Although in these systems the risk of highly variable inflows is partially internalized in the design process by letting these reservoirs have large storage capacities, operation of these systems is challenged by changing demand levels, changing purposes and objectives, and climate change. Given their dimensions, these systems are often managed in a centralized fashion, with one authority planning allocations to different users. As inadequate management may operate the system either too conservatively or in a fashion unaware of the occurrence of long, multi-year dry spells, the decision-making process should be supported by appropriate decision support tools. When used on a regular basis, in a real-time mode, by updating current information on water resources status and using inflow forecasts, such support tools should be able to identify risky situations suitably in advance, suggest appropriate hedging policies and optimize the use of additional, costly, water resources that can help mitigate the impacts of sustained dry periods. One of the key factors for a successful management is obviously the ability to predict future inflows into the system suitably in advance. Presently, the state of the art of climate services is provided by seasonal climate forecasts over a six-month horizon, coupled with downscaling models to turn climate input into streamflow. Forecasts for further-reaching time periods are an active field of research, but are not currently used by industry. However, also downscaled seasonal forecasts are out of reach for most water agencies and utilities, as they imply the availability of ad-hoc skills, resources and facilities to acquire climate forecasts and turn them into inflow forecasts. It hence makes sense to develop simpler, data-driven forecast systems to be integrated into the decision support tools to improve the quality of decisions, while research progresses, and resources and skills for implementing hydrological forecast are being developed To this end, in this paper we introduce a multi-scenario mathematical programming (algebraic) model for real time management of a multi-reservoir system for water supply where consideration of a multi-year Forecasting Horizon (FH) is necessary given the physical features of the system. It is a linearized MIP (mixed integer programming) model that optimizes water allocations to municipal and irrigation demand centres driven by the objective of minimizing weighted total discounted costs (scarcity costs plus costs of additional water resources) along the multi-year FH, being the weights the occurrence probability of each of the three inflow scenarios. Constraints include 1) mass balances at system's nodes, 2) systems' topology, 3) component's capacity, 4) spills, 5) non-empty conditions on reservoir storage at the end of the FH. It is a multi-scenario optimization tool because future, uncertain inflows are modelled, until the end of the current water year, as three different inflow scenarios: low flows, normal flows and high flows. The optimization model...