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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Determination of drought characteristics for additional drought types

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A visual inspection of time plots of the annual Poudre River virgin

flovs (plots of both historic and stochastically generated data) revealed

that there are two types of droughts that routinely occur on the Poudre

which are not accounted for using the standard drought criteria. For one,

it is common for the year vith the lovest annual runoff to be grouped

together vith a number of other below-average years, vith many of the years

only moderately belov average. The average shortage over the course of the

drought is great but not as extreme as the average shortage in the worst

magnitude (run intensity) drought, which is normally only 1 or 2 years in

length. The situation is shown in Fig. 8, a bar graph of the historic

virgin flows on the Poudre River. Historically, the worst magnitude

drought was the drought of 1919; however. the drought that contained the

worst single year was the drought of 1953 to 1956 (containing the extremely

lov runoff year of 1954). The latter type of drought vould stress a water

supply relying only on direct flow decrees more than the former, primarily

because such a water supply must provide sufficient water during the worst

single year, a more severe case than the worst average series of years.

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The other additional type of drought which toutinely occurs on the

Poudre is a long-term drought that occasionally contains a year or years

with above-average runoff. Such a drought occurred historically from 1930

to 1941 (also shown in Fig. B). The drought was 12 years in length and

contained a single above-average year in 1938. This would be considered

two separate droughts, using the commonly accepted definition of drought;

however the drought could be interpreted as one consecutive dry period by

using running averages instead of individual years. Such an extended

drought would stress a water supply relying heavily on long-term carryover

storage. A single above-average year in the middle of the drought may not

be sufficient to adequately refill the reservoirs and thereby bring an end

to the period of stress. It was decided, somewhat arbitrarily, that a

drought of this type would end whenever the 4-year running average was

above normal. In other words, the average of the runoff in the first wet

year and in the following 3 years must be above average in order to insure

that the reservoirs would refill and that the long-term drought would be

broken. Using the 4-year running average seemed to provide acceptable

criteria to make this determination.

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RE:liOURCE: CONliULTt>.NT<i INC

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