CWCB Wintertime Weather Mod Report

• Project Type: General OWC
• Applicant: Weather Modificaton
• Project Name: Colorado Weather Modification Program
• Title: CWCB Wintertime Weather Modification Report Fall 2002 - Spring 2003
• Date: 7/1/2003
• Water Conservation - Doc Type: Final Report

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An earlier estimate on March 20,2003 of the seeding effects for the San Juan Water Conservancy District's seeded
area using just the historical and current observations for the two Snotel sites on Wolf Creek Pass indicated an
additional 1.8S" of precipitation on the Southwest side of Wolf Creek Pass. This amount of extra water over the
Upper San Juan watershed would produce a potential additional 29,000 Ac.Ft of runoff for the San Juan River
through Pagosa Springs.

An analysis of the timing of the precipitation increases from the seeding efforts indicated that slightly more that
half (53%) of the precipitation increase over the Upper Animas and La Plata River Basins occurred during the
February through April 7, 2003 operational period. This same relationship was true for the Eastern San Jaun
Program Area (54%) even though the Program ended three weeks earlier than the Animas Basin Program. This
was primarily due to the nature of this winter's weather systems in that thefust three months of the Seeding
Program weather was in a drying trend and the last two months plus of the Program's weather was all near normal.

The procedure for estimating the seeding increases was bases on two research analysis conducted by Western
Weather Consultants, LLC of two separate seeding programs ** conducted over the Colorado mountains utilizing
similar seeding applications and weather event identification criteria. Both of these analyses produced extremely
similar evaluations of seeding responses to the systems being treated. A composite of these results utilizing a
conservative estimate of these seeding responses to the associated prevailing wind direction of the storm system
has been developed by Western Weather Consultants and forms the basis of this process to estimate seeding
increases.

Western Weather Consultants also followed the accumulation of the precipitation at the various Snotel Sites in the
Colorado Mountain Region throughout this winter's seeding season. We looked at how the precipitation was
accumulating as a percentage of what normally accumulates during the seeding season. Plots of the analysis of this
data are included for the periods ending February 1,2003, March 1, 20()3 and AprilS, 2003 to show the trend of
the winter's precipitation accumulation and the impact of the large storm of mid-March, 2003

** Precipitation increase estimates were made utilizing previous research results from the Colorado River Basin
Pilot Project (research randomized seeding program), June, 1976; and 10 and 16 Year Data Sets from two separate
precipitation sources for the VaillBeaver Creek Seeding Program (operational seeding program), March, 2001

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