FLOOD01774

• County: Otero
• Community: La Junta
• Title: Analysis of Hydrologic Factors that Affect Grond-Water Levels in the Arkansas River Alluvial Aquifer near La Junta Colorado 1959-99
• Date: 1/1/2000
• Prepared For: La Junta
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

The alluvial aquifer in the study area is hydrau-
lically connected to the Arkansas River. Accordingly,
the aquifer and the river can exchange water; if the
water-surface elevation in the river i,; higher than the
water-table surface in the surrounding aquifer, then the
river provides recharge to the aquifer and vice versa.
ln addition, the aquifer also can be recharged as a
result of modifications to the system made to accom-
modate irrigation procedures such as application of
water derived from surface-water diversions or from
ground water, or both. The aquifer also may discharge
water to local irrigation and water-supply wells.

Surface-Water Conveyances

There are two principal surface-water convey-
ances in the study area (fig. 1): the Fort Lyon Canal
and the Arkansas River. Records of discharge for the
fort Lyon Canal, that are from tbe gllging station
located downstream from the canal head gate were
obtained from the Colorado Division of Water
Resources. Records of dbcharge for the Arkansas
River at La Junta, a streamflow-gaging station that is
located at La Junta and is downstream from the diver-
sion at the headgate for the Fort Lyon Canal. are stored
in a USGS data base known as the Automated Data
Processing System lADAPS).
There are two facts about the Fort Lyon Canal
that need to be considered. First, the canal is unlined;
that is, the bottom and banks consist of native mate-
rials, and, as a consequence there are conveyance
losses (in this sense the Fort Lyon Canal provides
recharge to the local aquifer). In a [989 to 1990 study,
Dash ( 1995) estimated that losses could be as high as
about 15 acre-feet per day per canal mile,
Second, the Fort Lyon Canal gaging station is
located downstream from two structures that are some-
times used to divert water li'om the canal for mainte-
nance activities related to suspended sediment loads in
tbe canal. As a result. the gaging station does not
record wateT diverted through these structures.
Also, beginning 1975, about the middle of the
period being studied. Pueblo Reservoir began to
operate (Abbott, 1 985). Reservoir operations most
pertinent to the study described in this report are the
distribution of water stored in the reservoir for use at a
later time (winter water) and the distribution of water
available from transmountain diversion projects
(project water).

The discharge record for the Fort Lyon CanalIS
shown in figure 5 and table 4. Figure 5 includes the
annual total hydrograph, a smoothed curve of the
annual total hydrograph, and a line indicating the
long-term mean. A smoothed curve indicates general
patterns and is prepared using a technique called
locally weighted scatterplot smoothing (Cleveland and
others, (979), a cDlllputationally intensive technique
that is comparable to a moving average. Figure 5 indi-
cates that, generally, flow in the canal was greater than
the long-term mean from 1980 through 1989 during
the 1980's. This period includes one year of relatively
low flow, 1981, and also includes the highest annual
total flow for the period of record, 1985 (fig. 5). After
1989, figure 5 indicates that, generally. diversions to
the canal were less than the long-term mean, although
flow was greater than the long-tenn mean in 1996 and
1997 and the 1997 flow was almost as higb as the 1985
diversion. The mean-daily hydrograph (fig. 5) sbows
typical seasonal variation of flow for tbe period
studied and indicates that flow is highest in June.
The record for flow in the Fort Lyon Canal, as
well as flow in tbe Arkansas River, indicates a change
during the late 1970's. Accordingly, flow-duration
,malyses were prepared to describe differences in flc>w
characteristics for two periods, 1960 to 1980 and 1981
to 1997, The results for this study (table 5) list a
percentage of time. from the period of record being
analyzed, that the corresponding rate of flow was
exceeded.
The flow-duration statistics in table 5 indicatt
that there was flow in the Fort Lyon Canal more often
during the first period than the second. For instance,
there were very low flows (less than] cubic foot per
second) in the canal for only I to 3 percent of the first
period, whereas there were very low flows in tbe canal
for 10 to 15 percent of the second period. Perhaps
more importalllly, the flow-duration statistics indicate
tbat, due to large flows during the 1980's, flow in the
canal was greater, usually by a factor of I j to 2, than
in the first period. For example. diversions in the callal
were greater than about 345 cubic feet per second for
50 percent of the second period; in the first period the
50 percent exceedance flow was about 213 cubic fe'~t
per second.
The discharge record for the Arkansas River ,1t
La Junta is shown in figure 6. The annual total and
particularly the mean-daily hydrographs are generally
similar to the hydrographs for the Fort Lyon Canal
(fig. 5). The smoothed curve for the annual total flow

16 Analysis ot HydrologiC Factors. That Affect Ground~Water l.evels in the Arkansas River AUuviat Aquifer Near La Junta, ColoradO,
1959-99