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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The 50,000 years of generated data were used to determine the worst

average droughts that could be expected to occur on the Poudre River once

every 20, 50, 100, and 500 years. In the case of the l-in-500 drought, the

process consisted of subdividing the data record into 100 traces of 500

years each. The worst duration, severity, and magnitude droughcs that

occurred in each 500-year trace were then determined. The resulting

droughts (a total of 100 for each drought criteria) were averaged to arrive

at the worst average drought that could be expected to occur once every 500

years. The process was repeated for the l-in-l00, l-in-50, and l-in-20

droughts excepc that 500 traces of 100 years each, 1000 traces of 50 years

each, and 2500 traces of 20 years each were used, respectively. All the

analyses were performed using computer programs. The results are summarized

in Table 7. The results are based upon the standard drought terminology

and definition noted at the beginning of this report. An interpretation of

the table, for just the l-in-500 drought, is as follows:

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(1) The average l-in-500 drought, using worst duration criteria,

would be 10.8 years in length, although worst duration droughts

as short as 6 years and as long as 20 years were observed in the

100 traces of 500 years each.

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(2) The average l-in-500 drought, using worst severity criteria,

would show a total cumulative deficit of 890,000 ac-ft over the

duration of the drought. Worst severity droughts with deficits

'as little as 560,000 ac-ft to as much as 1,900,000 ac-ft were

observed in the 100 traces of 500 years each. The average length

of the worst severity drought would probably be similar to the

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10.8-year average of the worst duration drought.

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(3) The average l-in-500 drought, using worst magnitude criteria,

would show a shortage of 166,000 ac-ft per year for the duration

of the drought, which often would only he 1 or 2 years in length.

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As noted before, the drought characteristics determined from the

50,000-year record are similar to those observed in the historical record

and in the 400 years of record generated using tree rings. Table 8 is a

comparison of the drought characteristics resulting from the three data

records.

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RE:~OURCE: CON~ULTl\NT~ INC

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