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
7.3.3 Dolores/San Juan/San Miguel
Basin
7.3.3.1 Dolores/San Juan/San Miguel Surface
Water Supplies
The Dolores/San Juan/San Miguel basin StateMod
dataset incorporates the La Plata Compact, an interstate
compact that governs the distribution of water on the La
Plata River between Colorado and New Mexico.
Administration of the compact is dependent on the
stream flow at two USGS gaging stations: Hesperus
Station and Interstate Station. Colorado's allocation of
water per the Compact becomes limited if flow at
Interstate Station falls below 100 cfs between
February 16 and November 30, with the required
downstream delivery on the following day equal to one-
half of the mean flow at Hesperus. The DSS simulates
compact restrictions according to these rules.
Ten locations were selected for the Dolores/San Juan/
San Miguel basin to characterize supply availability using
StateMod datasets. The period of record for the
Dolores/San Juan/San Miguel StateMod datasets is 1909
to 1999 (full calendar years).
The selected locations, shown in Figure 7-34 are:
rivers that do not flow into New Mexico. For some of the
locations, however, such as the Dolores River near
Dolores, downstream priority calls are minimal.
Figure 7-36 shows minimum, median, and maximum
annual legally available flows for the period of record. A
wide range of annual flows at the locations indicates that
firm yield supply may be 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 7-37 through 7-56. 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. For most of the locations, a notable
extended drought period is evident from the late 1980s to
the early 1990s, with more acute droughts in the
mid-1930s, 1950s, and Iate1970s. Extended wet periods
evident in these figures include the early 1940s and the
mid-1980s.
^ San Juan River at Navajo
^ Piedra River at Arboles
^ Los Pinos River at Boca
^ Animas River at the state line
^ La Plata River at the state line
^ Mancos River at Towaoc
^ Dolores River near polores
^ Dolores River near Bedrock
^ San Miguel River near Placerville
^ San Miguel River near Uraven
Median annual legally available, physically available, and
naturalized flows are summarized for each location in
Figure 7-35. 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
With respect to the San Juan River and its tributaries, it
should be noted that approximately 85 percent of the
San Juan's flow originates in Colorado, and yet Colorado
must assure that adequate water passes the state line
for use in New Mexico. Consistent with the Colorado
River Compact, New Mexico's allocation under the
Compact cannot be satisfied by other Colorado compact
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$~ole'ri~ice Wo~e' $upplY Initia~ive
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 states that the most recent drought
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, which may not yet be over, is not yet included in
the StateMod datasets, and therefore, not represented in
the available flow numbers presented here.
Finally, Figure 7-57 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 San Miguel River
near Placerville. For the San Miguel River near
Placerville, even with very large volumes of storage, the
maximum annual firm yield is only approximately 77
percent of the average annual available flow. The curve
reaches an asymptotic value of 125,000 AFY at
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S:\REPORT\WORD PROCESSING\REPORT\S7 11-10.04.DOC 7-33