The Juba River Basin, a vital water resource for Somalia, faces increasing water shortages due to population growth, urbanization, and industrialization. Despite its importance, research on water allocation modeling under future development scenarios remains limited. This study utilizes the Water Evaluation and Planning (WEAP) system, incorporating the Soil Moisture Method, to model hydrological processes, sectoral demands, and allocation strategies under evolving socio-economic and climatic conditions. The model demonstrated satisfactory performance during calibration and validation phases, achieving a Nash-Sutcliffe Efficiency (NSE) of 0.64 and a coefficient of determination (R2) of 0.87. Simulation results reveal a concerning rise in unmet water demands. By 2030, domestic demand (430.22 Mm3) will experience a 5.79% shortfall, while livestock and irrigation sectors will face deficits of 12.7% and up to 1.32%, respectively, depending on irrigation efficiency. By 2050, unmet domestic demand will increase to 24.1%, with continued stress on other sectors. Scenario analysis demonstrates that enhancing irrigation efficiency (e.g., 60%) substantially mitigates these deficits. The study highlights the need for integrated water resource management strategies, including promoting efficient irrigation technologies, optimized deficit irrigation, evaporation control, population growth regulation, and sustainable land use planning. These insights provide a robust decision-support framework to guide policy interventions to ensure water security and climate resilience in arid and semi-arid river basins.Graphical AbstractThis study evaluates water demand, supply, and unmet needs in Somalia's Juba River Basin under reference (2002-2022), short-term (2022-2030), and long-term (2030-2050) development scenarios using the WEAP model. Results show increasing water demand, with unmet needs driven mainly by supply limitations and low irrigation efficiency. In the short term, irrigation dominates demand, and improving efficiency from 45 to 60% significantly reduces shortages. By 2050, total demand will nearly double, with rising deficits in the domestic and irrigation sectors. The Lug-Dolow sub-catchment faces the highest stress across all periods. The graphical abstract illustrates spatial and temporal demand trends, unmet needs, and the benefits of improved irrigation practices for enhancing water security.