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

yielded a value of about 32,000 acre-ft passing over 31 kIn of crestline in a 2-month period. That is
equivalent to 1.0 times the mean annual runoff when extrapolated to a 4-month winter (Supc::r et al., 1989).

Similar calculations for a 4-month period over the much wetter American River Basin in the Sierra Nevada
showed the flux was only about 13 percent of the 1984185 water year runoff (Heggli, 1986). However,
it was estima~d that 189,539 acre-ft of liquid water flux passed over the 60 kIn crestlim: of the basin,
which would be equivalent to almost 158,000 acre-ft for the 50 kIn length of the Tushars. That value is
larger than all the 5-month estimates of table 3.2, but only about a factor of two higher than the lowest
Tushar value. The Mogollon Rim value, when adjusted to 50 kIn and 5 months, would be about 129,000
acre-ft. Thus, similar seasonal flux amounts were estimated at all three locations. The American River
runoff was obviously considerably larger than that of the other drainages or its flux would represent a
much higher percentage of runoff.

Admittedly the entire winter SL W flux estimates of table 3.2 may be crude because of the underlying
assumptions and limited periods of observations. Nevertheless they strongly suggest that: a substantial
amount of excess SL W passes over the Tushar Mountains each winter unconverted to snowfall. Similar
observations made over the Wasatch Plateau dUJing early 1991 have yet to be analyzed. However,
amounts and durations of SL W observations appeared approximately comparable with those from the
Tushars and it is expected that similar flux estimates will result The important point from all these
estimates is that nature provides abundant SL W flux over the Sevier Basin Region that is not converted
to precipitation. Thus, the required "raw material" for cloud seeding exists in abundance. The challenge
for the future will be to detennine what fraction of the flux can be practically converted to snow on the
ground by an optimally designed and conducted seeding program.

Table 3.2. Summary of SLW flux each season compared with
mean annual runoff from the Beaver River.

Season

1985
1987
1989

Duration of
SlW data
(months)

1.5

1.0

1.5

SlW flux for
pe riod of data
(acre-ft)

21,430

23,360

42,475

Est. flux for 5-
month winter
(acre-ft)

71 ,430

116,800

141,585

5-month flux as % of
Beaver River me::ln
runoff

187

305

370

3.2.2 T&D of seeding material. . Given that ~bundant SL W has been documented, we will turn our
attention to the T &D of the AgI seeding agent commonly used in winter orographic projects, and to
infonnation on stonn stage and structure that relates to both SL W and T &D.

The effectiveness of ground-based AgI seeding of winter orographic clouds is still an open question. Hill
(l982b), Long (1984), Long (1986), Huggins (1990a) and Huggins (199Ob) all discuss results that suggest
the low level generators used in the Utah operational seeding may not adequately target the SL W in some
stonn conditions. Even the use of high altitude generators does not guarantee adequate coverage by
seeding plumes as shown by Reynolds et al. (1989). The delivery of appropriate concentrations of ice
nuclei to supercooled cloud regions is likely the most difficult problem facing winter orographic cloud
seeding. Discussion of some of the uncertainties involved with T&D has been given by Rangno (1986),
Super (1990) and others. Because of these uncertainties, investigation of the T&D of groood-released
seeding material was the main focus of the early 1991 field project in the Wasatch Plateau, located in the
northern portion of the Sevier River Basin.

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