9596

• UCREFRP Catalog Number: 9596
• Author: Ayers Associates.
• Title: Yampa River Basin Research Final Synthesis Report.
• USFW Year: 1999.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

Appendix B

4

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Daily gaged or synthesized flows first were expressed as a percentage of monthly flows:

Dail % = Daily cfs (gaged or s~thesized data) x 100
y Monthly sum of all daIly cfs values

Daily percentages were applied to each monthly CRDSS volumetric value (AF) to arrive at a
daily discharges in AF/day, which were converted to cfs as follows:

cfs = AF/day x

43.560 cubic feetl AF
86,400 seconds/day

We calculated the frequency, magnitude and duration of transgressions of an average daily flow
target of93 cfs for each of the four demand scenarios. We defined transgression frequency
(transgression-days) as the total number of days each year which failed to meet the flow target.
We defined the duration of transgressions as the greatest number of consecutive transgression-
days each year and defmed the magnitude of transgressions in terms of their "deficit" of the 93
cfs target. We calculated the monthly deficits in AF as the sum of the daily deficits (AF/day)
according to the formula:

AF/day = (93 - Q cfs) x

86.400 seconds/day
43,560 cubic feetl AF

Where Q is the estimated average daily flow less than 93 cfs. We calculated gross deficits as if
93 cfs were a minimum flow. To approximate historic flow conditions, we calculated net deficits
by subtracting gross historic deficits from gross 2025 or 2045 deficits. We did not calculate
daily surpluses for values of Q > 93 cfs, because surpluses were not used to offset deficits.

Results and Discussion

Statistically, the 17 years of the CRDSS are normally distributed, with 6 "dry", 6 "wet" and 5
"normal" years, where "normal" is defined as the mean annual historic discharge :!: Yz standard
deviation of the historic discharge of all 17 years. Wet years are those whose discharge is greater
than Yz standard deviation above the mean annual historic discharge, while dry years are those
whose discharge is greater than ~ standard deviation below the mean annual historic discharge.
Nine (53%) of these years exhibited no transgressions under either natural or historic demand
conditions. However, under 2045 demand conditions, 12 years (1975-81 and 1987-91) had at
least one transgression of93 cfs during July-October, including all of the dry and normal years,
and even one of the wet years (1978).

Summaries of daily flow data under natural, historic, 2025 and 2045 demand scenarios for the
median year (1975), driest year (1977), wettest year (1984) and the year with the driest low-flow
period (1990) at the Maybell gage (Tables 1-4) above the Maybell Canal diversion (Tables 5-8).
Daily flows were compared to the 93 cfs flow target to calculate frequency, magnitude and
duration of transgressions of the flow target. Transgressions are summarized in Tables 1-8 on
both a monthly basis and an annual basis. We also plotted 1975, 1977, 1990 and 1984 annual
hydro graphs for each of the four demand scenarios at the Maybell gage (Figures 1-4) and above
the canal (Figures 5-8). Tables 9-12 summarize total annual transgressions and deficits ofthe
flow target for all 17 years at the gage (Tables 9-10) and above the diversion (Tables 11-12).