WSPC04476

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-RAY
• State: CO
• Water Division: 5
• Date: 12/29/1993
• Title: Hydrosphere-Colorado River Model-Technical Overview
• Water Supply Pro - Doc Type: Report/Study

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Hydrosphere's Colorado River Model

December 29, 1993
Page 20

As it is currently implemented in CRSM and Hydrosphere's Colorado River Model, the
602(a) storage level is the quantity of active storage above minimum power pool in Lake
Powell which would guarantee n years' (n equals 12 in the baseline configuration) of Upper
Basin depletions plus n years' of compact deliveries to the Lower Basin if the next n years'
inflow were as low as the driest n years on record.

Equalization

Equalization of storage between Lakes Powell and Mead is provided for in the Operating
Criteria. It calls for releases from Lake Powell into Lake Mead to equalize the September 30
contents of the two reservoirs. Equalization releases are required if: (I) the forecasted end-Qf-
water-year (EOWY) contents in Powell are greater than the Mead forecasted EOWY contents;
(2) the Upper Basin contents are greater than the 602(a) storage criterion of that date; and (3)
the Mead forecasted EOWY vacant space satisfies flood control requirements.

Equalization releases from Powell are distributed evenly over the remaining months of
the water year to bring about the equalization of Powell and Mead storage by September 30.
Equalization releases only affect Lower Basin supplies to the extent that Mead elevations are
kept above shortage or surplus levels.

Declaration and quantification of shortage

In January, if the volume of Lake Mead is below a specified shortage elevation a
shortage is declared and CAP is cut back to a minimum annual delivery, also referred to as the
CAP "shortage" delivery. There are no intermediate shortage levels for CAP. Either it
receives the full delivery, the scheduled delivery, or it receives the minimum delivery.

Thus, CAP cannot exercise its scheduled delivery against Lake Mead when the storage
in that reservoir is below the shortage elevation. This subordination of low priority CAP
requests reflects one interpretation of the 1963 Supreme Court decision in Arizona vs.
California and the Colorado River Basin Project Act (Schuster, 1988, p. 73). The shortage
elevation and shortage delivery used by the USBR and included as the standard configuration
of the Colorado River Model have been set empirically so that subsequent operations will be
unlikely to draw Lake Mead below the nominal minimum power pool elevation (1083 feet).
This not only protects hydropower production, but also reduces the chance of future shortfalls
to consumptive uses of higher priority (e.g., California and other mainstem uses). The
experiments that led to the adoption of the currently-used shortage elevation were undertaken
using the historic period as a basis, thus the shortage elevation may require adjustment for
different hydrological conditions. Alternative assumptions regarding shortage elevation and
shortage deliveries can be accommodated by Hydrosphere's Colorado River Model.

Shortages equal to 4% of the CAP shortage are also imposed on Nevada. No other
shortages are imposed on the Lower Basin unless: (I) there are local shortages (these are
sometimes observed on the Virgin River); or (2) Lake Mead empties. If Lake Mead empties,
shortages fall first on CAP and then on MWD and shortages are imposed on Mexico in
proportion to the total, predicted U.S. shortage.

Declaration and quantification of surplus

The surplus strategy is the intended release of surplus water in scheduled amounts to
increase the benefits of water use above those that would be realized with unscheduled flood
control releases. Under the standard assumption a surplus is declared in October if the

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