8. Accounting of Inflow and Outflow From Reservoir Storage

When gain or loss in storage of a reservoir and the inflow to a reservoir are known, the outlet discharge may be computed. Conversely, when the storage, gain or loss, and the reservoir outlet discharge are known, the inlet flow may be computed.

In each of these computations, the gain or loss in storage during a given time period may be read from reservoir capacity charts and tables. These charts and tables generally give the reservoir volume in acre-feet for various gage heights of water. The change in reservoir volume for the time period is converted to cubic feet per second (ft3/s). The change of reservoir discharge increased by the inflow or decreased by the outflow gives the average discharge or inflow, respectively.

Bank storage causes indeterminate deviation. Storage will tend to cause a slow drop in reservoir water surface when the net rate of outlet flow is low and will retard rise in water surface during a slow increase in storage. These changes usually are imperceptible to an observer. Adjustments for evaporation and wind effect on gage readings may be necessary in reservoirs of large areas.