WSP12671

• File Number: 8146.600.30.A
• Description: Purgatoire River Study
• State: CO
• Basin: Arkansas
• Date: 1/24/2007
• Author: USDOI/SGS Lisa D Miller
• Title: Losses and Gains for Eight Unlined Canals Along the Purgatoire River Near Trindad, Colorado 2000 - 2004
• Water Supply Pro - Doc Type: Publication

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wetting) and then tend to decrease and stabilize. However, in
canals that have alternate wetting and drying cycles (do not
flow continuously throughout the entire irrigation season)
seepage losses would be variable.
Loss and gain measurements were made during 2000,
2001, 2003, and 2004. During 2002, drought conditions in
the Purgatoire River Basin substantially reduced the stream-
flow. The reduced streamflow affected the amount of project
storage water that was available for surface-water diversions.
Because inflow to the project canals could not be held steady
for several consecutive days during May 2002, it was decided
that the loss and gain measurements would not be representa-
tive and the field investigation scheduled for May 2002 was
canceled. Table 2 shows that the annual total acre-feet diverted
from the Purgatoire River to Picketwire, Baca, El Moro,
Chilili, Enlarged Southside, Model, John Flood, and Hoehne
canals in irrigation years 2000 through 2004 ranged from
6,409 acre-feet in irrigation year 2002 (Nov. 1,2001, through
Oct. 31, 2002) to 60,228 acre-feet in irrigation year 2000
(Nov. 1, 1999, through Oct. 31,2000).

10 Losses and Gains for Eight Unlined Canals Along the Purgatoire 'River near Trinidad, Colorado, 200o-:Z004

Loss and Gain Computations

The canal loss or gain was computed between main-
canal measurement sites (subreach) during steady-state condi-
tions by equating inflows to outllows plus 1l0w loss or gain in
the subreach (Slade and others, 2002):

Qj + Qt + Q, + Qu = Qo + Qd + Qe + QVg (1)

where
Q =
I
Q, =
Q, =
Qu =
Qo =
Qd =
Qc =
Ql/g =

reach inflow at upstream end of subreach, in cubic
feet per second;
streamflow from tributaries into subreach, in cubic
feet per second;
return flows to subreach, in cubic feet per second;
ground-water underflow, in cubic feet per second;
reach outflow at downstream end of subreach,
in cubic feet per second;
diversions from subreach, in cubic feet per
second;
evapotranspiration from subreach, in cubic feet
per second; and
loss (positive) or gain (negative) in subreach,
in cubic feet per second.

In computations for this report, underflow (llow parallel
to stream [canal] through shallow channel-bed deposits), evapo-
transpiration, tributary inllow, and direct surface-water irriga-
tion return 1l0w to the canals are considered negligible. Under-
1l0w is considered negligible for this study for several reasons:
(1) it is assumed that the local ground-water table is well below
the canal beds, (2) the weathered shale that underlies the canal
beds is less permeable than the alluvium in the streambeds, and
(3) the canal-bed elevations are higher than the Purgatoire River
streambed (except at diversion points). Evapotranspiration is
considered negligible because loss-gain studies were conducted
for short subreaches over short-.periods of time (that is, hours)
in canals with widths mostly less than 15 feet. No inllow from
intermittent tributaries was observed because gain-loss studies
were conducted during dry peri.ods. The canals studied do not
have points of direct surface-water irrigation return llows (that
is, irrigation water is not returned back to an individual canal).
Thus, equation (1) can be reduced to:

QVg = Qi - Qo - Qd

Many factors may affect the measured losses and gains
along the canals such as canal size, construction, maintenance,
infiltration rate of the bed material, the wetted perimeter, and
the head (depth of water) in the canal. Changes in application
of irrigation water on individual fields may potentially pro-
duce large variation in loss and gain measurements over short
distances and timeframes. Losses and gains also may vary
within the irrigation season. Generally, seepage losses would
be expected to be highest in the early part of the season (initial

Canal Losses and Gauns

This section presents the measured losses and gains in
Picketwire (Baca Joint), EI Moro, Chilili, Enlarged Southside,
Model, John Flood, and Hoehne canals for the 2000, 2001,
2003, and 2004 loss and gain measurements. Losses and gains
in canal subreaches were computed by subtracting the sub-
reach outflow and diversions within the subreach (leakage is
included as a diversion) from the subreach inflow.
Occasionally, estimates of discharge were made at some
diversion sites because the flow was too low to measure, a
suitable measurement section could not be found, or estimates
were determined to be just as accurate as manual measure-
ments. Estimates of discharge at Parshall flumes were deter-
mined using standard rating tables, and estimates at splitter
laterals were made by measuring 1l0w in the main canal just
upstream from the splitter box and proportioning the total flow
on the basis of the width of the lateral (see method description
in "Discharge Measurements" section of this report).

Pitr:ketwire (Baca Joint) Canal

(2)

Water diverted into Picketwire Canal from the Purgatoire
River at Trinidad is used to supply Picketwire, Baca, and
El Mora canals with water for irrigation. Picketwire Canal
flows along the northwest side (left bank) of the Purgatoire
River (fig. 4). An 8-ft Parshall flume with a free-flow capacity
of about 140 ft3/s is used to measure irrigation diversions at the
Pic:ketwire headgate, and a 9-inch (0.75 ft) Parshall flume with
a free-flow capacity of about 9 ft3/s is used to measure irriga-
tion diversions at the El Moro headgate. There is no single
headgate for Baca Canal. Delivery is made to the Baca lands
through a series of lateral turnouts from the main Picketwire