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

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140,000 F- 120,000 L5 W LL T 100,000 U a Z ui 80,000 F_ Z w Z Z 0 80,000 U 0 > 40,000 w U) w 0C 20,000 ❑ 1955 -79 1980 -94 JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC was stored in separate storage accounts for Colorado and Kansas. One State can call for the release of its stored water indepen- dently of the other State. This change in operating practices generally resulted in downstream irrigators waiting until later in the irrigation season to release their stored water to the river; therefore, the 1980 oper- ating plan has probably contributed to increased storage in John Martin Reservoir (fig 8). Storage of water in John Martin Reservoir also has benefited from increased inflow, as noted in the "Arkansas River at Las Animas" section. At Las Animas, located about 3 mi upstream from the reservoir, median annual stream- flow of the Arkansas River increased 142 percent from 76,400 acre -ft in 1955 -79 to 185,200 acre -ft in 1980 -94. JOHN MARTIN RESERVOIR OPERATIONS HAVE AFFECTED SPECIFIC CONDUCTANCE IN THE ARKANSAS RIVER Specific- conductance data from two Arkansas River sites located down- stream from John Martin Reservoir (fig. 1) were analyzed for changes that might have occurred after the adoption of the 1980 operating plan. These two sites are downstream from John Martin Reservoir and at Lamar. Data collected at each site were separated into two seasons: growing season (April 1— October 31) and winter - storage season (November 1— March 31). Data collected prior to 1980 were compared to data collected from 1980 through 1994. Arkansas River Below John Martin Reservoir After the adoption of the 1980 operating plan, streamflow at the site downstream from John Martin Reservoir increased substantially during the growing season and decreased slightly during the winter- storage season. The largest increase in the daily mean streamflow for any month was in July; the daily mean streamflow increased from 490 cubic feet per second (ft3 /s) in 1955 -79 to 878 ft3 /s in 1980 -94. The overall increase in streamflow for 1980 -94 was attributable to increased inflow into the reservoir and subsequent reservoir releases. In addition to increased inflow, a large increase in streamflow dur- ing the middle of the growing season proba- bly was attributable to the new operating 4,000 cr ww U J � � W 3,500 UZFn =) W J DUW ZCE U L � w 3,000 Uww W2p w CL Z O N 0= ¢ 2,500 U wg 2Z 2,000 plan, which allows downstream irrigators to release the stored water later in the growing season rather than at the very beginning of the growing season. Specific conductance changed appreciably after the implementation of the 1980 operating plan (fig. 9). During the growing season, the median specific con- ductance decreased from 2,180 µS /cm in 1955 -79 to 2,050 µS /cm in 1980 -94. Dur- ing the winter- storage season, the median specific conductance decreased substan- tially from 3,650 µS /cm in 1955 -79 to 2,640 µS /cm in 1980 -94. The overall decrease in specific conductance was a result of two main factors. The first was decreased specific conductance in the reser- voir inflow, as indicated at the Las Animas site. At Las Animas, the median specific conductance for the growing season decreased from 2,655 µS /cm in 1961 -79 to 1,855 µS /cm in 1980 -94, and the median specific conductance for the winter - storage season decreased from 3,500 µS /cm in 1964 -79 to 2,710 µS /cm in 1980 -94. The second factor was increased reservoir inflow and reservoir storage and a corre- sponding increase in mixing of seasonally low- specific- conductance water and season- ally high- specific- conductance water in the reservoir; thus, the range in specific conduc- tance narrowed after 1980. Arkansas River at Lamar Changes in streamflow and specific conductance measured at the Lamar site were very similar to changes at the site downstream from John Martin Reservoir. Sreamflow at Lamar increased substan- tially during the growing season; the largest increases occurred during July. However, streamflow also increased slightly during the winter - storage season. 1955 -79 1980 -94 GROWING WINTER - SEASON STORAGE SEASON Figure 9, Specific conductance for the Arkansas River below John Martin Reservoir, 1955-94.