WMOD00470

• Project Name: Project Skywater
• Title: Environmental Impacts of Precipitation Management: Results and Inferences from Project Skywater
• Date: 6/6/1977
• Weather Modification - Doc Type: Report

494

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4) VEGETATION

With respect to the vegetational characteristics of the
environment, increasing summer-convective precipita-
tion is accompanied by a gradual transition from desert
shrubland, to short-grass prairie, to tall-grass prairie, to
a saban a of mixed grass and deciduous forest, and
finally to forest. This transition has, of course, been
drastically modified by cultivation and land use, by
irrigated cropland and grazing in the drier part, and by
dryland crops and row crops in the progressively moister
parts. Precipitation management would tend to shift
the very diffuse boundaries of these grand divisions
somewhat westward.
Each vegetational province is characterized by a spec-
trum of plant species, including many specimens of
species adaptable to both wetter and drier climates and
warmer and colder climates. With the normal short- and
middle-term variations in climate, both relative and
absolute abundance of different species often vary greatly
from one epoch to another, but the absolute range of
most species is little affected. Thus, it was found that
during the drought of the 1930s the more moist-adapted
species were suppressed over wide areas but did not
disappear. When the drought ended, the dry-adapted
species, which had suffered less, at first grew exuberantly
but were later crowded down and suppressed (but not
extinguished) by recovery of the moist-adapted species.
Precipitation management, to the extent that it may
moderate the intensity of extreme droughts, will cause
the natural vegetation of each locality gradually to
resemble that of regions now slightly moister and may
moderate the secular changes in species composition
that take place in response to normal climatic fluctua-
tions.
Respecting vegetation, it should be noted that ac-
culturation has profoundly modified species composition
and may continue to do so. Overgrazing of the High
Plains in the late 19th century resulted in drastic and
apparently permanent change in the species composi-
tion of the grassland communities, with disappearance
of the forage species preferred by the cattle and, in
many places, displacement of grasses by desert shrubs
and forbs. In some moister locations, the original spe-
cies structure has been supplanted or greatly modified
by the introduction of improved pasture grasses. It is
highly probable that such cultural practices will con-
tinue to exert much greater influence upon the char-
acter of vegetational communities than will precipita-
tion management and will probably entirely mask the
latter.

5) ANIMAL POPULATIONS

The effect of precipitation management on animal pop-
ulations is likely to be mainly indirect, through its
influence on habitat, rather than directly on the or-
ganisms. Particularly in the case of birds and small
mammals, populations depend more on the presence of
suitable cover, nest sites, and food supplies than on the
weather. Though severe storms at critical times may
occasionally decimate some'species, there is little expec-

I

Vol. 58, No.6, June 1977

tation that preCIpItation management would affect the
frequency of such occurrences.
Although it has been proposed that outbreaks of
disease, weeds, and insect pests are facilitated by any
major ecosystem disturbance, the interaction between
the environment and precipitation management will
effect only gradual change and adjustment. It will not
cause disturbances like those of acculturation or like a
multitude of natural acute disturbances such as fire and
drought. In fact, to the extent that precipitation man-
agement reduces the intensity of drought, an important
source of ecosystem stress will be diminished.

6) SUMMARY

The best expectation presently available of the impact
of summer-convective precipitation management is that
each present environmental compartment would gradu-
ally come to resemble neighboring compartments on the
moister side of the precipitation gradient, with no ap-
parent risk of severe disturbances accompanying this
transition.

d. Impact on the natural environment-
Winter orograPhic

The setting for winter-orographic preCIpItation manage-
ment, comprised mostly within the more remote parts
of mountain ranges, is affected less by acculturation than
are other regions. Nevertheless, such activities as lum-
bering, mining, livestock grazing, game management,
recreational development, and fire protection have al-
ready greatly modified the aboriginal environment and
continue to do so.
By virtue of the environmental impact studies begun
in the early 1970s in connection with Skywater winter-
orographic experiments and recently reported upon, the
relationship of snowpack augmentation to certain pro-
cesses considered to be most significant environmentally
has been established or at least estimated.

I) SNOW COVER EXTENT AND DURATION

The most important impact of precipitation manage-
ment, through which other environmental compart-
ments are affected, is on the extent and duration of
snow cover as it retreats in the spring. The snow cover
release date establishes the beginning of soil warming
and of the growing season for most vegetation and may
affect the duration of adequate moisture supply for
growth. From Forest Service studies, it appears that a
snowpack 150% of normal delays the disappearance of
snow cover at a given location by "-'5 days, leading to
an estimate of 1-3 days' delay resulting from precipita-
tion management. This estimate is probably too high
for windswept areas and too low for the much smaller
areas where snowdrifts accumulate.

2) AVALANCHE HAZARD

The relation between snowpack augmentation and
avalanche hazard is greatly complicated by other factor>
affecting the release of avalanches and affecting the
prediction, warning, and application of countermeasures
that mitigate this hazard. In chutes where they seldom