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<br />. <br /> <br />. <br /> <br />. <br /> <br />:.?P:Ci ~-I~fl;Ij;:,QaC.'w ~,=;:,n,:, <br />l ,I.-....J.., ~'ir~r.; ,.\-" .... <br /> <br />1950 <br /> <br />j~ l-.....'"T.. ......._ - '!' <br /> <br />Figure 17 near here. <br /> <br />Table 9 near here. <br /> <br />Although the range in specific-conductance values at station 07124000 changed linle after 1974 (fig. 18), the <br /> <br />median specific conductance decreased significantly (p<O.O I) from 3,000 IlSlcm in 1961-94 to 2,500 IlSicm in <br /> <br />1975-94. Specific-conductance values were noticeably lower in most months during 1975-94 (fig. 19). but the dif- <br /> <br />ferences are not statistically significant except in March, April. and June (table 10). The relatively small number <br /> <br />ofsignificanr differences between 1961-74 and 1975-95 specific-conductance values is somewhat surprising. in <br /> <br />light of the large increases in monthly discharge. A possible explanation for this situation is that the majority of <br /> <br />discharge in the Arkansas River between Pueblo Reservoir and Las Animas is typically diverted for irrigation, <br /> <br />except during extremely high discharge periods, and that irrigation return flow comprises much of the discharge in <br /> <br />the downstream end oftbis reach during most pans of many years. Through the process of evapotranspiration, the <br /> <br />dissolved-solids concentration in the irrigation return flow is increased. Therefore, the specific conductance of the <br /> <br />river at station 07124000 has not been subject to significant decreases and has remained elevated since the con- <br /> <br />strUction of Pueblo Reservoir. The small decrease in specific conductance has not changed the salinity hazard c!as- <br /> <br />sification of the water. The water has retained the same C3 (high salinity hazard) to C4 (very high salinity hazard) <br /> <br />hazard classifications that were common to the period prior to the construction of Pueblo Reservoir. <br /> <br />Figure 18 near here. <br /> <br />Figure 19 near here. <br /> <br />Table 10 near here. <br /> <br />30"'- <br />