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

I
I
1
1
1
I'
I'
1
I
1
I
I
I
1
I
I
1
I
I

003~13

The drought study focused primarily on droughts with the greatest total

deficit (the third type of drought in Fig. 1) as being the most stressful

condition for water supplies on the Poudre River.

Summary of procedures

It was necessary in the study to generate a long-term stochastic

record of virgin flows on the Poudre River. A stochastic record is a

synthetically generated long-term streamflow record which has the same

statistical 'parameters as the historic record. A long-term record was

needed to determine average characteristics of l-in-20, l-in-50, l-in-l00,

and l-io-500 droughts. The historic period of record was too short for

such a determination, especially for the 1-in-500 drought. The procedure

consisted of calculating the statistical characteristics of the historic

record and then using this information in an appropriate stochastic model

to generate a much longer synthetic record of annual virgin flows on the

Poudre. The synthetic record was next subdivided into increments of 20,

50, 100, or 500 years. The worst droughts that occurred in each increment

were determined and then averaged together. The results represent droughts

that can be expected to occur, on the average, once every 20, 50, 100, or

500 years. Stochastic techniques were also used to generate long-term

synthetic records for virgin flows in adjacent watersheds and for imports

to the Poudre River basin. Comparisons to long-term tree-ring data were

used to help check the validity of the stochastically generated data.

Four sets of droughts were selected from the synthetic record as being

representative of typical 1-in-20, 1-in-50, l-in-l00, and l-in-500 condi-

tions. Statistical disaggregation techniques were used to estimate monthly

flows from the annual data. Monthly flows were generated for the Poudre

River virgin flow and for various sources of imported water to the basin.

The stochastic modeling was performed under the direction of Dr. Jose Salas

from Colorado State University in conjunction with personnel from Resource

Consultants, Inc. Dr. Vujica Yevjevich of George Washington University

provided input and reviewed the results of the stochastic modeling portion

of this study.

4

RE:'iOURCE: CON'iUlT~NT) INC