S7_11-15-04

• Basin: Statewide
• Title: SWSI Phase 1 Report - Section 7 Availability of Existing Water Supplies
• Date: 11/15/2004
• Author: CWCB
• SWSI - Doc Type: Final Report

Section 7
Availability of Existing Water Supplies
7.3.2 Colorado Basin
7.3.2.1 Colorado Basin Surface Water Supplies
Six locations were selected for the Colorado basin to
characterize surface water supply availability using
StateMod datasets. The period of record for the Colorado
StateMod datasets is 1909 to 1995 (full calendar years).
The selected locations, shown in Figure 7-16 are:
^ Blue River below Green Mountain Reservoir
^ Roaring Fork River near Glenwood Springs
^ Colorado River near Kremmling
^ Colorado River near potsero
^ Colorado River near Debeque
^ Colorado River at the state line
Median annual legally available, physically available, and
naturalized flows are summarized for each location in
Figure 7-17. Differences between legally and physically
available flows indicate that much of the physically
available flow at upstream locations may not be available
for development due to senior downstream water rights.
Figure 7-18 shows minimum, median, and maximum
annual legally available flows for the period of record.
The wide range of annual flows at the locations between
wet and dry periods and an analysis of potential reservoir
yields indicate that firm yield supply is significantly less
than average yield supply.
To better represent the effects of seasonal and year to
year hydrologic variation, monthly (for minimum,
maximum, and median years) and annual time series of
legally available flows for the periods of record are
shown in Figures 7-19 through 7-30. The median annual
flow and 3-year running averages are also included on
the annual time series plots. The monthly analyses
highlight the fact that available flows vary greatly with
season, with the greatest amounts of water available in
the summer months and a sharp decline in flows in the
autumn and winter. The annual time series plots also
show large variation with a notable extended drought
period from the late 1980s to the early 1990s and more
acute droughts in the mid-1930s, 1950s, and late 1970s.
Extended wet periods evident in these figures include the
late 1920s and the mid-1980s.
The interpretation above is in general agreement with the
CWCB Drought Study (HDR 2003), which summarized
the history of drought in Colorado and identified
significant drought periods in the last 100 years. The
Drought Study notes that the most recent drought
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analyzed for years 2000 to 2003 exceeds many of the
drought records established during the 20th century. It
should be noted that the drought period of the past few
years is not yet included in the StateMod datasets, and
therefore, not represented in the available flow numbers
presented here.
Finally, Figure 7-31 is provided to further quantify the
impacts of seasonal and year to year hydrologic variation
and to illustrate the difference between average annual
available flow and the potential annual firm yield. This
chart shows firm yield as a function of total available
storage for legally available flows at the Colorado River
near potsero, where even with very large volumes of
storage, the maximum annual firm yield is approximately
65 percent of the average annual available flow. The
curve reaches an asymptotic value of 350,000 AFY at
approximately 2,000,000 AF total storage, beyond which
no significant gains in firm yield can be achieved with
increased storage. At the asymptotic value, all excess
water is captured, stored, and used, but supply is still
limited (below the average annual) by the timing of the
available flows and reservoir evaporation and seepage.
The critical (limiting) periods for this analysis are the late
1970s for smaller storage volume yields, and the early
1990s for larger storage volume yields. Note that yields
would likely be significantly higher if some value or
frequency of shortages, greater than zero, were
acceptable. Potential limitations to the projected supply
availability as a result of the Colorado River Compact are
evaluated in Section 7.4. The Compact includes potential
limitations for the Colorado, Gunnison, Dolores/San
Juan/San Miguel, and YampaNVhite/Green Basins
combined as well as additional limitations on certain
subbasins.
In addition to the Colorado River Compact, there are
other factors not reflected in the data presented that may
further limit future supply availability as summarized
below. The future development of existing conditional
water rights are not included in the DSS datasets. These
water rights could eventually be developed resulting in
less available water for the rest of the basin.
Development of conditional rights, however, must be
applied to beneficial use and meet a water need.
Conditional water rights, by basin, are summarized in
Section 10.
Since the current Colorado DSS dataset covers the
period from 1909 to 1995, once the current drought has
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