WSP10225

• File Number: 8283.100
• Description: Colorado River Computer Models - Colorado River Simulation System
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 6/1/1985
• Author: John Burke
• Title: CRSS Vs CRSP - Analysis of Data and Results from Two CAP Water Supply Studies - January 1981-CRSP and December 1984-CRSS
• Water Supply Pro - Doc Type: Report/Study

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Parker Dam included a component representing historical average water loss
between Parker and Imperial Dams, and gains between Imperial Dam and the
Mexican boundary. Water losses and gains were computed by an accounting
procedure similar to that used in developing the CRSS Lower Basin hydrology
data base. The CRSP model's reach losses and gains below Parker Dam were
based on records from the years 1963 to 1978.

CRSP, in addition to its reach loss or gain, assigned an
additional depletion of 200,000 af downstream from Parker Dam as an
estimated credit for unmeasured return flows. This was done to reconcile
Supreme Court Decree accounting estimates of consumptive use from past
years (which were used in the loss/gain accounting procedure) with the
future estimates of consumptive use employed by the CRSP model. In
general, the Supreme Court Decree accounting does not include unmeasured
return flow in its estimates of consumptive use.

In CRSS, a hydrology data base for the Lower Basin reaches of the
river is employed. One component of this data base for each reach from
Lees Ferry to Imperial Dam is reach loss or gains. This figure is
comparable to the loss or gain used in the CRSP model. It represents
unexplained, minor, or miscellaneous components of water loss or gain
between two stations on the river, and is derived from an accounting
procedure that also employs Supreme Court Decree accounting estimates of
consumptive use. Unmeasured return flow is a component of reach loss and
gain in the CRSS hydrology data, just as it was in CRSP. In CRSS, no
adjustment is wade to account for unmeasured return flow.

This difference in the treatment of unmeasured return flow
probably accounts for a portion of the large difference that exists in the
two models' average inflow between Parker and Imperial Dams. The
January 1981 CRSP study assigned 130,000 af of operational loss per year in
this reach to offset the credit for unmeasured return flow. Below
Imperial Dam, another 70,000 af of operational loss was assigned to offset
unmeasured return flow credit.

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Part of the difference in Parker to Imperial inflow can also be
attributed to the consideration of phreatophyte consumptive use in the
development of the CRSS hydrology data.

Because of changes in land use over the years from 1935 to the
present, there has been a downward trend in phreatophyte consumptive use. ;y, ,P'
A substantial water savings has occurred, and this is reflected in the
estimate of inflow to the Parker-Imperial reach in CRSS.

In the future, a slight additional reduction in phreatophyte
consumptive use is projected in the CRSS. The average annual water savings
over the comparison period 1985-2040 is about 262,000 af for the Parker to
the Imperial reach. This figure is derived by subtracting the average
future phreatophyte consumptive use (170,000 af) over the comparison period
of 1985-2040 from the average past phreatophyte consumptive use
(432,000 af), as cited in the Data Base Refinement of October 1982 for the
years 1935 to 1978.

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