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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J)33:8

Although the equation is simple, a relatively large work effort was required

to collect and interpret the diversion record data. Table 2 is a listing

of the diversion records necessary for calculating virgin flows at each of

the five gaging stations. Previously prepared summaries were used, to the

extent possible, to reduce the work effort and to serve as checks on the

results. Data in a report on the Cache la Poudre project study prepared by

Tudor Engineering Company (Ref. 1) were useful in this regard, as were

annual summary tables of diversion records prepared by the local Water

Commissioner.

The resulting annual virgin flows for the five gaging stations

spanning the period of historic records are listed in Table 3. It should

be noted that the calculated virgin flows are, for a number of reasons,

only approximate. The results are sufficiently accurate, however, for the

purposes at hand, namely to estimate the statistical characteristics of the

historic record.

Cross-correlation of virgin flows

Statistical tests revealed that the annual virgin flows at the five

gaging stations, from Table 3, were cross-correlated with one another. The

calculated cross-correlation coefficients are summarized in Table 4; the

results are based on data for the period 1911 to 1983. The best correla-

tion is between the Poudre and Big Thompson rivers although virgin flows on

the Poudre are also correlated with the other river basins.

The extent to which runoff on the Poudre River is related to runoff on

the Colorado River is demonstrated graphically by Fig. 3, a bar graph of

the Colorado River virgin flows overlaying the Poudre River virgin flows.

The Colorado River was chosen for this figure since it is the major source

of imported water to the Poudre (via the C-BT system and Grand River

Ditch). Major droughts occurred on the Poudre in the 1930s, 1950s, 1960s

and 1970s. As can be seen from the figure, droughts also occurred on the

Colorado River during the same general periods although their length and

degree of severity were somewhat different.

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