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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002112

Hydrosphere's Colorado River Model

December 29, 1993
Page 19

Flood control operations at Lake Mead are of two types, depending on the season.
From January through July, releases from Mead are made at a rate such that a minimum vacant
space is available in Mead. The release considers the anticipated releases at Lake Powell
flowing into Mead, plus the maximum forecasted reach inflow between Powell and Mead for
the current month through July. Anticipated releases are estimated based in part on available
space in Powell. Flood releases are set at one of six standard flow rates.

From August through September, releases are made sufficient to meet specific, monthly
space requirements in Lake Mead. These space requirements are calculated based in part on
creditable space in Powell, Flaming Gorge, Blue Mesa and Navaho reservoirs. There are no
restrictions (other than physical capacity) set on flood releases during this period.

Flood releases are distributed to CAP and the Metropolitan Water District of Southern
California (MWD). Each entity is given half of the flood release up to its capacity. Any
excess flood release is given to the entity which has leftover capacity. Flood releases in excess
of the combined capacity of CAP and MWD are given to Mexico.

Objective release

The terrn "objective minimum release" refers to the annual release of 8.23 maf from
Lake Powell to the Lower Basin according to the terms of the Operating Criteria. Although
the Operating Criteria are not an interpretation of the 1922 Colorado River Compact, the 8.23
maf objective minimum release is derived from the terms of the 1922 Compact and the
Mexican Treaty of 1944.

The Compact obligates the Upper Basin to deliver to the Lower Basin a sum of 75 maf
over every ten-year period. Thus, a release of 7.5 maf in every year would meet the terms of
the compact. The Mexican Treaty requires the U.S. to deliver 1.5 mafto Mexico each year.
If half of this obligation is to be met by the Upper Basin, the total average annual release
required to meet the terms of the Compact and Treaty would be 8.25 maf. The Paria River
delivers an average of 20,000 af at the compact point below Lake Powell, so the average
annual release can be reduced to 8.23 maf.

Releases in excess of the objective minimum release can be made to equalize Lakes
Powell and Mead or to deliver surplus to the Lower Basin.

Inflow forecasting

JnfIow forecasts are required for equalization and flood control. Hydrosphere's
Colorado River Model, like CRSM, does not make forecasts from primary inforrnation such as
snowpack water content and soil moisture. Forecasts are simulated from known future
inflows. Although CRSM; has the capahility to applying an error terrn, which includes a
random component, to the known future inflows, this approach is not norrnally used.
Hydrosphere's Colorado River Model currently does not use an error terrn so inflow forecasts
are based on perfect knowledge of future flows.

Calculation of the 602(a) storage criterion

The 602(a) storage criterion, according to section 602(a) of the Colorado River Basin
Project Act (Public Law 90-537), is that quantity of storage necessary to insure that the Upper
Basin can meet its future deliveries to the Lower Basin without impairing Upper Basin
consumptive uses.

Hydrosphere Resource Consultants 1002 Walnut Suite 200 Boulder, Colorado 80302