WMOD00011

• Applicant: Don Griffith North American Weather Consultants, Inc.
• Sponsor Name: City and County of Gunnison
• Project Name: Cloud Seeding Evaluation
• Title: The Development, Conduct and Evaluation of a Cloud Seeding Program for the Gunnison River Basin, Colorado During the 2003-2004 Season
• Prepared For: City and County of Gunnison
• Prepared By: North American Weather Consultants
• Date: 8/1/2004
• County: Gunnison
• Weather Modification - Doc Type: Report

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Contamination of target area sites from other cloud seeding programs is a consideration
as it is in selecting comrol sites. Unfortunately, our geographic range is vcry limited compared
to that for control sites since the target area is fixed. ~ormal1y one attempts to use all available
target sites unless there arc data quality problems. The Gunnison County project is in a peculiar
situation in that a cloud seeding program has been conducted over the Grand Mesa and at
times over the West Elk Mountains for a significant number of winter seasons (31 prior to the
2002-2003 winter season). We. therefore. have to accept the possibility of contamination
affecting our evaluations.

Seeding was conducted last winter from November 15th through April 15th. NAWC has
traditionally used whole seeded months in our evaluations of other operational cloud seeding
programs. since some historical data are not available at mid-month. This is especially true
when dealing \\'ith historical manual snO\\I course measurements. As a consequence. this
(Gunnison) analysis will focus upon the four-month period of December through March. As
stated earlier. separate regression equations were developed for the target set including and
excluding McClure Pass.

4.1.3 Regression Equation Development. December - March Period

~10nthly precipitation values were totaled at each gage site in the control and target areas
for the December-March period in each of the selected historical water years and averages for
each group were obtained. The predictor equation was developed from these data for the four
month period. The correlation coefficient (r) between the two groups for December-March was
0.87 when McClure Pass was included as a target site. suggesting a reasonably accurate predictor
equation. in this case accounting for about 76% of the difference between the predicted and
actual target area average precipitation when used for evaluation of seeding effects. The
correlation coefficient was 0.83 when ~1cClure Pass was excluded from the target group. which
means that the regression equation accounts for about 67% of the difference between the
predicted and actual target area average precipitation.

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