Conjunctive Use Model S-1

• Description: Conjunctive Use Model S-1
• State: CO
• Title: Conjunctive Use Model S-1
• Water Supply Pro - Doc Type: Report/Study

There was discussion at the January 21 technical meeting as to what should be used for
the reduced objective minimum (ROM) release from Lake Powell. There were
suggestions of 7.48 maf and 6.5 maf. For the first set of S -1 model runs, 7.48 maf was
used as the ROM release.
The water supply trigger (WST) elevation overrides the continuation of the ROM release
from Lake Powell. If the water surface elevation of Lake Powell is below the LPT,
releases from Lake Powell are reduced to the ROM release only so long as Lake Mead
remains above the WST elevation. For the first model runs of S -1 elevation 1,050 feet at
Lake Mead was used as the water supply trigger (WST) elevation. When Lake Mead
drops below 1,050 feet, releases from Lake Powell return to the 8.23 maf (unless releases
need to be higher than 8.23 maf for equalization), even if the elevation of Lake Powell is
below the LPT elevation.
In the S -1 model, the LPT elevation is evaluated against the projected September 30 (end
of the water year) water surface elevation at Lake Powell. Releases are scheduled to be
the ROM release when Lake Powell is projected to end the year below the LPT elevation
(so long as Lake Mead is projected to remain above the WST elevation). The WST is
also evaluated against the projected September 30 water surface elevation at Lake Mead.
The S -1 model imposes a temporary equalization level (TEL) at Lake Powell once
releases have been reduced to the ROM release. The TEL provides a method for the
Lower Basin to recoup water without implementing a specific accounting scheme. The
TEL is an elevation at Lake Powell that is greater than the LPT elevation, but lower than
the 602(a) Storage level.
The S -1 model allows releases from Lake Powell to be reduced to the ROM release when
Lake Powell is below the LPT elevation, so long as Lake Mead is above the WST
elevation, but then as Lake Powell recovers, water is shared with Lake Mead at a level
less than the 602(a) Storage level. If there is at least one year where the ROM release
have been implemented in the model, equalization releases subsequently take place at the
TEL. The TEL is then used until active storage in the Upper Basin exceeds 602(a)
Storage on September 30, and Lakes Powell and Mead are equalized under "normal
operations." Once the 602(a) Storage level has been reached on September 30 and full
equalization has been achieved, the TEL is inactivated. Achieving equalization of
storage between Lakes Powell and Mead at the 602(a) Storage level "wipes the slate
clean." Only though a subsequent period of low inflow, with Lake Powell again
dropping below the LPT elevation (with the implementation of the ROM release), does
the TEL again become active. Under no circumstance are equalization releases made in
the S -1 model when active storage in Lake Powell is less than active storage in Lake
Mead on September 30.
The method to determine the elevation of the TEL was an iterative approach. After the
initial version of the S -1 model was developed, the TEL was adjusted iteratively until a
TEL was found where the sum of all water released from Lake Powell for all traces in the
S -1 model is equal to the sum of all water released from Lake Powell for all traces under