PROJ00441

• Contract/PO #: C153459
• Contractor Name: Loveland, City of
• Water District: 0
• County: Larimer
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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o Duration (in months)

o Probability of occurrence

o Amount of water available during the drought (or magnitude of
defie! ts)

The probability of occurrence can be stated as a "recurrence interval" of a

specified time period. The recurrence interval is the average amount of

time between droughts of similar duration and water availability. A

drought with a lO-year recurrence interval would occur, on average, once

during any ten year period.

In the past, the reliability of a water supply system was usually
determined by comparing the monthly demands on the system to the flow
available for a specific drought; this drought was then referred to as the
"design" drought. The design drought may have been selected from historic
droughts experienced in the region or estimated from statistical analysis
of available streamflow or precipitation data. The choice of which
particular drought of all drought possibilities to use as the design
drought was based on a combination of hydrologic and non-technical factors
such as:

o The frequency and severity of water supply shortages experienced in
the pas t.

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o The frequency of large variations in streamflow.

o Policy decisions, i.e., a City may decide to protect against a
certain drought event as a matter of policy.

o The availability of alternate sources of supply during drought
conditions.

Recent advances in computers has allowed the use of statistical and

stochastic techniques to supplement the analysis of historic data. These

techniques are used to generate a long-term (several hundred or thousand

years) record of "synthetic" streamflow. These synthetic streamflow

histories maintain the monthly, seasonal, and annual characteristics of the

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