WSP09785

• File Number: 8142.600.30.A
• Description: Trinidad Project - Studies - Other Studies - Purgatoire river transit loss study
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1993
• Author: USGS
• Title: Evaluation of Streamflow Traveltime and Streamflow Gains and Losses Along the Lower Purgatoire River Southeastern Colorado 1984-1992
• Water Supply Pro - Doc Type: Report/Study

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models. A numerical flow model could be developed
to provide a reliable means of determining travel-
times and transit losses for this river, but accurate
estimates require accurate data to be available for
each river reach.

SUMMARY

The lower Purgatoire River flows more
than 160 miles from Trinidad Dam and Reservoir
to its confluence with the Arkansas River near
Las Animas, Colorado. Water supplies in the
area are overappropriated and, at times, shortages
of irrigation water occur when the demand by canal
systems exceeds the available streamflow. A better
knowledge of streamflow traveltime and streamflow
gains and losses along the lower Purgatoire River
would enable more informed management decisions
about the availability of water supplies for irrigation
use in southeastern Colorado. In 1994--95, the
U.S. Geological Survey, in cooperation with the
Purgatoire River Water Conservancy District
and the Arkansas River Compact Administration,
began an evaluation of streamflow traveltime and
streamflow gains and losses using historical records
for 15 surface-water sites and daily diversion data
for 13 canals in the study area. The irrigation canals
divert streamflow from the Purgatoire River usually
from April through the middle of November; peak
diversions generally are largest from June through
August. Eleven canals diverted water in reach 1
(60.6 miles), there were no surface-water diversions in
reach 2 (40.1 miles), and two canals diverted water in
reach 3 (58.5 miles). At times, there was little surface-
water flow in some river reaches because the irrigation
canals diverted all the river water.
Two streamflow-gaging stations in reach 1 were
used to evaluate streamflow entering the study area
during two relatively long-term periods of time before
and after the commencement of operations at Trinidad
Dam in 1979. Seasonal comparisons of daily stream-
flow during two time periods (1957-67 and 1984-92)
indicated that regulation of the Purgatoire River at
Trinidad Reservoir had affected the seasonal quantity
and timing of streamflow into the study area. Daily
median streamflow during 1984--92 (site Ql) was
larger than during 1957-67 (site QIA) during the
summer, but was generally smaller during the other
three seasons.

The availability and completeness of streamflow
data for tributary streams in the study area varied
seasonally and spatially in the three river reaches.
Analysis of daily mean streamflow data for two gaged
tributaries in reach 1 and for five gaged tributaries in
reach 2 indicated that tributary streams contributed
streamflow during many days of the year; however,
tributary streamflow that exceeded 10 percent of
mainstem streamflow occurred infrequently. Most
of the time, the total gaged tributary streamflow in
reaches I and 2 contributed less than 10 percent of the
mainstem streamflow, indicating that tributary streams
generally were not a source of substantial streamflow
(inflow) to the lower Purgatoire River. No tributary
streamflow data were available for reach 3.
Traveltime analyses were used to determine
when streamflows would arrive at three downstream
sites. The frequent diversion of streamflow for irriga-
tion purposes and substantial ungaged streamflow in
the most upstream reach prevented the tracking of
streamflow through reach 1. Therefore, an estimation
of streamflow traveltime for the 60.6 miles of river in
reach 1 could not be made using the available data.
Hourly streamflow data from 1990 through 1994 were
used to estimate traveltimes for 31 (reach 2) and 30
(reach 3) suitable streamflow events for about
100 miles of the lower Purgatoire River. In reach 2,
the traveltime of streamflow for the 40.1 miles ranged
from about 11 to about 47 hours and in reach 3, travel-
time for the 58.5 miles ranged from about 6 to about
61 hours. The traveltime in the river reaches generally
increased as the streamflow decreased, but also varied
for a specific streamflow in both reaches.
Streamflow gains and losses were estimated
using daily streamflow data at the upstream and
downstream sites, available tributary data, and daily
diversion data. The difference between surface-water
inflows and surface-water outflows in a river reach
determined the quantity of water gained or lost. In
reach 1, difficulties in establishing streamflow travel-
time prevented the determination of streamflow gains
or losses. From 1984 through 1992, more than
2,900 daily estimates of streamflow gains or losses
were made for reach 2 and more than 3,200 estimates
were made for reach 3 that indicated daily gains and
losses in streamflow were common during all seasons
of the year. Although some large daily streamflow
gains and losses were computed, most daily estimates
indicated small gains and losses in streamflow. Daily
median values of streamflow gains and losses for

SUMMARY 23