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Headwater Benefits

Energy Gains Method
Flow Duration Curves The controlled retention and release of water by a headwater storage reservoir alters the downstream river flow. The impact of storage regulation on the flow available for energy production can be illustrated by flow duration curves that show the percent of time the river flow is equaled or exceeded during a given period of record.

The unregulated flow curve illustrates the large percentage of high and low flows exhibited by an unregulated river. For example, with seasonal fluctuations there may be:

  • high flows during spring runoff and heavy rains; and
  • low flows during the drier summer months.

The regulated flow curve illustrates the effects of regulation of the same river provided by headwater storage projects where:

  • high flows are reduced by storing water; and
  • low flows are augmented by later release of this stored water.

When this regulation of flows increases the percentage of time the river flow falls within the range which a downstream hydropower plant can generate energy, i.e., maximum and minimum turbine range, the plant can generate more energy than without regulation by the headwater storage project. The difference between the kilowatt-hours of energy produced at a downstream hydropower plant with and without the benefit of storage regulation is called ENERGY GAINS, and is represented by the shaded area between the two curves.

The Commission uses various analytical methods to determine energy gains. The method chosen depends upon the complexity of the river basin and the potential headwater benefits. Complex river basins that include a large number of headwater and downstream projects often require a detailed computer analysis using the Commission's HWBEG computer model, which requires large amounts of input data. River basins that are not complex, or in which the headwater benefits are expected to be small, may be studied using a flow duration methodology to determine energy gains.

Updated: August 7, 2012