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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Mesa and West Elks programs. A new' regression equation was developed for this data set and
then used to estimate the December. 2003 through March. 2004 target area precipitation. The
indicated effect of seeding using this approach was then 8% instead of 3%.

~.2 Snowpack (\\'ater Equh"alent) A.nalysis

Historically. the Soil Conservation Service (SCS). now the Natural Resources
Conservation Service (!\RCS). routinely measured the mountain snowpack depth and sno\~/
water content at snowcourses once or twice a month. usually starting in January and continuing
until about June 1. ~1easurements were made by visiting the snowcourse. taking a series of core
samples of the snow and determining the water content and depth of the snow. This type of
sampling is still being done at some sites. However. in more recent years (since about 1980). the
advent of NRCS's SNOTEL measurement and telemetry system has allowed daily measurements
of snow water (and precipitation) at many \vestern mountain sites. With the use of a snow
pillow, the \'later equivalent of the snov,;pack can be detennined by remotely sensing the weight
of the snow on the snow pillow.

The water content within the snov.'Pack is important since. after consideration of
antecedent soil moisture condilions, it ultimately detennines how much water ","ill be available to
replenish the supply when the snow melt occurs. Hydrologists routinely use lhe sno\'" water
conlent measurements to make forecasts of streamflow during the spring and early summer
months. Consequently. it was judged important to evaluate the effects of seeding on snowpack
water content as well.

Colorado has excellent historical snowcourse and S~OTEL snow pillow data collected
by the NRCS. ~any of the same mountain reporting sites are available for both precipitation
and snowpack measurements. However, there are some pitfalls with snowpack measurements
that must be recognized when using snow water content to evaluate seeding effectiveness. One
potential problem is that not all winter stonns are cold for their full duration, and sometimes rain
as well as snow falls in lhe mountains. This can lead to a disparity between precipitation totals.
which theoretically measure eWT)1hing that falls. and snowpack water content which measures

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