PROJ00273

• Contract/PO #: C153368
• Contractor Name: Cache LaPoudre Water Development Study
• Water District: 0
• County: Larimer
• Bill Number: SB 81-439
• Loan Projects - Doc Type: Feasibility Study

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amount imported into the upper basin through the transbasin import facilities
to the west. '!he consumptive uses and effects of regulation asslDUed to be
minor were not included in the adjustment. The resulting flows are plotted in
Figure 1II-6.

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The decived annual native flows exhibit the same general trends as do
the recorded annual flows discussed earlier in this Chapter. The early pe-
riod, prior to 1905, is "wet". Then the records move through a transition to
a dryer period beginning in 1925. '!he estimated native flows for the dryer,
period, represented by the 56-years from 1925 to 1980, averaged 271,000 acre-
feet per year.

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4. Selection of the Hydrologic Study Period

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It appears that the Cache la Poudre River is still in a period of a
long cycle which is "dryer" than normal. As long-term climatic changes are
not predictable, it is prudent to select the long, dry period as the norm for
any study. Thus, the statistics for the estimated native flow at the mouth of
the canyon from 1925 through 1980 have been assumed in this study to represent
what future native flows from the upper basin will be.

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Since a 56-year record is exceedingly long for the purposes of recon-
naissance level reservoir operation studies, a shorter period, 1949 through
1971 was selected as representative of the long term conditions. Called the
hydrologic study period, this 23-year portion of the total record also has an
average annual estimated native flow of 271,000 acre-feet. Along with the
average annual flow, the shorter 23-year hydrologic study period exhibits
other statistical characteristics similar to the longer 56-year period. The
hydrologic study period also contains the driest year of record, 1954, for
which the estimated native flow at the mouth of the canyon was only 121,000
acre-feet, and the driest four consecutive years of record, 1953 through 1956.

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5. Derivation of Monthly Rative Flows

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Native streamflow at strean~aging stations in the upper basin and at
all existing and potential damsites considered in this study were estimated
for the hydrologic study periOd by correlation with the flow records at the
streamgages within the basin. The correlations were done using HEC-4, the
U.S. Army Corps of Engineers' generalized computer program for streamflow
s imulat ion.

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The native streamflows were derived as monthly flows and extrapolated
as necessary to the length of the study per iod. In determining the stream-
flows at the existing reservoirs and potential damsites, consideration was
given to the relation between annual precipitation; the size, shape, orienta-
tion and elevation of the watershed; and direction of storm movement. The
correlations with the HEC-4 model produced monthly values for the length of
the study period for each existing reservoir and potential damsite. The
monthly flow records and results of these derived streamflows are. available in
computer files.

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1II-3

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