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I I I I I I I I I I I I I I I I I I I 5. Is driven by available records. 6. Reports monthly discharges in acre-feet for the Yampa River above the Maybell Canal diversion and at the Maybell gage CRDSS Inout - Monthlv Average Instream Flow Targets If daily flows were normally distributed, they would exceed the average monthly flow as often as they fall below it. Recognizing that day-to-day flow variations have occurred historically (both pre- and post-human influence), we modified the flow target in the CRDSS.to account for the normal distribution of daily data below the mean. The CRDSS model applied these modified flow targets as "demands" for fish and assigned them a priority junior to all currently existing water rights. These monthly mean values were based on the 93 cfs flow target plus 1f2 standard deviation of the daily variability of the lower 20th percentile months (by volume) during the entire period of record at the Maybell gage for each of the low-flow months as follows: August 93 + 1f2 (66) = 126 cfs September 93 + 1f2 (44) = 115 cfs October 93 + 1f2 (44) = 115 cfs We set fish demand at zero cfs during the remainder of the year (November-July). Because the CRDSS output is a monthly discharge and because daily data in the lower 20th percentile months may not be normally distributed, we only used these modified flow targets as screening criteria to identify those months whose minimum daily flows were likely to violate the daily average flow target (93 cfs) at some time under either historic or future demand conditions. Months whose average flows fell below these criteria, as well as adjacent months both within and outside August-October, were examined in detail for daily transgressions of the 93 cfs flow target. In all cases, months failing to meet their specific screening criterion exhibited one or more daily transgressions of 93 cfs. A preliminary analysis revealed that in one CRDSS year (1977), daily average flows~auring the month of July frequently fell below the 93 cfs flow target. Therefore, we expanded the low-flow period under evaluation to include July 1977. CRDSS Output and Data Analysis Methodologv To provide continuity during the July-October low-flow period under evaluation, we reformatted the CRDSS water-year output to a calendar year basis, excluding October-December 1974. Data for calendar years 1975-1990 and January-September 1991 are reported below. We distributed the CRDSS monthly discharge on a daily basis in the same pattern as the gaged or synthesized6 historic daily flows, converted daily AF discharges to average daily flows in cfs, and compared these data for July-October of each year with the flow recommendations of Mod de et al. 6 Daily flows above the Mayben Canal diversion were distributed based on the Mayben gage discharge plus concurrent Mayben Canal diversions during the CRDSS period of record. Appendix B 3