<p>Floods and droughts resulting from extreme hydrological events are among the most devastating natural hazards. However, reservoir operations, can significantly alleviate flood and drought effects induced by extreme events. In this study, we quantified the flood and drought regulation capacity of the Grand Ethiopian Renaissance Dam (GERD), the largest dam in Africa. We applied a model parameter confinement strategy from 20,000 Monte Carlo parameter samples. Accordingly, we found 87 reduced parameters sets that are obeying the calibration criteria based on KGE, and relative error. By using these reduced parameter sets alongside climate projections, we simulated future inflows to the dam. Based on these simulations, reservoir operation rules were applied to model outflows through both turbines and the spillway. We conducted analyses under two scenarios: i) without GERD and ii) with GERD operation, to assess the dam’s flood and drought regulation capacity both in historical and future periods. We identified flood characteristics at various recurrence intervals (RI). Additionally, we applied a Standardized Streamflow Index (SSI) to determine hydrological drought and its characteristics. The findings indicate that GERD operation has a substantial regulatory effect on both flood and drought conditions. In the far-future period, the largest 2-year RI flood volume of 12.86 bcm is regulated by 72%. Across all periods, peak flood magnitudes are consistently reduced, with a maximum reduction of 4,572.3 m³/s. Furthermore, GERD’s operation yields a significant reduction in peak drought, achieving a maximum reduction of 40.35% in the mid-future period. Among various drought characteristics, drought intensity is notably reduced by GERD operation, with a maximum reduction of 14.21% in the near future. This study demonstrates the potential of reservoirs for flood and drought regulation as climate change adaptation tools, benefiting water-sharing countries in transboundary settings by strengthening resilience to climate change.</p> Graphical Abstract <p>As illustrated on the graphical abstract, in this study we investigated the flood and drought regulation capacity of the Grand Ethiopian Renaissance Dam (GERD), Africa’s largest dam under climate change. The whole methodological framework includes three main parts: i) the hydrological modeling part, which illustrates the hydrological model, forcing and model parameter confinement for streamflow simulations; ii) the reservoir operation part, which shows the reservoir operation under the actual operation rules and iii) the flood and drought index calculation for the natural flow and regulated flow from the dam in the historical to future period due to climate change. This study aimed to investigate GERD’s capacity for flood and drought regulation by comparing two flow scenarios: natural flow (without GERD) and regulated outflow from the reservoir (with GERD operation). Using the calibrated HBV hydrological model and bias-corrected climate models, inflows to the GERD reservoir was simulated in both historical and future periods. Outflows from the dam, were determined based on actual reservoir operation rules. Additionally, flood and hydrological drought events were identified from both natural flow and regulated outflow to assess GERD’s regulation capacity under climate change. The findings indicate that GERD operation has a substantial regulatory effect on both flood and drought conditions. In the far-future period, the largest 2-year RI flood volume of 12.86 bcm is regulated by 72%. Furthermore, GERD’s operation yields a significant reduction in peak drought, achieving a maximum reduction of 40.35% in the mid-future period. This study demonstrates the potential of reservoirs for flood and drought regulation as climate change adaptation tools, benefiting water-sharing countries and enhancing cooperation in transboundary settings.</p>