When a tandem or parallel reservoir system is defined, the model determines the priority and the amount of release to make from each reservoir in order to operate towards a storage balance among the system reservoirs. For every decision interval, an end-of-period storage is first estimated for each reservoir based on the sum of beginning-of-period storage and period average inflow volume, minus all potential outflow volumes. The estimated end-of-period storage for each reservoir is compared to a desired storage that is determined by using a system storage balance scheme. The priority for release is then given to the reservoir that is furthest above the desired storage. When a final release decision is made, the end-of-period storages are recomputed. Depending on other constraints or higher priority rules, system operation strives for a storage balance such that the reservoirs have either reached their Guide Curves or they are operating at the desired storage (percent of the active storage zone).
The storage balance across reservoir systems is based on the definition of System Storage Zones. These zones divide each system reservoir into comparable zones, across which to balance. There are two methods by which the desired storage balance is determined: implicit (default) and explicit (user-defined). The implicit method delineates the default storage balance scheme for the reservoir system. The explicit method is optional and allows a user defined storage balance scheme for the reservoir system. Detailed descriptions and examples are presented in "Implicit System Storage Balance Method" and "Explicit System Storage Balance Method" and demonstrating the functionality of implicit and explicit storage balance methods.