WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

-Lower Soil Te~erature

A noticeable decline in forest populations of small
mammals occurred ~ following winters of heavy snowfall.
This was most evident in the numbers of deer mice
(Peromyscus maniculatus) but it was also found in
chipmunks (Eutamias minimus) and in Microtus spp.
The basic reason for the population decline derives
from the delayed growth of essential spring foods and
results primarily from a delay in breeding so that
fewer litters 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 (Thomomys
talpoides) populations.

-Deeper Snowpack

As snow depth increased, elk (Cervus candensis) moved
to areas where snow was shallower than 40 em. They
avoided regions with more than 70 em of penetrable
snow depth. A 15 percent increase in snowpack may
decrease available elk winter range by 8 percent.
Spring movement of elk was more strongly associated
with the start of new growth in mountain meadows than
directly with the receding snowline. Ample habitat
was available for elk calving below the snowline and
without regard to snow depth.

Microbes

Plant litter in the tundra decreased in areas of
deeper snowpack. This presumably reflects removal by
the additional flux of water through the system rather
than an increase in decomposition rates.

Abiotic Factors

The effects of the snowpack on soil erosion resulted
from the greater area of bare soil left by the decrease
in vegetative cover around snowbanks. Those predicted
for the tundra area of the San Juan Mountains derive
from either the additional snow depth or the additional
meltwater produced by cloud seeding.

-Deeper Snowp-ack

If the area of long-lasting snowbanks expand after an
increase in snowfall, local changes 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 vicinity of already ex-
isting snow drifts and so should not be important re-
gionally.

~Additional Meltwater

Theoretical considerations suggest that an increased
streamflow generated by cloud seeding should be
associated with an increase in the total amount of
dissolved material removed from the target area. This
is supported by empirical observations of rock weather-
ing rates and stream water quality in the tundra
environment. However, this effect will be slight and
should not be concentrated spatially.

-Historical Climatolo~y

Temperature fluctuations over the last 120 years in-
dicate a cooling trend from the late 1860's to about
1930 when a reversal occurred; precipitation trends
are inversely related to temperature trends. These
trends are quite different from thosenoted in many
parts of the Northern Hemisphere. Seasonal precipita-

tion fluctuations show rapid transitions from relative-
ly dry to relatively wet modes.

-511 ver

No significant increase in silver concentration were
found in the target area, except in small areas near
generator sites, after four winters of seeding. No
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-
its of water in the United States Southwest. In 1968
Congress authorized the Bureau of Reclamation 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 research findings indicated that
the most likely chance of success would come from
winter seeding of orographic storms over some of the
high mountain areas situated in the Upper Colorado
River Basin. Grant et al. (1968) tested this approach
on a limited scale at Climax, Colorado, and found
significant increases in snowfall when seeding exper-
iments were performed under specific weather situations.
Knowledge accruing from this study, and from two others
in Colorado (Steamboat Springs and Wolf Creek Pass),
and from a National Academy of Sciences Report (1964),
led to the conclusion that carefully managed winter
cloud-seeding techniques could produce a 10 to 30
percent increase in mountain snowfall. The Bureau of
Reclamation designed the Colorado River 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 Howell (this ~hapter).

The San Juan Mountains of southwestern Colorado were
chosen as the site of the cloud seeding pilot project
because of their strategic location for intercepting
winter orographic storms deriving from the southwest
which would also ensure relatively frequent occurrence
of a cloud-top temperature condition calculated to be
within the most effective seeding range of above -26 C.
The original area destined 20 receive 2he cloud-seed-
ing experiment was 8,550 km (3,300 mi ) on the west-
ern side of the Continental Divide, and above 2,900 m
(9,500 ft) elevation (Fig. 1)..

ALPINE SITES
I. [Id%do Lok.
1- Wllliaml La"
FOREST SITES
$- MUllonory AldOl
~- Wolf Cr..k
,. Rico

Figure 1. San Juan cloud seeding target area, and
intensive study sites.

2