WSP07449

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 7/1/1985
• Author: Resource Consultants
• Title: Droughts and Their Effect on the Water Supplies for the City of Fort Collins Colorado
• Water Supply Pro - Doc Type: Report/Study

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JJ333l\

tree-ring growth to determine if conditions in the past several hundred

years, as measured by old trees, could have been significantly different

from conditions within just the past 100 years. Studies by other investi-

gators, primarily on the Colorado River, have shown correlations between

annual tree-ring indices and annual virgin flows.

Computerized tree-ring data were obtained from the Laboratory of

Tree-Ring Research (Ref. 3) for the following stations in the vicinity of

the Poudre River basin: South Platte, Estes Park, North Park, North

Platte, Owl 'Canyon, and Horsetooth Reservoir. Numerous regression analyses

were performed correlating tree-ring indices with virgin flows in the

Poudre River. A correlation was found using the Estes Park station; the

correlation was improved by also including the tree-ring index for the Owl

Canyon station. The resulting regression equation is as follows:

Ln (PRVF) = 8.29 + 0.321 Ln(ESTE) + 0.298 Ln (OWL1)
(R2 = 40%)

where Ln = natural log. PRVF = annual Poudre River virgin flow in ac-ft,

ESTE = annual tree-ring index for Estes Park (mean of 1000) and OWLl

annual tree-ring index for the Owl Canyon 1 station (mean of 1000).

The extent to which annual runoff is related to the tree-ring indices

is further demonstrated by Fig. 6. The figure shows the average tree-ring

index for the two stations overlaying the annual Poudre River virgin flows

for the years 1884 to 1964. These are the years common to both data

records. As can be seen, extended periods of below average tree-ring

growth generally correspond to periods., of maj or drought on the Poudre,

although the correlation is not perfect.

The regression equation noted above was used, together with a random

component, to generate a set of virgin flows for the Poudre River corre-

sponding to the period of historic tree-ring records, in this case the

years 1572 to 1964. The results are shown graphically in Fig. 7. In the

figure, a bar graph of the average tree-ring index at the two stations is

placed above a bar graph of the corresponding generated virgin flows. The

length of record is about 400 years.

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RE:C;OURCE: CON~ULT~NT~ INC