Safe yield

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The Virginia State Water Control Board's definition of safe yield for a complex water supply intake (reservoirs in conjunction with streams) was adopted in the January 2004 Gannett Fleming Safe Yield Study, which states: "The safe yield is defined as the maximum withdrawal rate available to withstand the worst drought of record in Virginia since 1930. If actual gauge records are not available, correlation is to be made with a similar watershed and numbers synthesized in order to develop the report."[1]

The Rivanna Water and Sewer Authority has defined safe yield as follows: "the maximum amount of water that can be withdrawn from a reservoir or a river water source over the duration of a drought. It is determined by what comes in (stream flow and precipitation), what goes out (evaporation, seepage, downstream releases, etc.) and the usable stored water within the reservoir system."

Safe yield of a reservoir can be enhanced by releases from upstream reservoirs in some cases. For example, the water supply plan has evaluated releases from Beaver Creek Reservoir in Crozet to supplement the South Fork Rivanna Reservoir.

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