Maximizing Reservoir Yields for a Dual Supply Utility

Fairfax County Water Authority (FCWA), serving the Virginia suburbs of Washington, D.C., draws water from both the Potomac River and the Occoquan Reservoir. Deciding how much water to treat from the river versus the reservoirs is a multi-dimensional problem including variables such as historic reservoir inflows, time of year, historic and current Potomac flows, a regional operating agreement, current regional demands, and current reservoir storage. The challenge was to develop graphical tools that could easily and quickly be used by planners, water managers, and operators in both drought and normal years to maximize the reliability and use of the reservoir resource. The resulting graphical tools exploit the flexibility allowed by dual-supply systems, allowing personnel to choose an appropriate reservoir release rate given different reservoir storage levels and time of year. The curves helped guide release decisions during the drought of 1999, allowing managers to maximize reservoir water supply withdrawals while ensuring adequate reliability of reservoir refill and preserving emergency storage at appropriate reliabilities. Managers understood that the "cost" of fully utilizing the reservoirs during the drought was to incur a 1% chance that withdrawals would have to be reduced during the winter, when the free flowing Potomac is able to more than meet demands.