highlands, had reduced flooding at Sudan's Khashm el-Girba Dam.
The reservoir, located in the temperate Ethiopian Highlands and up to 200 m deep, will experience considerably less evaporation than downstream reservoirs such as Lake Nasser in Egypt, which loses 12% of its water flow due to evaporation as the water sits in the lake for 10 months. Through the controlled release of water from the reservoir to downstream, this could facilitate anincrease of up to 5% in Egypt's water supply, and presumably that of Sudan as well.[37]