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

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Table 6. Step -trend results on the daily mean streamflow at station 07109500 (Arkansas River near Avondale) between 1969 -74 and 1975-94 [ft3 /s, cubic feet per second; N, number of values; p value is the significance level of the test; <, less than; NS, trend not statistically significant; I, statistically significant increasing trend; D, statistically significant decreasing trend] Month 1969 -74 Median streamflow (ft%) N 1975 -94 Median streamflow (ft%) N p value Significance January 500 186 332 620 <0.01 D February 462 168 365 560 <01 D March 414 186 480 620 <01 I April 452 180 636 600 <01 I May 1,220 186 1,050 620 .73 NS June 1,950 180 2,410 600 <01 I July 1,560 186 1,650 620 .09 NS August 993 186 1,090 620 <01 I September 497 180 540 600 .40 NS October 386 186 472 620 .04 I November 560 180 403 600 <.01 D December 540 186 298 620 <01 D to statistically significant trend was defined as having a p value less than or equal to 0.05 From 1969 to 1995, the discharge from the Colorado Springs wastewater - treatment plant increased from about 25 ft3 /s to about 50 ft3 /s (V.L. Card, Colorado Springs Utilities, written commun., 1996; Edelmann and Cain, 1985). Although the median daily stream- flow of Fountain Creek at Pueblo during November through February increased from about 63 ft3 /s in 1969 -74 to about 87 ft3 /s in 1975 -94, the increase was not large enough to offset the seasonal decrease in streamflow of the Arkansas River at Avondale, which resulted from the storage of WWSP water in Pueblo Reservoir. Specific conductance at station 07109500 had less variability between the pre - Pueblo Reservoir (1969 -74) and post - Pueblo Reservoir (1975 -94) periods (fig. 14) than specific conductance at the upstream station 07099400. Although the annual minimum and maximum specific conductance tended to increase after 1974, the median specific conduc- tance for 1969 -74 (778 µS /cm) was not significantly different (p =0.16) from the median specific conduc- tance for 1975 -94 (812 µS /cm). The annual minimum specific conductance increased from a range of about 200 to 400 µS /cm in 1969 -74 to a range of about 250 to 600 µS /cm in 1975 -94 (fig. 14). The annual maximum specific conductance increased from a range of about 1,000 to 1,100 µS /cm in 1969 -74 to a range of about 1,000 to 1,450 µS /cm in 1975 -94 (fig. 14). Monthly specific conductance differed between 1969 -74 and 1975 -94 (fig. 15), but most of the differences were not statistically significant (table 7). The small number of significant differences partly may be due to the small amount of specific - conductance data in 1969 -74 (table 7). On a seasonal basis, the median specific conductance in the growing season increased from 580 µS /cm in 1969 -74 to 700 µS /cm in 1975 -94; the increase was not statisti- cally significant (p= 0.07). In the winter - storage season, the median specific conductance increased significantly (p50.01) from 900 µS /cm in 1969 -74 to 1,050 µS /cm in 1975 -94. Significant increases in specific conductance occurred during June, August, and December (table 7). The increase in the median specific conductance in December (900 to 1,130 µS /cm) coincided with a significant decrease in streamflow (table 6). The increased median specific conductance in June (338 to 455 µS /cm) and in August (470 to 550 µ&cm) coincided with significant increases in streamflow for those months (table 6). This relation between streamflow and specific - conductance trends is unusual because increased streamflow generally is expected to RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 25 IN THE ARKANSAS RIVER