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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002100

Hydrosphere's Colorado River Model

December 29, 1993
Page 7

INFLOW HYDROLOGY

Several sets of inflow hydrology have been developed for use with Hydrosphere's
Colorado River Model. These are 1) the historical 1906-1983 trace, 2) two synthetic stochastic
traces with mean annual flows at Lees Ferry of 15.0 and 13.8 maf, 3) a reconstructed pre-
historic streamflow trace, 4) two changed climate traces representing warming of 2 degrees and
4 degrees Celsius,S) a trace developed for analysis of a severe drought and 6) multiple
indexed-sequential traces. A brief description of each inflow set is given below. The model
will also accommodate inflow sets which have been developed by our customers.

Historical Streamflow Hydrology

Historical natural flows for 29 inflow points in the Colorado River Basin were obtained
from the CRSM inflow data file. Some of the inflows represent major tributaries, like the
Green and San Juan Rivers, and some of them represent gains along major tributaries or along
the mainstem. The inflows span the years 1906 through 1983, a period of 78 years. Most of
the Upper Basin inflows are natural, or virgin, flows. The Lower Basin inflows which
represent tributaries, like the Bill Williams, are actual, gaged flows. The Lower Basin inflows
which represent gains are natural flows calculated by backing out upstream operations.

Many of the Lower Basin inflows do not have complete records. To fill in missing data
the USBR has chosen to use average monthly flows for those years, Average flows make up
the first 25 or 30 years of some of the Lower Basin inflows. Throughout the Basin, there are
frequent negative inflows which represent stream losses.

The 1906 to 1983 hydrologic trace includes the high streamflow years at the beginning
of the century. The mean annual flow at Lee Ferry for the 78 years is 15 maf. However, it is
generally agreed that the mean of these 78 years, indicated from tree ring analyses, is higher
than the true long-term mean (Stockton and Jacoby, 1976). The tree ring analyses indicate that
the long-term average annual flow on the Colorado River at Lee Ferry is about 13.5 maf. The
1906 through 1930 streamflow trace has been shown to be "... the longest period of
predominantly high-flow years in the entire 450 years of reconstructed record..." (Stockton
and Jacoby, 1976),

The use of only historic traces does not allow for the testing of designs and policies
against a range of sequences that are likely to occur in the future. By testing designs and
policies against a range of sequences that could occur, the variability and range of possible
future performance is better understood and better system designs and policies can be selected.
This is particularly true for systems having large amounts of over-year storage, such as the
Colorado River basin (Loucks et al.' 1981).

Because of the limitations associated with using historical streamflow traces, some water
agencies have used an empirical procedure, the indexed sequential method (see discussion
below), for generating streamflows. The indexed sequential method uses the historical flow
sequence to generate new sequences (Frevert and Cheney, 1988),

Hydrosphere Resource Consultants 1002 Walnut Suite 200 Boulder, Colorado 80302