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~ ~ l .-. . ~ ~ ''1;..1. for water years 1970-93, 1980-93, and] 986-93 (table 6). Only the trend for 1980-93 had a significant p value (0.05 or less). The large variability of the Grand Valley dissolved-solids load (table I) and small sample size made it more difficult to detect statistically significant trends. The annual Grand Valley dissolved-solids load should be a sensitive variable for determining the effects of the salinity-control projects on dissolved- solids load from irrigated areas. However, the Grand Valley dissolved-solids load is not directly measured; it is a residual term calculated from a mass balance of dissolved-solids loads in the Colorado and Gunnison Rivers. The range and variability of annual Grand Valley dissolved-solids load (table I) seem unrealistic because the irrigated acreage and amount of water applied each year in the Grand Valley do not have large year-to-year changes. Therefore, the variation in the Grand Valley dissolved-solids load does not seem related to actual physical changes in the Grand Valley. The annual Grand Valley dissolved-solids load was a function of the magnitude and errors associated with Ihe annual loads in the rivers, especially of loads at station 09163500. For example, a 3-percent error in the annual load for water year] 988 at station 09 I 63500 would change the computed Grand Valley dissolved- solids load for 1988 by about 88,000 tons, or about a 19-percent change. There is a significant correlalion coefficient of 0.6 I between the annual Grand Valley dissolved-solids load and the annual dissolved-solids load at station 09163500. That correlation implies thaI the magnitude of the Grand Valley dissolved-solids load is dependent on the magnitude of the annual load at station 09163500. If the errors associated with deter- mining the Grand Valley dissolved-solids load are randomly distributed through a relatively long time period, then the mean value might be a reasonably real- istic approximation of the annual dissolved-solids load over the time period. Because of the large year-to-year variability, the annual Grand Valley dissolved-solids load might be less suitable than other variables for use in trend tests that have the purpose of showing effects of the salinity-control project in the Grand Valley. Trends in Selected Major-Ion Concentrations Dissolved solids, or salinity, is a measure of the dissolved inorganic constituents in water. In the Colorado and Gunnison Rivers in the study area, nearly all the dissolved solids consist of calcium, magnesium, sodium, sulfate, chloride, and bicarbonate ions, with Table 4. Monotonic trends in annual dissolved-solids loads 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 (Periods are in water years; slopes are in Ions per year; percent is the slope c:\pressed as percent change per year; p value is Lhe significance level of the teSI; SL. significance levels. which are: HS is highly significant. p is less Ihan or equal to 0.01; S is significanl, p is greater than om and less than or equal to 0.05; MS is marginally significant, p is greater than 0.05 and less than or equal to 0.10; and NS is not significant, p is greater than 0.10; <,Iessthan] Stallon Period Unadjusted annual load Slope Percent p value SL 09095500 I 97(}"93 -5,020 -{).34 0.537 NS t98(}"93 -26,000 -1.74 .265 NS 1986-93 -{;9.000 -4.96 .130 NS 09152500 I 97(}"93 -{;.650 -.53 .383 NS 1 98(}"93 -t4,5oo -1.16 .469 NS 1986-93 -27.000 -2.29 .548 NS I 97(}"93 -17,400 -.52 .331 NS 198(}"93 -56,500 -\.52 .237 NS t986-93 -127,000 -4.13 .191 NS 09163500 Flow~adjusted annu81108d Slope Percent p value SL -1,350 -{).09 0.605 NS -2,960 -.20 .689 NS -4 t ,000 -2.95 <.001 liS -{;,350 -.50 .027 S 2,220 .18 .676 NS -8.620 -.73 .413 NS -16,100 -.49 .004 HS -7.770 -.23 .497 NS -{;2.3oo -2.02 .003 HS TREND ANALYSIS FOR THE COLORADO AND GUNNISON RIVERS 21