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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A great deal of information is presented in the report, especially in

Sections II and III. A technical addendum contains additional backup data.

A briefer summary report is available that highlights only the major

aspects and conclusions of the study.

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Drought definition

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A decision made early in the drought study was to utilize, to the

extent possible, drought terminology as found in published literature on

the subjec.. A drought is usually d~fined as a year or series of years

with below normal runoff. This definition is basad on the use of stream-

flow data as opposed to other parameters. such as precipitation or soil

moisture indices. The drought must be consecutive, starting in the first

year with below average runoff and continuing through a series of years

until the drought is broken by a year with above average s.reamflow. An

extended dry period, that happens to include within it a single year of

above average runoff, is not classified as one continuous drought. Instead,

it is considered to be two separate droughts. at least according to the

commonly accepted definition.

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It is also standard to classify droughts according to the following

three criteria: duration, cumulative deficit, and average annual shortage.

Duration is the total number of years spanned by a drought and is also

referred to as run length (RL). Cumulative deficit is the total shortage,

as compared to the average, experienced over the course of the drought; it

is also referred to as severity or run sum (RS). The average annual

shortage is equal to the total deficit.,divided by the drought duration; it

is also referred to as magnitude or run intensity (RI RS/RL).

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The drought criteria are shown graphically in Fig. 1. As can be seen

from the figure, it is possible in any given period for a drought with the

greatest total deficit to be different from a drought with the longest

duration and different from a drought with the worst average annual shortage.

It is somewhat common. however, for the drought with the worst total

deficit to be the same as the drought with the longest duration. The

drought with the worst average annual shortage is usually much shorter in

length that the other two, often spanning a period of only 1 or 2 years.

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RE:C)OURCE: CONC)UlTt:\NTC) INC

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