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

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1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997

Figure 4. General trends in ground-water levels.

Regression Characterization

General change in water levels through time
also can be characterized with simple linear regres-
sion. The results of regression analyses can be tabu-
lated and are more convenient to work with than a
large number of hydrographs. Linear regression tech-
niques relate a dependent variable (for this study,
measured depths to water) to an independent variable
(for this study, time). The results of regression anal-
yses can be characterized with an equation, which
includes a term describing the slope for a straight line.
For regressions of depths to water with time, a nega-
tive slope indicates a general decrease in depths to
water with time. A decrease in depth to water repre-
sents a net increase in water-level elevation.

When using results from linear regression to
characterize monotonic changes such as an increasing
or decreasing slope, the significance of the slope term
in the regression equation can be used to screen slopes
that are not considered significant. For the purposes of
this report slopes that were not at least 95 percent
signiikant were considered insignificant. Regression
slopes for the 141 wells evaluated are listed in table 3
and are characterized as increasing or decreasing and
significant or insignificant. The regression slopes indi-

cate that, for wells that have significant slopes, two-
thirds of the slopes indicate increasing ground-water
levels through time.

Most wells that have increases in water levels
also have records that are representative of the period
being studied; that is, they span most of the study
period. When the regression results are plotted on a
map, wells with increasing water levels are distributed
uniformly throughout the study area. Although most
of the wells have increases in water levels, some have
decreases in water levels. Wells that have significant
decreases in water levels generally have records that
end before the 1980's or have records for only a few
years. The hydrographs in figure 4 indicate a promi-
nent period of water-level increase that began about
1980 that is common to wells that have record for this
period. Because wells that have decreases in water
levels are missing periods of prominent increases or
represent only a small part of the period studied,
excluding them from a characterization of general
trends for water levels in the study area is reasonable.
However, they do indicate that the phenomenon of
water-level increases in the study area has a temporal
element to it and that there can be short-term devia-
tions from the general long-term trend of water-level
Increases.

14 Analysis of Hydrologic Factors That Affect Ground-Water Levels in the Arkansas River Alluvial Aquifer Near La Junta, Colorado,
195!l-99