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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It appears that whenever low flows occur in the Poudre River drainage,

low flows can be expected in the other river drainages. This is unfor-

tunate from a water supply standpoint since it would be quite beneficial to

rely on excessive runoff in the surrounding watersheds whenever runoff is

low on the Poudre. However, just the opposite is generally the case.

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Basic statistical analvsis of annual virgin flows

The historic annual virgin flows in Table 3 were statistically analyzed

using two different time periods: (1) the 100 years from 1884 to 1983 and

(2) the 73 years from 1911 to 1983. The latter was used because the

calculated virgin flows for the Poudre River were less accurate in the

early years of record (primarily due to a lack of diversion records) and

because statistically there was a non-homogenity at this station between

the early and remaining years of record. Basic statistics for the virgin

flows at the five gaging stations are summarized in Table 5. The results

are for the time period 1911 to 1983.

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The relatively significant skewness coefficient for the Poudre River

(1.046) is indicative of a non-normal distribution. In fact. the Poudre

River annual virgin flows follow more of a log-normal distribution as

demonstrated by Fig. 4, a probability plot where the annual Poudre River

flows for the years 1884 to 1983, on a log scale, have been plotted against

probability of occurrence.

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The annual virgin flows at the five gaging stations were also tested

for any auto-correlation with the runoff in the preceding year or years.

The results are summarized in Table 6.. The virgin flows were found to be

essentially not correlated with previous runoff, however lag 1 auto-

correlation coefficients of 0.137 and 0.141 were calculated for the Poudre

and North Fork of the Michigan rivers. respectively. These results are

based on data for the period 1884 to 1983. The auto-correlation coeffi-

cients are quite small indicating that the probability of one dry year

following another is only slightly greater than what could be expected from

a purely random standpoint. The coefficients are so small that statisti-

cally it can not be concluded that there is any auto-correlation at all.

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

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