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

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Table 7. Step -trend results on specific conductance at station 07109500 (Arkansas River near Avondale) between 1969 -74 and 1975-94 [gS /cm, microsiemens per centimeter at 25 degrees Celsius; 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] lA statistically significant trend was defined as having a p value less than or equal to 0.05. The duration frequencies of specific conduc- tance at station 07109500 for 1969 -74 and 1975 -94 were compared to the salinity- hazard classifications for irrigated crops (fig. 16). The salinity hazard is a relation developed by the U.S. Salinity Laboratory (Richards, 1954) that describes the qualitative effect of saline water on irrigated crops. The hazard is based on the specific conductance of the water and is divided into four classes of salinity hazard ranging from low (Class CI) to very high (Class C4). Water at station 07109500 generally was Class C2 or C3. Class C2 water (250 -750 µS /cm) is defined as having a moderate salinity hazard and can be used on crops having a moderate salt tolerance without needing special irrigation practices for salinity control. Class C3 (750 -2,250 µS /cm) is defined as having a high salinity hazard. The changes in specific conduc- tance that occurred at station 07109500 after 1974 did not result in a substantial change in the salinity- hazard classification of the water (fig. 16). At Las Animas Station 07124000 (Arkansas River at Las Animas) is located about 120 mi downstream from Pueblo Reservoir (fig. 1). Streamflow at this station is substantially smaller than at station 07109500 (fig. 3) because several large irriga- tion canals divert most of the streamflow in the 96 -mi reach between the stations. Irrigation -return flow composes a substantial fraction of the stream- flow at station 07124000 (Cain, 1987); therefore, the specific conductance is considerably higher than at upstream sites (fig. 4). Streamflow and specific- conductance data were available at station 07124000 for 1961 -94. The median annual streamflow at station 07124000 increased significantly (p =0.01) from 77,200 acre -ft/yr in 1961 -74 to 149,400 acre -ft/yr in 1975 -94. This difference represents, on average, an increase in the daily mean streamflow of about 100 ft3 /s. The daily mean streamflow at station 07124000 increased in every month after 1974 (fig. 17); the differences in streamflow were significant for all 12 months (table 8). The increased streamflow prob- ably is attributable to a combination of factors, including the WWSP and associated changes in growing- season and nongrowing- season irrigation practices and the increased importation of water from the western slope for irrigation. The effects of these factors are greatest at Las Animas because it is the farthest downstream station between Pueblo Reservoir and John Martin Reservoir. 28 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado 1969 -74 1975 -94 Median Median Month specific specific p value Significancel conductance N conductance N (RS/cm) (gS /cm) January 970 6 1,090 16 0.17 NS February 950 5 960 13 .66 NS March 900 4 844 20 .56 NS April 925 6 852 27 .66 NS May 600 7 734 29 .38 NS June 338 10 455 25 <01 I July 320 5 454 21 .15 NS August 470 7 550 27 .01 I September 850 7 718 24 .60 NS October 950 7 827 17 .37 NS November 840 7 890 17 .22 NS December 900 7 1,130 18 <01 I lA statistically significant trend was defined as having a p value less than or equal to 0.05. The duration frequencies of specific conduc- tance at station 07109500 for 1969 -74 and 1975 -94 were compared to the salinity- hazard classifications for irrigated crops (fig. 16). The salinity hazard is a relation developed by the U.S. Salinity Laboratory (Richards, 1954) that describes the qualitative effect of saline water on irrigated crops. The hazard is based on the specific conductance of the water and is divided into four classes of salinity hazard ranging from low (Class CI) to very high (Class C4). Water at station 07109500 generally was Class C2 or C3. Class C2 water (250 -750 µS /cm) is defined as having a moderate salinity hazard and can be used on crops having a moderate salt tolerance without needing special irrigation practices for salinity control. Class C3 (750 -2,250 µS /cm) is defined as having a high salinity hazard. The changes in specific conduc- tance that occurred at station 07109500 after 1974 did not result in a substantial change in the salinity- hazard classification of the water (fig. 16). At Las Animas Station 07124000 (Arkansas River at Las Animas) is located about 120 mi downstream from Pueblo Reservoir (fig. 1). Streamflow at this station is substantially smaller than at station 07109500 (fig. 3) because several large irriga- tion canals divert most of the streamflow in the 96 -mi reach between the stations. Irrigation -return flow composes a substantial fraction of the stream- flow at station 07124000 (Cain, 1987); therefore, the specific conductance is considerably higher than at upstream sites (fig. 4). Streamflow and specific- conductance data were available at station 07124000 for 1961 -94. The median annual streamflow at station 07124000 increased significantly (p =0.01) from 77,200 acre -ft/yr in 1961 -74 to 149,400 acre -ft/yr in 1975 -94. This difference represents, on average, an increase in the daily mean streamflow of about 100 ft3 /s. The daily mean streamflow at station 07124000 increased in every month after 1974 (fig. 17); the differences in streamflow were significant for all 12 months (table 8). The increased streamflow prob- ably is attributable to a combination of factors, including the WWSP and associated changes in growing- season and nongrowing- season irrigation practices and the increased importation of water from the western slope for irrigation. The effects of these factors are greatest at Las Animas because it is the farthest downstream station between Pueblo Reservoir and John Martin Reservoir. 28 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado