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 stochastically generated traces in the technical addendum (5

traces of 500 years each) were reviewed in order to determine the average

characteristics of the two additional types of drought, i.e., the drought

containing the worst single-year and the 4-year running average drought

with the worst long-term deficit. The results are summarized in Table 9.

In the case of the l-in-500 drought, the procedure consisted of manually

determining the worst droughts that occurred in each of the 5 traces and

then averaging the droughts to arrive at the worst average dtought that

could be expected to occur once every 500 years. The procedure was

repeated for the l-in-l00 drought except that 25 traces of 100 years each

were used. The l-in-20 droughts were determined using only data set 2 with

25 traces of 20 years each. Characteristics were not determined for the

l-in-50 droughts, but they can be estimated based on the 1:20 and 1:100

droughts.

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It should be noted that the results in Table 9 are only approximate.

More accurate figures could be obtained using the entire 50,000 years of

generated record and using computer programs. The results based on only 5

traces of 500 years each are sufficiently accurate, however, for approxi-

mating the general characteristics of the two additional drought types.

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Selection of typical l-in-20, l-in-50, l-in-l00, and l-in-500 droughts

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It is very difficult to select typical l-in-20, l-in-50, l-in-l00, or

l-in-500 droughts from the long-term synthetic record. That is because

every drought is different in its o~ way and because a drought that may be"

extreme in one respect, such as severity (or total deficit), may not be

that bad in another, such as magnitude (or low average annual flows). It

was decided, from a water supply standpoint. that three types of drought

should be considered: (1) a drought containing the worst single year,

(2) a drought with the worst severity (or total deficit over consecutive

dry years), and (3) a 4-year running average drought with the worst long-

term deficit (where an occasional above-average year does not necessarily

end the drought). The three types of drought were considered because it

was believed that the first would stress direct-flow water rights, the

second would stress short-term reservoir storage, and the third would

stress long-term reservoir storage.

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