Reservoirs represent a small component of the global water balance but play a significant role in modifying the natural water cycle to meet human demands. The increasing connections of Energy-Water-Land (EWL) systems and climate change at the global scale pose challenges for reservoirs to reliably meet water demands and attenuate droughts. In this study, we implement a representation of reservoir water storage in the Global Change Analysis Model (GCAM) to explore the future role (e.g., expansion) of reservoir water storage globally in meeting demands and evolving in response to interactions with the energy and land systems. GCAM represents 235 global water basins, operates at 5-year time steps, and uses supply curves to capture economic competition of supplying water through reservoirs, groundwater, and desalination. Our approach consists of developing a generic Linear Programming (LP) model that uses the concept of virtual reservoirs to lump distributed reservoir storage in each basin into a single storage unit. The LP model develops a capacity-yield curve for each basin’s virtual reservoir that maximizes annual water yield from each basin’s virtual reservoir based on evolving intra-annual hydrologic inflow and demand patterns and dynamically updates GCAM in the form of supply curves in each model period. The updated supply curve, which specifies the unit cost to supply increasing quantities of water, enables GCAM to capture the evolving levelized regional costs to supply water from reservoirs, forecast reservoir storage capacity expansion potential, and evaluate the impact of increased storage capacity on multisectoral feedbacks through an economic lens. This modeling framework allows us to explore diverse water storage questions previously unexplored by GCAM (and similar tools), such as the implications of climate impacts on reservoir water supply, and how land use change might alter reservoir expansion pathways. Overall, this work investigates what effects future expansion of reservoir storage capacity could have on the evolution of global EWL systems through the development of increasingly robust projection tools.