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

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500 450 Z U 400 r¢ w F- 7 Z J W ¢ Z a 350 o a ❑ CC LD 0 300 U J w Z > — O 250 U) 0 200 150 t r 300 400 500 600 700 SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS Figure 2. Relation of dissolved - solids concentration to specific conductance in water samples collected in the Arkansas River at Pueblo, 1994 -93. Specific Conductance in the Arkansas River The specific conductance of water in the lower Arkansas River markedly increases between Pueblo Reservoir and Lamar, about 160 miles (mi) down- stream (fig. 3). Low- specific- conductance snowmelt runoff (generally less than 200 µS /cm) from the upper Arkansas River Basin is the primary source of streamflow in the lower Arkansas River. Downstream from Pueblo Reservoir, the large increase in specific conductance (fig. 3) is attribut- able to natural and human effects, but the primary cause is the continual use and reuse of water for agricultural irrigation (Cain, 1987). When water is used for irrigation, part of the water evaporates or is consumed by plants, concentrating the original amount of dissolved solids in less water; thus, the dissolved- solids concentration and the spe- cific conductance in the remaining water is increased. The remaining higher specific - conductance water reenters the river as irrigation -return flow. The rate of increase in specific conductance in the lower Arkansas River tends to increase down- stream because high- specific- conductance irrigation -return flows compose an increas- ingly large percentage of streamflow (Cain, 1987). Specific conductance also varies during the year as a result of the temporal variability of streamflow. Specific conduc- tance generally is lowest in May— August, when streamflow generally is largest, and increases with decreasing streamflow in the fall, winter, and spring (fig. 4). PUEBLO RESERVOIR OPERATIONS Storage began in Pueblo Reservoir in 1974, and the dam was completed in 1975 on the main stem of the Arkansas River near Pueblo. The operation of Pueblo Reservoir, 4,000 Z — rt U Fw 3,000 Z w cn UZJ ❑UU 0 w w 2,000 Vaw U) CC u W w dwUn U) F� Z¢ ¢ ¢ 1,000 5U w� 2 0 0 50 in particular the Winter Water Storage Program (WWSP), has a notable effect on streamflow in the lower Arkansas River. The WWSP allows participating irrigation canal companies, which have historically diverted winter flows from the Arkansas River, to store water during the winter in Pueblo Reservoir, in John Martin Reservoir, and in several small off - channel reservoirs. Prior to the WWSP, farmers diverted Arkansas River streamflow onto fallow fields for winter irrigation. This practice was followed to increase soil moisture for later use by crops during the growing sea- son. Winter irrigation is inefficient, how- ever, and the WWSP replaced this practice. Water stored during the winter as part of the WWSP can be released to the river and used by participating canal companies throughout the year at times when natural streamflow is insufficient to meet irrigation - water needs. The WWSP began in 1975 and has been in operation every year since then, except for the 1977 -78 winter - storage period. During 1975 -94, the median annual volume of WWSP water stored in Pueblo Reservoir was about 42,200 acre -feet (acre -ft) (Thomas C. Simpson, Southeastern Colorado Water Conservancy District, written commun., 1997). PUEBLO RESERVOIR OPERATIONS HAVE AFFECTED SPECIFIC CONDUCTANCE IN THE ARKANSAS RIVER Specific- conductance data col- lected at three Arkansas River sites located between Pueblo Reservoir and John Martin Reservoir (fig. 1) were analyzed for changes that might have occurred after Pueblo Reservoir was constructed. These three sites are Pueblo, Avondale, and 100 150 170 MILES DOWNSTREAM FROM PUEBLO RESERVOIR Figure 3. Median specific conductance in the Arkansas River, 1994 -93.