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

303315

I

I

II. Generation of Stochastic Record
and Selection of Typical Droughts

Calculation of historic annual virgin flows

I

I

The first step in the drought study was to assimilate historic stream-

flow records and to adjust the records for upstream diversions, imports,

ann changes in reservoir storage. The resulting adjusted records are

commonly referred to as ,virgin flows and are representative of streamflows

that would have occurred without any regulation by man-made facilities. It

was necessary to use virgin flows so that the actual severity of historical

droughts could be determined. The statistical characteristics of the

historic virgin flow data would he used in the generation of a long-term

stochastic record.

I

I

I

I

It was decided to consider not only the historic virgin flows in the

Poudre River basin but also the flows in the adjacent watersheds. This is

because rather substantial imports from the Laramie, Michigan, and Colorado

rivers supplement the native runoff and help to lessen the impact of

drought on the Poudre. A relatively small amount of water is also

occasionally diverted from the Big Thompson River as part of the C-BT

(Colorado-Big Thompson) system. An evaluation of droughts on the Poudre

should therefore consider concurrent conditions in the surrounding

watersheds.

I

I

I

I

The gaging stations selected for calculating virgin flows on the

Poudre, Laramie, Michigan, Colorado, and Big Thompson rivers are listed in

Table 1; their locations are shown in Fig. 2. The approximate locations

where water is imported to the Poudre River basin are also shown in the

figure.

I

I

The following basic equation was used in calculating virgin flows from
the gaged streamflow records:

I

Virgin flow = Gaged streamflow + upstream diversion
- upstream imports
+ change in reservoir storage

I

I

6

RE:)OURCE: CON)ULT~NT) INC

I