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

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1,500 w wF- UW BPD z- p V W 1,000 ZC, U 0aw () W U � W d 2 500 U) N < Cr w2 2z GROWING WINTER - SEASON STORAGE SEASON Figure 6. Specific conductance for the Arkansas River near Avondale, 1969 -94. streamflow combined with the high specific conductance in Fountain Creek resulted in an estimated 58- percent increase in the dissolved - solids load contributed to the Arkansas River between 1969 -74 and 1975 -94. Pueblo Reservoir operations decreased streamflow in the Arkansas River during the winter - storage season. As a result, the increased dissolved - solids load from Fountain Creek was subject to less dilution at the site near Avondale; therefore, specific conductance increased during the growing season between 1969 -74 and 1975 -94 (fig. 6). During the growing season, specific conductance in the reservoir outflow increased and the dissolved - solids load from Fountain Creek increased, which combined to increase specific conductance near Avondale. Arkansas River at Las Animas Streamflow at Las Animas is substantially smaller than at the upstream sites above Pueblo and near Avondale. The downstream decrease in streamflow is attributable to irrigation diversions upstream from Las Animas. Much of the streamflow at Las Animas is from irrigation -return flow (Cain, 1987). After 1974, streamflow at Las Animas increased substantially in every month. The increase in streamflow at Las Animas probably was caused by a combination of factors, includ- ing the WWSP and associated changes in seasonal irrigation practices and the increased importation of Colorado River Basin water for irrigation. Prior to the construction of Pueblo Reservoir and the beginning of the WWSP, most winter streamflow was diverted and consumed upstream from Las Animas. With the adoption and full implementation of the WWSP and the increased use of trans - mountain water for irrigation, streamflow at Las Animas increased significantly. Prior to 1978, about 14 percent of the annual streamflow at the upstream end of the lower basin flowed past Las Animas. After 1978, that percentage increased to about 26 percent. The increase in streamflow at Las Animas increased the dilution potential and resulted in substantial decreases in specific conductance between 1961 -74 and 1975 -94 (fig. 7). The median specific con- ductance decreased during the growing season from 2,820 µS /cm in 1961 -74 to 2,250 µS /cm in 1975 -94 (fig. 7). Similarly, the median specific conductance decreased during the winter - storage season from 3,300 µS /cm in 1961 -74 to 2,730 µS /cm in 1975 -94 (fig. 7). The increase in stream- flow and dilution potential at Las Animas apparently was large enough to offset the increasing specific- conductance trend in the Arkansas River measured farther upstream near Avondale. 3,400 W W z w 3,200 U Z cq Z U U 3,000 0nw.w U u) W 2,800 FL Z 0 W 0 W U) F, N 2,600 Z ¢¢ g 2,400 z 2,200 JOHN MARTIN RESERVOIR OPERATIONS John Martin Reservoir was completed in 1948 on the main stem of the Arkansas River between Las Animas and Lamar. Storage of irrigation water in John Martin Reservoir is by agreement between the States of Colorado and Kansas, under the Arkansas River Compact, which was signed in 1948. The Compact ensures that both States receive their percentage shares of Arkansas River flows (Abbott, 1985). The Compact dictates a winter- and a summer- storage period. During the winter - storage period, November 1 to March 31, most of the reservoir inflow is stored. Prior to 1980, provisions were made for the release of stored water, without reference to the volume of stored water assigned to each State. To ensure that each State received its share of stored water, release demands by each State were made concurrently. Although the Compact was to ensure that Colorado and Kansas irrigators received their legal shares of Arkansas River water, it did not "result in the most efficient utiliza- tion possible of the water in its control' (Arkansas River Compact Administration, 1980, p. 40). Prior to 1980, after the winter - storage period, reservoir storage usually was drawn down to empty or almost empty very early in the irrigation season, often by the middle of April (fig. 8). From 1955 through 1979, reservoir storage was com- pletely depleted by April 30 in 15 of the 25 years. To help alleviate the problems associated with historical reservoir opera- tions, a new reservoir operating plan was adopted in 1980 (Arkansas River Compact Administration, 1980). Under the new plan, water not immediately called for and released to downstream irrigators 1961 -74 ® 1975 -94 GROWING WIN I Eli - SEASON STORAGE SEASON Figure 7. Specific conductance for the Arkansas River at Las Animas, 1961 -94.