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<br /> <br />14,000 <br /> <br />I\J <br />f\.") <br />f0 <br />.c:.. <br /> <br />c <br />z <br />8 <br />~ 12,000 <br />0: <br />W <br />a. <br />III <br />~ 10,000 <br />() <br />ii'i <br />:> <br />() <br />~ 8.000 <br /> <br />ui <br />C!l <br />0: <br />< <br />J: <br />g 6,000 <br />o <br />::; <br />ili <br />0: <br />!ii <br />z <br />ili <br />::; <br />;;! <br />:> <br />z <br />z <br />< <br /> <br />4,000 <br /> <br />I3-EI STATION 09095500 <br />A...I:;, STATION 09152500 <br />G-0 STATION 09163500 <br /> <br />Horizontal lines are the mean annual <br />stream discharges. water years 1970-93 <br /> <br />2,000 <br /> <br />....... .IJ... ":. on uAum....... <br />... ... '..""..A. <br />.. .. " <br />'i <br /> <br />o <br />1968 1970 1972 1974 1978 1978 1980 1982 1984 1988 1988 1990 1992 1994 <br /> <br />WATER YEAR <br /> <br />Figure 3. Annual mean stream discharge for gaging stations 09095500 Colorado River near Cameo, <br />09152500 Gunnison River near Grand Junction, and 09163500 Colorado River near the Colorado-Utah <br />State line, water years 1970-93. <br /> <br />METHODS OF TREND ANALYSIS <br /> <br />Trend analysis can be used to determine if <br />stream-water quality has changed over time. 1\vo <br />general types of trend tests, monotonic and step-trend <br />tests, were used to examine dissolved-solids and major <br />constituent data associated with the salinity-control <br />projects. A monotonic trend means that the water- <br />quality variable of interest has changed oyer time. <br />A monotonic trend test will not specify if the change <br />occurred continuously, linearly, or in abrupt or discrete <br />steps (Hirsch and others, 1991). Step-trend tests are <br />used instead of monotonic tests where a known event <br />or change occurred at a specific time in a watershed that <br />could have significantly altered constituent concentra- <br />tions or loads. In such cases, the data can be divided <br />into "before" and "after" periods relative to the known <br />event. An example of such a known event is the weIl <br />plugging for the Meeker Dome Unit. Another case for <br />use of step-trend tests is when the water-quality record <br />is broken by a relatively long time gap, Conversely, <br />step-trend tests would not be used if water-quality <br /> <br /> <br />changes were incremental over a rather long time <br />period. For a long-term effect, such as the salinity- <br />control projects in the Grand Valley and lower <br />Gunnison River Basin, monotonic trend tests are more <br />appropriate for trend analysis. <br /> <br />Monotonic Trend Analysis Using the <br />Seasonal Kendall Test <br /> <br />Monotonic trends on periodic concentration data <br />and monthly dissolved-solids loads were examined <br />using a computerized procedure developed by the <br />USGS called EStimate TREND (ESTREND) (Schertz <br />and others, 1991). The program is designed to investi- <br />gate trends in water-quality data that often have non- <br />normal distributions, seasonality, outliers, missing <br />data, or have censored data (values reported as less <br />than). The program is written in FORTRAN language <br />for the USGS Prime minicomputer system (Schertz and <br />others, 1991). <br /> <br />10 Trend Analysis of Selected Water.Quallty Data Associated With Salinity-Control Prolocto In the Grond Valley, <br />In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado <br />