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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only the ""'aler left in the snO\~'pack. Also. ""'arm periods can occur betv.'een snowstorms. If a
significant warm period occurs, some oflhe snow may melt. Thus. some of the sno\l.' water may
not be recorded at the end of the month. even though some of the melted soo..... may have gone
into the ground to recharge the ground water. This can also lead to a disparity between SOO\l,"
water measurements at higher elevations (where less snow will melt in wann weather) and those
at lower elevations.

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Another variable that can affect the indicated results of the sno\'\."pack evaluation is the
date on which the snowcourse measurements are made. These measurements are generally made
near the end of the month. Since the advent of SNOTEL. daily measurements are available al
many of the sites. However, prior to S!\OTEL. and at those sites where snowcourses are still
measured by visiting the site, the measurement is recorded on the day it was made. In some
cases, because of scheduling or stonny \\:eather. these measurements have been made as much as
10 days before or after the end of the month. This can lead to a disparity in the snowpack water
content readings when comparing one group (such as a control) with another control or target

group.

In order to address the potential di fferences in the types of observations discussed above.
NA WC adopted the following procedure. Most of the snowpack data used in this analysis are
from sites that were originally snowcourse sites and became SNOTEL sites after approximately
1980. NA WC recognized that this could present a problem because of potential differences
between the snowcourse and S~OTEL measurement techniques. The !'\RCS has also recognized
and addressed this potenlial problem. Their solution \\.'as to obtain concurrent data at the newly
established S~OTEL sites using both (collocated) measurement techniques for an overlap period
of approximately 10 years in duration. They then developed correlations between the two types
of measurements and applied a site-specific correction factor at each site that converted the
previous monthly snowcourse measurements to estimated values as if the S!\'OTEL
measurements had been available at these sites. The !':RCS also attempted to correct the timing
problem in these estimates to reflect first of the month values. In other words, if an historical
year had a measurement taken on the ~;Ih of Januarv instead of the first of Februan.', the i\RCS
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used adjacenl precipitation data to estimate the snow water content on the first of February. The

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