Firm Yield Model For Reservoirs

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Firm Yield Model For Reservoirs November 2008
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Oklahoma-Texas Area Projects

• 11 reservoirs with a total capacity
• of 4.2 million acre-feet.
• Total water rights from projects
• MI - 571,000 acre-ft/yr to about 2.7 million
  customers.
• Irrigation - 110,600 acre-ft/yr for about 63,000
  acres.
• Annual recreation use of over 8 million
  visitor-days each year.

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Reservoir Firm Yield

• The amount of water that can be withdrawn from a
  reservoir each year including the years of the
  most critical drought of record.
• The firm yield is affected by the level of
  streamflow depletion in the basin upstream of the
  reservoir and the amount of water that must be
  bypassed through the reservoir to satisfy
  downstream requirements. Any changes to these
  conditions will change the firm yield.

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Withdrawal of Unappropriated Waters

• Project sponsors identify a water need and
  Congress authorizes planning studies
• United States/Reclamation makes withdrawal
  request to the OWRB for all unappropriated
  waters in the basin under Title 82, Chapter 1,
  Section 105.29
• Withdrawal ensures water will be available for
  diversion, storage, and delivery if reservoir
  is constructed
• Withdrawal also provides a basis (i.e., all
  unappropriated waters) to calculate the projects
  firm yield
• Reclamation completes studies and Congress
  authorizes the project
• Dam and conveyance facilities are constructed,
  and sponsor operates project and relies on
  water availability to meet repayment obligation
  to the United States

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Determination of Firm Yield

• Calculated using an operational model
• Water balance on a monthly time step
• Based on historic inflow and evaporation data
• Initial Vol. Total In Total Out Final Vol.
• 100 years of sediment

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Surface Water Permits for Reclamation Projects in
Oklahoma

• OWRB issues a surface water permit to the project
  sponsor equal to the firm yield of the reservoir
• All withdrawn water is required to produce that
  yield
• Issuance of additional upstream surface water
  permits will reduce the firm yield and impact the
  project sponsors ability to meet contractual
  water delivery requirements during periods of
  extended drought
• Storage quantity required to deliver the project
  firm yield far exceeds the firm yield itself due
  to evaporation losses and other variables

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Storage and Yield Comparisons
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