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<br />
<br />space-building or drawdown season, which extends from August through
<br />December, the objective is to drawdown gradually the reservoir
<br />system to create space for next spring's snowmelt runoff. During
<br />the runoff forecast season, from January through July, the
<br />forecasted maximum inflow is routed through the reservoir using
<br />predetermined release rates, so that the reservoir system is full by
<br />July 1. .
<br />
<br />During the space-building season, the flood control regulations
<br />specify minimum monthly vacant storage requirements for Lake Mead.
<br />These requirements may be partially satisfied by considering
<br />available storage space in some of the other upstream reservoirs.
<br />The vacant storage space requirements in Lake Mead are as follows:
<br />
<br />August 1
<br />September 1
<br />October 1
<br />November 1
<br />December 1
<br />January 1
<br />
<br />1,500,000 acre-feet
<br />2,270,000 acre-feet
<br />3,040,000 acre-feet
<br />3,810,000 acre-feet
<br />4,580,000 acre-feet
<br />5,350,000 acre-feet
<br />
<br />The regulations allow for Lake Powell, Flaming Gorge plus
<br />Fontenelle, Blue Mesa, and Navajo Reservoirs to provide a portion of
<br />these requirements. The maximum amount of space creditable to Lake
<br />Mead at each of these storage locations is 3,850,000: 1,507,200:
<br />748,500: and 1,035,900 acre-feet, respectively. The system is
<br />operated to limit releases at Hoover Dam during the drawdown season
<br />to a maximum of 28,000 ft3/s unless modifications are approved by
<br />the Corps of Engineers to increase the release.
<br />
<br />From January 1 to July 31, during the runoff forecast season, the
<br />minimum Lake Mead flood control release requirements are determined
<br />from the maximum forecasted inflow volume and the effective storage
<br />space in Lake Mead and Lake Powell. The required Hoover Dam flood
<br />control release during the current month (R=) in ft3/s can be
<br />determined from solution of the following algorithm:
<br />
<br />FI=SSM+SSP+BS+EV-l,500,OOO+59.5(R~+Rr~rm)
<br />
<br />where, FI is the forecasted inflow volume to Lake Mead in acre-feet
<br />during the current month: SSM is the current storage space in Lake
<br />Mead in acre-feet below elevation 1229 feet: SSP is the current
<br />storage space in Lake Powell in acre-feet below elevation 3700 feet:
<br />the bank stora~e (BS) is equal to 0.065(SSM - 1,500,000): EV is the
<br />net evaporation loss to Lake Mead in acre-feet through August 1; Rrm
<br />is the Hoover Dam release rate in ft3/s during all remaining months
<br />through August 1; and N= is the number ,of remaining months until
<br />August 1, excluding the current month.
<br />
<br />Lake Mead inflow forecasts are prepared by the Colorado River
<br />Forecasting Service and are adjusted to account for effective
<br />storage space in upstream reservoirs, flow depletions, and potential
<br />forecast errors. Flood control releases below Hoover are made
<br />for five discharge levels - 19,000, 28,000, 35,000, 40,000, and
<br />73,000 ft3/s. The algorithm is solved iteratively to determine
<br />the release from Hoover Dam.
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