WMOD00570

• Title: Precipitation Enhancement Effects of Sorghum, Using a Daily Growth Model
• Date: 4/1/1982
• Weather Modification - Doc Type: Report

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Because it is soil water and water stress, not precipitation, t~at directly
affect crop yield, additional precipitation that builds up soil iwater early
in the season will reduce or eliminate water stress and usually lead to
increased yield, even in years with above-normal precipitation. I Built-up
soil water can be especi ally important in a year with a pronounqed dry period
during the crucial grain-filling stage. There is evidence sugg~sting that
even small amounts of precipitation induced on otherwise rainles!s days may be
usefu"], part icul arly if there are many such days.

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The studies also show that temperature stress and soil type can have a
pronounced effect on yield. Temperature stress (almost always oaused by low
temperatures for the sites used in this study) can offset the p~sitive effect
of adequate or even abundant precipitation, and it is more significant at the
northerly sites. Soil characteristics apparently determine, to ia large
extent, the relationship between an increment of precipitation ~nd an incre-
ment of yield. I

Analysis of the results from the precipitation enhancement scenarios suggests
that those operating on a broad spectrum of natural condition a~e~ in general,
more effective at increasing yield than those affecting only celltain cate90ries
of events. The most effective scenarios (6, 7A, and 7B) affect ~ore events,
which implies both more total precipitation and a greater proba~ility of
adding precipitation at the "right" time. These "broad spectru~" precipita-
tion E~nhancement scenarios also provide significant yield increases (i.e.,
increases of at least 6 percent) in more years than the other sdenarios
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do. I
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Among the scenarios that operate on only certain categories of ~vents (small
storms, large storms, trace days, etc.), there is no consistent Itendency for
one scenario to produce significantly greater yield increases tHan another.
Year-to-year variations and site-to-site variations in yield res1ulting from
these scenarios seem to be a function of the number of storms, ~hich implies
total amount of precipitation, rather than the nature of the ev~nts themselves.

The results of this study have implications for both planning a~d operation
of precipitation enhancement projects intended to affect agriculture. A
major element in planning a project is development of a seedability clima-.
tology - that is, establishing the expected frequency of seedable events
according to some hypothesis and estimating the amount of additional precipi-
tation that would result from seeding. Our studies show that there is a
trade--off between timing and amount (though not explored quantit11ativelY) such
that the same yield can be achieved by adding a moderate amount of precipita-
tion near the start of grain filling or by adding a large amount[ earlier in
the season. Therefore, t he expected impact of a prec i pitat ion enh ancement
project should consider both the amount of precipitation likely [to be achieved
and the timing of that precipitation relative to plant growth and development.
It should be noted that optimal times for additional precipitatilon vary among
different crops. Planners should focus on the principal crop(s)1 of the
project area and, to the extent possible, tailor the project to Ithe crop, not
the weather. Planners should also recognize that varying soil types will
affect the transferability of expectations from one area to ano~her, even
when the areas are close and the climatologies similar. In addi1tion, the

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