WSP09401

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 8/28/1986
• Author: CDM
• Title: Phase I - Drought Study - City of Loveland Raw Water Supply System
• Water Supply Pro - Doc Type: Report/Study

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o Duration (in months)

o Probability of occurrence

o Amount of water available during the drought (or magnitude of
deficits)

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 past.

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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