Assessing Hydrological Impacts through Scenario-Based Modelling in the Bahr El Jebel Basin, South Sudan

The Bahr El Jebel Basin, one of the major Nile sub-basins, is a hydrologically complex system created by the extensive Sudd Wetland. It has high climatic variability, low river gradient, and high evapotranspiration. Water availability and flow stability remain at risk from continued increasing demands under climatic, land use, and population change and are leading to questions about long-term sustainability of basin operations. This research is based on a simulation scenario approach using the RIBASIM model to evaluate hydrological responses under a range of forecasting conditions. The assessment highlights the sensitivity of the basin and its implications for sustainable water management by focusing on the variations in the river discharge, wetland water balance and evapotranspiration losses. Results show that climate change is the dominant driver of hydrological variability in the Bahr El Jebel Basin. The Climate Change (+5%) scenario is the highest discharge scenario with the highest increases in the wet-season between July and October of around 25% of the discharge, indicating higher flood risk. Development scenarios involving hydropower expansion, agricultural growth, and population increase yield smaller net increases in discharge (1.5–5%), as water abstractions partially offset the effects of climate change. In contrast, the combined Development–Climate Decrease (−5%) scenario results in an annual discharge reduction of about 3.8%. The effects of climate change outweigh those of development, suggesting a highly sensitive response and enabling further growth. It has a relatively moderate hydrologic response in poor conditions, suggesting opportunities that can be achieved while maintaining environmental protection.