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

however, they are less than the long-term mean and
indicate that there are relatively low levels of
discharge due to ground-water withdrawals from the
aquifer, a condition that contributes to increases in
water levels.
Water levels in well I, a lowland well, are below
the long-tenn mean before about 1980 and above the
long-term mean after 1980. The smoothed curve for
the daily-mean /low in the Arkansas River has a
similar, nearly step-like pattern indicating the general
/low conditions in the river may affect water levels in
lowland wells. The step-like increase described here
coincides with the appearance of project water in the
system.
The elevation for the Arkansas River bed has
one of the most consistent patterns of all the hydro-
logic factors, one of steady increase through the period
of record. The pattern is similar to the smoothed curve
for water levels in the lowland well indicating that as
the river-bed elevation increases local ground-water
levels increase too. In addition, the relatively high
river-bed elevations of the 1990's combined with rela-
tively low amounts of ground-water withdrawals are
likely related to increases in ground-water levels in
well I observed in the 1990's.
The water level in the lowland well also clearly
indicates that it can be affected by short-term condi-
tion in the river. The water level in well I measured
June I, 1987, is the highest water level measured at the
well. The daily mean flow in the Arkansas River
peaked at 6,840 cubic feet per second 10 days earlier:
on June I the flow was 1.680 cubic feet per second.
According to the flow-duration statistics in table 5,
these are relatively high daily mean flows for the
Arkansas River that are exceeded less than I and
5 percent of the time during I 981 to 1997. The relation
between high flow in the river coinciding with high
water levels in well I is clear in figure 7 when
comparing the daily mean /low in the Arkansas River
and the water levels in weill during June 1987.
The affects of hydrologic factors, some of which
appear to have good visual correlation with changes in
ground-water levels, can be quantified with regression
techniques. For instance, simple linear regression tech-
niques can be used to explain about 6 I percent of the
variability measured in upland wells based on ground-
water withdrawals and about 70 percent of the vari-
ability measured in lowland wells based on river-bed
elevation. Other, more advanced regression techniques
such as stepwise regression, which automates the

procedure of selecting the most effective algorithm for
several independent variables (in this case, the inde-
pendent variables would consist of all hydrologic
factors) can be implemented. When stepwise regres-
sion was implemented as part of this study, it was not
possible to explain more than about 90 percent of the
variability observed in either the upland or lowland
wells. That is, the results of stepwise regression using
all the hydrologic factors to explain changes in water-
levels, provided relatively modest improvements over
regression models using only ground-water with-
drawals for upland wells and river-bed elevation for
lowland wells.

One way to improve the current (1999) level of
knowledge might be to introduce a data collection
effort specifically designed to document losses from
the Fort Lyon Canal and the effects of surface-wakr
application in the short term. Such efforts could most
likely make use of existing wells to define cross
sections of the water table between the canal and the
Arkansas River.

SUMMARY

High ground-water levels in the alluvial aquifer
between the Fort Lyon Canal and the Arkansas River
in the vicinity of La Junta, Colorado, from the Fort
Lyon Canal head gate east to the Otero County line,
can impair the use of agricultural land and personal
property. Water-table and depth-to-water maps made
on the basis of water levels measured during March
1999 indicate that the local alluvial aquifer received
recharge from the Fort Lyon Canal and the Arkansas
River and that the water table was within 5 feet of the
land surface in about 31 percent of the study area. In
general, water levels are shallowest near the river and
become deeper closer to the canal; areas of relatively
shallow water levels are fairly widespread in the
western part of the study area.

A visual analysis of hydrographs for five wells
with water-level measurements from about 1965 to
1995 indicates a general tendency for water levels to
have increased. A more detailed examination of the
hydrographs indicates;

':

"

. Water levels in the two lowland wells near the
Arkansas River are relatively close to the land
surface.

24 Analysis of Hydrologic Factors That Affect Ground~Water Levels in the Arkansas River Alluvial Aquifer Near La Junta, Coloradcl,
195&-99