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

Bulletin American Meteorological Society

velopment (or assigned to the hydroelectric compartment
of multiple-use water projects) will tend to lend addi-
tional justification to marginal hydroelectric projects,
including pumped-storage projects where there is an
opportunity to supplement the pumped supply with
streamflow augmented by precipitation management. It
does not appear likely, however, that precipitation man-
agement will be more than a subsidiary factor in the
economic considerations involved.

3) MUNICIPAL AND INDUSTRIAL WATER SUPPLY

Substantially increased demands for water for this pur-
pose are projected in certain areas of limited supply.
It is unlikely that precipitation management will dis-
place other responses such as water conservation and
reuse. It is likely, however, that a mix of strategies will
emerge that includes precipitation management in the
role of a low-cost but limited capability alternative
utilized to a limited extent.
A principal component analysis of agricultural and
economic characteristics of six counties arrayed across
the precipitation gradient from Colorado to Illinois
(Howell, 1976) showed strong precipitation-related dif-
ferences among them. Cluster analysis showed that the
wetter counties had a high proportion of land in har-
vested crops, large investments in farm equipment, and
a dense population, while dry counties had much idle
(fallow) cropland and produced hay and cattle. Between
the extremes, there was a set of counties characterized by
a mixture of dry farming and irrigation, where fallow
cropland and production of small grains were prominent.
The analyses suggest that precipitation management
would tend to shift these existing patterns of land use
and related human activities slightly westward without
much affecting the character of the patterns themselves.

4) SUMMARY

Human activities in the fields of agricultural, hydro-
electric, and municipal-industrial use of water are likely
to undergo adjustment to precipitation management
through a mixed-strategy approach that will not place
an inordinate degree of dependency on the continued
ability of precipitation management to meet perform-
ance targets.

c. Impact on the natural environment-

Summer convective

Over most of the civilized world, the natural environ-
ment is profoundly acculturated and bears few traits of
wilderness. In considering the natural environment, one
must, therefore, regard the environment as it is exempli-
fied by the real landscape. Except for a few pockets of
wilderness, the environment is the product of an on-
going symbiosis between the land and humankind
(Dubos, 1976). It is, nevertheless, useful to make the
distinction between the direct, intentional impact of
precipitation management on a cultural element such
as agriculture and the complex of indirect effects that
may impinge on other elements of the landscape and
biosphere, be these "natural" or cultivated.

493

If precipitation management is applied to agricultural
cropland in subhumid and semiarid regions, the largest
receptor of indirect effects will be the surrounding and
interspersed grasslands, scrublands, and breaks. The
western Great Plains, from Montana to Wisconsin, from
Colorado to Illinois, and from New Mexico to Missis-
sippi, are remarkable in displaying an environmental
continuum across a gradient of precipitation that in-
creases '-'10% with each eastward displacement of
60 km. This landscape is, therefore, a permanent exhibit
of the potential eventual influence of widespread and
prolonged precipitation management on the environ-
ment.

1) RUNOFF AND GROUNDWATER

The abiotic characteristics of the environment, such as
runoff, groundwater, erosion, and soils, are greatly in-
fluenced by physiography as well as precipitation. Run-
off, which is intermittent at the dry end of the gradient,
increases gradually with increasing summer-convective
rainfall (in absolute quantity per unit area and as a
proportion of the precipitation), and the coefficient of
variation decreases. Groundwater recharge also tends to
increase, but effects are overwhelmed by those of
physiography and underlying geology.

2) EROSION

.Erosion has been found to be greatest ('-'28 g m-2 yr-1)
where the rainfall is '-'270 mm yr-1 (Langbein and
Schumm, 1958). In regions of lighter rainfall, the
amount of erosion is dependent upon the amount of
energy expended on the soil by falling rain and running
water. Where the precipitation is greater, vegetation
that tends to bind and protect the soil and susceptibility
of particular soils to erosion are the dominant factors.
About 83% of the variation in eroded soil has been at-
tributed to vegetative cover in combination with litter,
slope, and soil organic content (Meeuwig, 1970). In
regimes of heavy rainfall, erosion is usually evident
only where vegetation is disturbed. Since, under the
assumptions previously stated, it is unlikely for economic
and practical reasons that precipitation management
will be applied in regions of <270 mm annual precipita-
tion, its effect will be to decrease erosion except in
regions of disturbance. Since agriculture represents by
far the most widespread disturbance, the actual effect
of precipitation management will be determined princi-
pally by the efficacy of control measures practiced in
connection with agricultural land use, and the effect
of precipitation management will be quite secondary.

3) SOIL TYPES

These are recognized as the end product of accommoda.
tion among geological, vegetational, animal, and climatic
factors. Within the Great Plains, two or three diffuse
boundaries between soil types may be found that are to
some degree related to summer-convective rainfall
amounts. Precipitation management would presumably
cause an eventual slight shift in the position of these
diffuse boundaries over a very long period of time.