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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nr!3334

It should be stressed that the virgin flows shown in Fig. 7 are not an

exact representation of historic conditions. This is because the calculated

regression equa~ion only accounted for 40 percent of the observed variation.

The remainder is accounted for by a random component. Somewhat different

results would have been obtained if a different set of random seeds had

been used. Tree-ring data for the stations at Estes Park and Owl Canyon

are included as part of the technical addendum, together with the virgin

flows generated from the data.

Several statistical checks were performed on the 400 years of virgin

flow data generated from the tree rings and it was concluded that the

record statistically matched quite well the 50,OOO-year record that had

been previously generated using purely stochastic methods. The two data

records had essentially the same mean, variance, and, most importantly, the

same type of drought patterns. It appears that the runoff patterns which

were observed over the past 100 years are statistically similar to patterns

which probably occurred in the past 400 years, which in turn are statisti-

cally similar to patterns in the 50,000 years of stochastically generated

data.

v

Determination of drought characteristics

Numerous methods for drought definition are found in the literature.

Since the City of Fort Collins water supply is dependent primarily on the

river flows generated by snowmelt runoff, the droughts for this study were

defined by annual flows on the Poudre River. A drought can therefore be

defined as a year or series of years of flow below a given level. For this

study the average annual flow was used as the level for drought definition

(i.e., flows less than average are considered drought flows). The idea of

using levels less than average (e.g., 50% or 75% of average) for drought

definition was studied, but since average flows are more commonly used for

drought definition, it was decided the average annual flow concept would be

used for definition purposes. Also, due to the method of water supply

analysis used, the results of the study would not be significantly affected

by the drought definition used.

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