| description abstract | Ethiopia has begun seriously developing their significant hydropower potential by launching construction of the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile River to facilitate local and regional growth. The GERD, located just upstream of the border with Sudan, is the first dam ever to be constructed directly on the main stem of the Blue Nile and will become the largest dam in Africa. Although this has required substantial planning on Ethiopia’s part, no policy dictating the reservoir filling rate strategy has been publicly issued. This filling stage will have clear implications on downstream flows in Sudan and Egypt, complicated by evaporative losses, climate variability, and climate change. In this study, various filling policies and future climate states are simultaneously explored through a linked set of models (rainfall-runoff, routing, and hydropower) to infer potential streamflow reductions near Sudan’s Gezira Scheme, one of the largest irrigated areas in the world, and at Lake Nasser behind Egypt’s High Aswan Dam. Simulations across 2011–2060 are evaluated at a monthly time step. Given intermediary tributaries and hydroclimatic variability, a clear nonlinear relationship exists between GERD filling rates and streamflow at key locations in Sudan and Egypt; this study quantifies these differences. For example, impounding 10% (25%) of monthly streamflow behind the GERD produces a 6% (14%) average reduction in streamflow entering Lake Nasser during the first 5 years; changes at the Gezira Scheme are even larger owing to fewer contributions from other tributary flows in between the GERD and the Gezira Scheme. Climate changes may shift this notably. A filling policy contingent on the GERD impounding water only if streamflow exceeds the long-term historical average produces a 7% reduction at Lake Nasser over the same 5 years, with a larger expected variance. The goal of this work, however, is not to prescribe a particular filling policy; rather it is to provide regional decision makers with a set of plausible, justifiable, and comparable outcomes for negotiation and consensus. | |
