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Analysis of Historical Discharge Records 11 w U Z W lr Iwi 0 LL_ W Q ~ -2 Z w U W w Il -4 EXPLANATION o Upper far outlier x Upper outlier Upper adjacent 75th percentile Median 25th percentile Lower adjacent x Lower outlier o Lower far ouTlier (See text for additional explanation of boxplo[ values.) -8 -6 Whitewater Below-Redlands- Redlands-Canal station dam station station (See figure 1 and table 1 for station details) Figure 6. Distribution of percentage differences for applied shifts for measurements made at stations along the Gunnison River study reach, water years 1995-2002. rily is due to the different types of y-axes-Logarithmic in figure 7A and arithmetic in figure 7B. Distributions of daily differences between the moving 3-day daily mean discharge sum at the two downstream stations and the moving 3-day daily mean discharge at the upstream station and the percentage differences during WYs 1995-2002 are shown in figure 8. Excluding WY 2003, differences between the downstream and upstream 3-day daily mean dis- chargesranged from about -200 to +100 ft3/s during one-half of each year (the values within the IQR), but the differences had absolute values as large as about 500 to 1,000 ft3/s during the other one-half of the year (the values outside the IQR). Daily percentage differences each year (1995-2002) almost always ranged from 0 to -10 percent within the IQR and were as small or large as about -60 to +50 percent outside the IQR. The below-Redlands-dam station does not have a bridge or cableway nearby from which high-discharge measurements can be made, and the highest wading discharge measurement made at the station during WYs 1995-2002 was about 1,800 ft3/s (fig. 4). Consequently, discharge rating shifts might have con- siderable error when discharge is larger than about 1,800 ft3/s. Therefore, a subset of the moving 3-day daily mean discharge data was analyzed that only included those data for which the 3-day daily mean discharges at the below-Redlands-dam station were less than 2,000 ft3/s. For the subsetted data, the height of the IQR box (between 25th and 75th percentiles) for both the discharge differences and the percentage differences (fig. 9) is somewhat smaller than for all the data (fig. 8). The range of the discharge and percentages differences outside the IQR is nearly as large for the subsetted data (fig. 8), as for all the data (fig. 9), although the number of values each year is smaller for the sub- setted data. Overall, differences between the subsetted data and all the data are not large, indicating that the differences can be large even during times of smaller discharge. Results for Water Year N10~i Near the end of WY 2003, water-resource managers that used the real-time data at the three stations noticed that the general trend of decreased downstream discharge observed dur- ing the previous years seemed to have changed to an increase in downstream discharge (Coll Stanton, Bureau of Reclamation, oral commun., 2003). The reasons for the apparent change in downstream discharge were not known, but perhaps the change might be attributed to some changes in the equipment at the below-Redlands-dam station made earlier during WY 2003. At the station, the nitrogen bubbling system, the nitrogen line feeder tubing, and the orifice end cap were replaced on February 21, 2003; the new bubbling system automatically purges the nitrogen line to the stream every 15 minutes, decreasing the possibility of a plugged orifice (Jerry Thrush, Colorado Division of Water Resources, written commun.,