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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these observations included the National Weather Service and the National Resources
Conservation Service. The four complete months with seeding activities (December through
March) were evaluated using SNOTEL precipitation observations from the target area. April 1st
snow water content values from SNOTEL or NRCS manually observed snow course sites were
utilized in a separate evaluation. Six sites in the target area (five used in last year's evaluation.
plus Slumgullion which was added to include infonnation from the new portion of the target
area) were correlated with eleven precipitation sites. Nine snow waler observation sites in the
target area (comprising five sites used last year, plus two sites unavailable last year (Crested
Butte and Keystone). plus t\\.o new sites from the new southern portion of the target area) were
correlated "lith five sites in non-seeded control areas. Historical periods were selected to
exclude earlier seeding projects that may have impacted the observations. Individual station
records were examined for data quality. Data from potential control sites were averaged together
in dilTerent groupings and correlated (using linear regression techniques) with the average values
from the target sites to detennine the best set of control stations. one that provided both a high
correlation with the target and also provided some geographic "bracketing" of the target area.
Reasonably good correlations were established between control and target areas. with r values
ranging from 0.83-0.87. These equations were then used to estimate the amount of precipitation
and snow water content that would be expected for the 2003-2004 winter program based upon
the observations at the control sites. These estimates were then compared to the observed
average precipitation and snow water content at the target sites.

The precipilalion e\'aluation discussed in seclion 4.JA indicated a seedin~ dfeel of
+3~%. The snon water contenl evalualion (seelion 4.2.2) su~~esled a lar~er effecl~
approximalelya 13-14% increase. It is unclear why the indicated increases in precipitation are
less than those indicated in the snow water contents. It is noted that a similar result was
indicated last year. NA WC's report for last year indicated a potential problem in the
precipitation measurements related to under catch of snowfall due to strong winds at the near- to
above-timberline locations in the target area. Examination of the plots of precipitation and sno\\"
water content from this past season did not seem to indicate such an effect. One possible
difference between the precipitation and snow water content evaluations that may partially
explain the different outcomes is the length of the historical periods used to develop the

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