WSP12246

• File Number: 8210.310
• Description: Colorado River Basin Organizations and Entities - Colorado River Water Conservation District
• State: CO
• Basin: Western Slope
• Date: 1/1/1996
• Author: David L Butler
• Title: Trend Analysis of Selected Water-Quality Data Associate With Salinity-Control Projects in the Grand Valley in the Lower Gunnison River Basin and at Meeker Dome Western Colorado
• Water Supply Pro - Doc Type: Report/Study

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Figure 3. Annual mean stream discharge for gaging stations 09095500 Colorado River near Cameo,
09152500 Gunnison River near Grand Junction, and 09163500 Colorado River near the Colorado-Utah
State line, water years 1970-93.

Horizontal lines are lhe mean annual
stream discharges, water years 1970-93

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METHODS OF TREND ANALYSIS

Trend analysis can be used to determine if
stream-water quality has changed over time. Two
general types of trend teslS, monotonic and step-trend
tests, were used to examine dissolved-solids and major
constituent data associated with the salinity-control
projects. A monotonic trend means that the water-
quality variable of interest has changed over time.
A monotonic trend test will not specify if the change
occurred continuously, linearly, or in abrupt or discrete
steps (Hirsch and others, 1991). Slep-trend tests are
used instead of monotonic tests where a known event
or change occurred at a specific time in a watershed that
could have significantly altered constituent concentra-
tions or loads. In such cases, the data can be divided
into "before" and "after" periods relative to the known
event An example of such a known event is the well
plugging for the Meeker Dome Unit Another case for
use of step-trend tests is when the water-quality record
is broken by a relatively long time gap. Conversely,
step-trend tests would not be used if water-quality

changes were incremental over a rather long time
period. For a long-term effect, such as the salinity-
control projects in the Grand Valley and lower
Gunnison River Basin, monotonic trend tests are more
appropriate for trend analysis.

Monotonic Trend Analysis Using the
Seasonal Kendall Test

Monotonic trends on periodic concentration data
and monthly dissolved-solids loads were examined
using a computerized procedure developed by the
USGS called EStimate TREND (ESTREND) (Schertz
and others, 199]). The program is designed to investi-
gate trends in water-quality data that often have non-
normal distributions, seasonality, outliers, missing
data, or have censored data (values reported as less
than). The program is written in FORTRAN language
for the USGS Prime minicomputer system (Schertz and
others, ]991).

10 Trend Analysis 01 Selected Water-auality Data Associated With Sallnity-Control Projects In the Grand Valley,
In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado