WMOD00071

• Applicant: Western Weather Consultants
• Project Name: Vail Pass
• Date: 11/1/1984
• Weather Modification - Doc Type: Application

_Lower Soil Te~erature

A noticeable decline in forest populations of small
mammals occurred followinR winters of heavy snol../lal1.
This was most evident In the numbers of deer mice
(Perornyscus maniculatus) but it was also found in
chipmunks (Eutamlas min!mus) and in Xicrotus spp.
The basic reason for the population decline derives
from the delayed growth of ess@nrial ~orin~ foorl~ ~nrl
results primarilY from a delay In breeding so that
~fewer littar9 are produced. The delayed growth of
plants was a function of lower soil temperatures and
the longer snow cover. No similar effect was found
on the toad (Bufo boreas), or pocket gopher (Thomonvs
talpoides) populat~

.-Deeper Snowpack

As snov depth increaged~ elk (Cervus candensisl ~
to areas where snow was shallower than 40 cm. They
avoided re ions with mare than 70 ~m at penetrasre-
snow depth. A 15 percent increase n snowpac may
decrease available elk winter range by 8 percent.
Sprin~ mo~ement of elk ~as more stron~ly associated
with the-start of n~w KrOwth in mountain meadows than
directly with the recedinR snowline. Ample habitat
was available for elk calvinK belov th~ snowline and
without re~ard to snow depth.

Microbes

Plant litter in the tundra dec~eased in a~eas oE
deeper snowpack. Jh1s oresumably reflects removal by
the additional' flux of water through the syste~ rather
than an increase in decomposition rates.

Abiotic Factors

The effects of the snovpack on soil erosion resulted
from the greater area of bare soil left by the decrease
_.in vegetative cover around snowbanks. Those predicted
Lor the tundra area ot the San Juan Mountains derive
fro~ etcher the additional snow dePth or the additional
meltwater produced by cloud seeding.

-Deep.er SnoWP.,ack

If [he area of lon2-1astina snowbanks expand after an
increase 1n snowfall. local chanRes in the rates of
soil erosion on the bare surface around them may in-
crease by a factor of 10. This effect should not
extend beyond the immediate v1cinit of alread ex-
at ng snoW r ta an ao a uld not be important re~
~ional1 Y.

'. -Additional Melt'Water

Theoretical con9ideratlons SU&KeSt that an increased
streamflow generated by cloud seeding should be
associated with an increase in the total amount of
dissolved caterial removed from the target area. This
is supported by emp~rical observations of rock weather-
ing rates and stream water quality in the tundra
environmeat. However~ this effect will ~e 51i~ht Dnd
should not be concentrated spatially.

-Historical Climatolo&y

r

Temperature flUCtuations over the last 120 y~ars in-
dicate a cooling trend from the late 1860's to abOut
1930 ~hen a reversal occ~rred; precipitatiun trends
are inversely rel::tted to temperature l:=-;:;nds. Thes>:,
trends are qul.::~ -:Hffe.rent fr0r.\ '::1"3en-Jt~d in .'1:1;'".::
,pa!'t"3- ;J.f the- ~o!"th,::,:-n' Hecisph~!'~.. - '''i:!:HonaL ?','.:~::::._b'~::,,-

tion fluctuations sho~ rapid transitions from r~lative-
ly dry to relatively wet ~odes.

-Silver

No significant increase in silver concentration were
found in the target area. except in small areas near
generator sites. atter four winters of seedin~. ~
deleterious effects of silver iodide additions have
been noted at concentrations which could be expected
due to cloud seeding.

HISTORY AND DESIGN

The San Juan Ecology Project was established as one of
a complex of investigations arising from the need to
explore. the possibilities of increasing the availabil-
it9 of water in the United States Southwest. In 1968-
Congress authorized the Bur~au of Recla@8tion to
initiate weather modification pilot projects to test
the feasibility of large scale winter cloud seeding
which showed promise of being an economical technique
for augmenting the flow of the Colorado River (Division
of Atmospheric Water Resources Management 1970).
Analysis of previOUS resea~ch findinRs indicated that
the most likely chance of success would corne from
_' winter seeding of orographic storms over 90t:le of the
high mountain areas situated in the Upper Colorado
. River Basin. Grant et al. (l96a) tested this approach
00 a limited scale at Climax. Colorado. and found
si~nificaot increases in snowfall when seeding eX?er-
!iments ~ere perforoed under specific ~eather situations.
'Knowled e accruin from this study. and from t~ others
in Colorado Steamboat Springs and Wel Creek Pass.
and from a National Academy of Sciences Report (1964),
led to the conclusion that caretully canaged w1nter
cloud-seeding techniques could produce a ~U to jU
percent increase in mountain sno~tall. Tne Bureau of
Reclamation designed the Colorado R1ver Basin PilOt
Project (part of Project Skywater) to test this
possibility on a sub-operational scale. The most re-
cent statement on the effectiveness of this design is
given in summary form by Howel~ (this _Cha,?ter).

The San Juan Mountains of so~thwestern COlorado ~ere
chosen as the site of the cloud seeding pilOt project
because of their strategic location for inte~cepting
winter orographic storms deriving from the south~est
which would also ensure relatively frequent occurrence
of a cloud-top tecperature condition calculated ~o be
within the I:1Ost effective seeding range of above -26 C.
The original area destined 20 receive 2he cloud-seed-
ing experiment was 8,550 km (3,300 c1 ) on the west-
ern side of the Continental Divide, and above 2.900 m
(9,500 ft) elevation (Fig. 1)~

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r~~un'~ r. San Juan cl';~d -s-:'!'eding targo!t area, and
iO~~~3ive stud: sito!s.

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