<p>Water demand is rising globally due to population growth, rapid urbanization, climate change, and the need for economic development. The need for additional water storage is therefore increasing to meet these demands. Water storage, both natural and built infrastructure, offers a way to manage the availability of water resources. Properly planning and optimizing diverse water storage options for increasing water demand requires an assessment framework. We developed a framework for assessing water storage gaps under current and future scenarios. It is applied in an African setting in the Tana-Beles sub-basin of the Blue Nile basin, one of Ethiopia’s economic growth corridors, where irrigation and hydropower development are planned. The volume of usable water storage in Lake Tana, groundwater, and built reservoirs was estimated and compared with irrigation, energy, domestic, livestock, and industrial water demands for the current and future periods. Results showed that the current annual water storage gap is 613 MCM. The storage gap increases to 3663 MCM by 2040s, about five times higher than in the current period. Besides, spatial disparity exists between where most water storage is available and where the water demand occurs. Particularly, demands for irrigation, domestic use, and livestock are spread throughout the sub-basin while there is a strong reliance on surface water sources, which are located in the upstream part of the basin. Given that these sources are subject to spatial and temporal limitations, diversifying natural and built water storage sources is recommended to address the increasing gap between supply and demand.</p>