WMOD00295

• Title: Guidelines for Cloud Seeding to Augment Precipitation
• Date: 1/1/1995
• Weather Modification - Doc Type: Report

16

CLOUD SEEDING

mostly to increase soil moisture and ground water flows (Changnon et al.
1988; Knapp et al. 1988).
In a related unique field experiment in Illinois, the actual effects of
enhanced rain on crop production were evaluated. Large (9 m X 48 m)
mobile, plastic-covered shelters with sprinkler systems are being used to
exclude natural rain but otherwise expose crop plots to the prevailing
weather. Watering is quantified and timed to the historical rain-day pre-
cipitation records for wet, dry, and average summers, and water is added
to simulate modification. Initial results indicated that rainfall increases of
10-40% in Illinois increase com and soybean yields by 4-20% if natural
rainfall is below or near average (Changnon and Hollinger 1988). Refined
results showed that, for 2.5 em of rainfall added during a hot, dry sum-
mer, yields increased "10 bu/ acre for com and 4 bu/ acre for soybeans."
In a summer of average rain, increases are less, about "5 bu/ acre for corn
and 3 bu/ acre for soybeans." Yields of both crops were shown to decrease
when summer rainfall exceeds 36 em. Rain increases of realistic percent-
ages applied with the sprinklers only on days when natural rainfall was
less than 0.25 em provided no detectable yield increases, whereas a 40%
rain increase on all rain days produced the greatest increase in crop yield.
Com yields responded well to added rain on days with 0.25-2.5 em of
natural rainfall (Changnon and Hollinger 1990).
Yield trends and stability influence both micro economic (e.g., single
farm) and macroeconomic (e.g., aggregate Com Belt) decision makers as
they set priorities for investment in new technologies, such as cloud
seeding (Garcia et al. 1987). It is not feasible for a single producer to have
a cloud seeding program, but groups of producers might do so. Thus, it
is appropriate to consider local and regional impacts (Dennis 1980), Le.,
the level of aggregation in effort, area seeded, and economic effect.
As yields have steadily increased with improving agricultural technol-
ogy other than cloud seeding, variability of yields and the absolute yield
risk to producers have also increased. These increases in variability and
risk could be due to a heightened sensitivity of technology to weather, or
to temporal increases in weather variability. These findings have implica-
tions for estimating the economic effects of a fluctuating climate and soci-
ety's response to mitigate adverse effects. Enhanced precipitation and
consequent moderated crop heat stress might reduce risk by alleviating
extreme year-to-year yield changes. However, yields are influenced by a
broad set of agriclimatic conditions that must be considered in estimating
the overall impact of weather. Therefore, differences in estimated weather
effects on yields for similar aggregation levels must be accounted for. Lack
of sensitivity to these differences can result in inappropriate measurement
of the distribution of economic gains from activities such as cloud seeding.
The effects of seeding are likely to be regionalized, and this can distort its
relative attractiveness to user groups representing differing spatial aggre-
gations such as a few farms, a few counties, or a state (Garcia et al. 1987).
To aid in policy making, an effort was made to determine the economic
beneficiaries of a functioning precipitation modification technology