Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

• Description: Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River
• State: CO
• Author: Lewis, Michael; Brendle, Daniel
• Title: Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River
• Water Supply Pro - Doc Type: Report/Study

Reservoir. The transmountain water is imported
during the summer and can be stored in several off -
channel reservoirs in the upper basin or in Pueblo
Reservoir. The imported water may be released from
storage to meet downstream municipal or irrigation
water - supply demands. Rainfall runoff, ground -water
inflow, and irrigation -return flow also contribute to
flow in the Arkansas River. A substantial amount of
the water in the Arkansas River is diverted and
consumptively used for irrigation in the study area.
During 1955 -94, the median annual streamflow
in the Arkansas River decreased downstream by
about 88 percent from Pueblo (448,000 acre - ft/yr)
to Lamar (53,700 acre- ft /yr), largely because of
irrigation diversions. Irrigation -return flow from
tributary streams, drainage ditches, and the alluvial
aquifer supplements flow in the Arkansas River, and
much of the streamflow in the river downstream from
La Junta consists of irrigation -return flow during parts
of many years (Cain, 1987).
METHODS OF TREND ANALYSIS
Trend analysis can be used to determine if
streamflow or water quality has changed over time.
In this study, step -trend analysis was used to deter-
mine streamflow and specific- conductance trends
in the Arkansas River. In a step -trend analysis, data
collected before a specific time are assumed to be
from a distinctly different data population than data
collected after that time. The difference between the
data populations is assumed to be one of location (for
example, mean or median), but not necessarily of scale
(for example, variance or interquartile range). The
step -trend analysis is much more specific than other
trend analyses (for example, monotonic trend analysis)
because step -trend analysis requires that a particular
fact, the time of the change, is known before any
examination of the data. It is imperative that the deci-
sion to use step -trend analysis not be based on prior
examination of the data because prior examination
would bias the significance level of the test. Signifi-
cance levels, as represented by the individual p values
of each test, were for the two -sided trend test because
no prior determination of the direction of trends was
made. For this study, a significant test was defined
at a 95- percent confidence level (p <- 0.05). The step -
trend analysis is particularly well suited to this study
because the purpose of the study was to determine
if streamflow or specific conductance at a particular
location on the Arkansas River changed after the
construction of Pueblo Reservoir (1975) or after
the implementation of the John Martin Reservoir
1980 operating plan.
Step -trend analysis was done using the non -
parametric Mann - Whitney - Wilcoxon rank -sum test
(Bradley, 1968). This nonparametric procedure was
selected because the data (streamflow and specific
conductance) generally were not normally distributed,
based on graphical data analysis. Nonparametric
procedures have more power (or efficiency) than para-
metric procedures in cases where there is a substantial
departure from normality (Helsel and Hirsch, 1988).
In addition to doing trend analysis on daily
mean streamflow data and specific- conductance data
that were grouped by month, specific - conductance
data were analyzed for trends with data grouped by
season. One problem with using monthly specific -
conductance data is that the sample sizes are smaller
compared to data sets consisting of several months of
data. The p value for hypothesis testing is affected by
sample size. For a given trend magnitude and vari-
ance, p values tend to increase as the sample size
decreases (Helsel and Hirsch, 1992); therefore, it
becomes more difficult to reject the null hypothesis
of no trend as the sample size decreases. Grouping
specific- conductance data by season increased the
sample sizes of the data sets being analyzed. The
seasonal grouping was based on the timing of water -
storage and release operations for Pueblo Reservoir
and John Martin Reservoir. For the streamflow -
gaging stations located between Pueblo Reservoir
and John Martin Reservoir, data were grouped by
growing season (March 16— November 14) or winter -
storage season (November 15 —March 15). For the
streamflow- gaging stations located downstream from
John Martin Reservoir, data were grouped by growing
season (April 1— October 31) or winter - storage season
(November 1 —March 31). A thorough description
of the factors affecting these seasonal groupings
is provided in the following "Water Administration
and Reservoir Operations" section.
Qualitative comparisons of streamflow and
specific- conductance data were made using boxplots.
Boxplots are useful because variability between data
sets and unusually large or small values in a data set
can easily be seen. For this report, boxplots were
constructed to compare monthly differences in daily
mean streamflow and specific- conductance data at
particular main -stem sites before and after implemen-
tation of reservoir operations. Boxplots contain the
6 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River,
Southeastern Colorado