The first filling of the Grand Ethiopian Renaissance Dam (GERD): hydrological and operational insights

The filling of the Grand Ethiopian Renaissance Dam remains highly contentious despite reaching 95% of storage capacity by the end of hydrological year 2024–2025, without major downstream impacts. This study assesses the first filling (2020–2024) by comparing the planned and actual filling trajectories. In the absence of data exchange, river inflows were estimated using a hydrological model, reservoir filling was reconstructed from satellite observations, and impacts were evaluated using a River Basin Simulation model. Actual (total) hydropower generation (2020–2024) is 3,000 GWh, compared with 12,000 GWh under the planned scenario, mainly due to delayed turbine commissioning. Exceptionally wet years and mitigation measures, largely avoided downstream impacts. Hydropower generation at Roseires slightly increased, Merowe remained largely unchanged, and floodplain agriculture in Sudan declined by 4 to 50%. High inflows and mitigation measures maintained high levels at High Aswan Dam, and comparable hydropower generation. The integrated modelling framework demonstrates how transboundary reservoir operation can be reconstructed and evaluated using independent public-domain data where formal data exchange is lacking. Exceptionally wet years and effective mitigation measures largely buffer the downstream impacts of the Grand Ethiopian Renaissance Dam’s first filling, while unilateral implementation without data exchange exacerbates mistrust among riparian states. Publicly available data and an integrated modelling proved effective in transboundary water management.