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12 Evaluation of Streamflow Losses Along the Gunnison River from Whitewater Downstream to the Redlands Canal Diversion Dam, near Grand Junction, Colorado, Water Years 199x2003 Table 2. Cumulative annual and annual mean discharges for streamflow-gaging stations at the upstream and downstream ends of the Gunnison River study reach and difference between these discharges, water years 1995-2003. [acre-ft, acre-feet; ft~/s, cubic feet per second; see figure 1 and table 1 for station details] ater ear V nits hitewater station (upstream) Discharge, for indicated water year, in indicated units Sum of Below-Redlands- Below- Redlands-Canal dam and Redlands-dam station station Redlands-Canal stations (downstream) Difference (down- stream - upstream) 1 Percentage difference 21995 acre-ft 2,803,400 2,475,800 318,400 2,794,200 -9,200 ft3/s 4,977 4,395 565 4,960 -16.3 -0.33 1996 acre-ft 2,007,600 1,400,000 546,300 1,946,300 -61,300 ft3/s 2,773 1,934 752 2,681 -84.5 -3.05 1997 acre-ft 2,817,000 2,226,900 558,000 2,784,900 -32,100 ft3/s 3,891 3,076 771 3,847 -44.3 -1.14 1998 acre-ft 2,093,000 1,505,600 542,200 2,047,800 -45,200 ft3/s 2,891 2,080 749 2,829 -62.4 -2.16 1999 acre-ft 1,692,700 1,051,300 571,600 1,622,900 -69,800 ft3/s 2,338 1,452 790 2,242 -96.3 -4.12 2000 acre-ft 1,466,400 863,000 542,000 1,405,000 -61,400 ft3/s 2,020 1,192 747 1,939 -84.5 -4.19 2001 acre-ft 1,170,700 547,300 555,700 1,103,000 -68,000 ft3/s 1,617 756 768 1,524 -93.5 -5.81 2002 acre-ft 805,100 296,900 479,500 776,400 -28,700 ft3/s 1,112 410 662 1,072 -39.6 -3.56 2003 acre-ft 860,100 522,300 333,800 856,100 -4,000 ft3/s 1,188 721 461 1,182 -5.43 -0.47 ~ Percentage difference =[(downstream discharge -upstream discharge) /upstream discharge] x 100 2For the March 1-September 30, 1995, period. 2004). In order to evaluate the apparent change in downstream discharge, it was decided to expand the previously completed mass-balance analysis to include WY 2003. In addition to using the published daily mean discharge data for the Whitewater and Redlands-Canal stations for the mass-balance analysis, daily mean discharges for WY 2003 that received some quality assur- ance also were computed for the below-Redlands-dam station (Jerry Thrush, Colorado Division of Water Resources, written commun., 2003). Methods for mass-balance analysis for WY 2003 were identical to those used for WYs 1995-2002, but the results were somewhat different. The difference between the sums of daily mean discharges at the two downstream stations and the daily mean discharge at the upstream station, about x,000 acre-ft, and the percentage difference, about -0.5 percent, for WY 2003 are substantially smaller than for any other water year, exclud- ing the 1995 partial year (table 2). Differences between the sum of the moving 3-day daily mean discharges at the two down- stream stations and the moving 3-day daily mean discharge at the upstream station, as well as the percentage differences, for WY 2003 (fig. 10) are substantially smaller than those for WY 1996 (fig. 7), which is representative of WYs 1995-2002. In addition to a smaller annual difference for WY 2003 (table 2), the variability in the differences between downstream and upstream moving 3-day daily mean discharge also was much less during WY 2003 than during the previous years (figs. 8-9). Although the IQRs for discharge difference and percentage difference for WY 2003 are not the smallest, 2003 is the only water year for which the medians are near zero (figs. 8-9). The near-zero annual difference (table 2) and near-zero medians for WY 2003 (figs. 8-9) resulted from generally negative differences during the first one-half of the year and