WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

altitude generators consistently produce AgI plumes over mountainous terrain within the SL W zone.
However, additional T &0 investigations are needed to enable improved delivery of adequate
concentrations of ice nuclei to SL W cloud regions.

A large percentage of winter storms are too warm for seeding with ground-released AgI which has limited
effectiveness at temperatures above about -8 oC. Even the expensive option of aircraft AgI seeding might
not be effective in many of these cases because the SL W is concentrated near the ground where the
temperature may be too wann for significant ice crystal formation. Options such as propane seeding
should be explored for wann storms so the effectiveness of seeding may be optimized.

Storms typically go through a series of stages in their passage over a mountain region. Studies of these
storm stages are reviewed with emphasis on the availability of SL W and the T &0 of ground-released Agl.
It is shown that seedability often varies considerably during a storm passage and also from storm to storm.

Long-term precipitation and temperature records are considered from a high altitude weather station in the
Sevier Basin, These provide estimates of typical stonn frequencies and precipitation amounts and how
these range from season to season. Precipitation days, which can be considered a proxy for days in which
seeding might be conducted, varied from 30 days during dry winters to 60 days during wet winters.
Although the number of precipitation days are variable, even dry winters appear to have enough for
frequent seeding operations. Temperature data indicated nearly 80 percent of the days with snowfall were
-6 oC or colder at the weather station.

Review of the available evidence indicates several uncertainties about the effectiveness of cloud seediitg
as currently practiced in Utah and the West in general. A number of options are available to deal with
these uncertainties as briefly discussed, The option recommended is believed to offer the most rapid
means of removing uncertainties associated with the Utah operational cloud seeding program, but it is a
relatively expensive option. The recommended approach is to conduct a demonstration program in the
Sevier River Drainage to validate winter orographic cloud seeding. Accordingly, design considerations
are reviewed and the best experimental area is selected for the basin. A two-stage program is proposed
that would emphasize cause-and-effect physical experiments over a limited area during the first phase.
The second phase would consist of a statistical/physical experiment to confinn the multi winter increases
that seeding could produce over a large area.

Preliminary benefit-cost estimates are given based on an assumed increase in seasonal. snowpack with
subsequent runoff calculated by a snow accumulation and ablation model. An economic analysis by the
Utah Division of Water Resources provided the value of the calculated additional water. Two costs of
operational seeding were used. The first reflects the costs of the Utah operational seeding program as
currently conducted, using widely spaced valley generators. The second estimate reflects the additional
costs of using closely spaced, high output generators well up the windward slopes to improve targeting.
The results indicate benefit-cost ratios from about 3:1 to more than 10:1, depending upon the assumptions
made. If winter cloud seeding can provide mountain snowfall increases approaching or exceeding 10
percent, the technology should ltave good economic justification.

iv