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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The combined effect of water surpl us and temperature stress was ~xamined
for Oklahoma City and Concordia. When no-temperature-stress yields were
correlated with total precipitation with the water surplus subtrhcted out,
the coefficients increased from 0.26 to 0.42 (significant at thel5-percent
level) at Oklahoma City and from 0.37 to 0.70 (significant at the I-percent
level) at Concordia. I

We ascribe the poor statistical relationship between total preci~itation
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amount and yi el d at Concordi a primarily to the effect of temperature stress.
At Oklahoma City, the poor relationship appears to be caused pri~arily by the
water surplus. Although Goodland experienced both temperature stress and
water surplus, its mean precipitation was lower and its mean wat~r stress
greater than those of the two easternmost sites, allowing its yi~ld to
respond more positively to precipitation amount. Neither Lubboc~ nor
San Angelo had water surplus, and temperature stress was negligHlle at both
sites. Thus, the relationship's between precipitation amount andlyield were
the strongest of the five sites. I

Precipitation Enhancement Scenarios. Analyses of the precipitat10n enhance-
ment scenarios show the strong relationship between yield and total preClpl-
tation. The analyses also show that the increase in yield is related to the
increase in precipitation. Table 4 presents the means of the normalized
values of yield and precipitation for 11 of the scenarios descri~ed in a
previous section.~/ Figure 5 presents the same information graPticallY.

Scenario 7B, which increases precipitation for most natural events and
induces precipitation on one-third of the days with a trace, proVides the
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most additional precipitation and results in the greatest yield increases at
all five sites. Scenario 7A (which differs from 78 only in the amount of
rain induced on trace days) is second highest for both precipita1:ion and
yield increases at the five sites. Scenario 3A, which increaseslonly the
larger natural precipitation events by 10 percent, provides the least addi..
tional precipitation and the lowest yield increases of the 11 scknarios; none
of the yi e 1 d i ncre ases exceed 5 percent. I

More scenar ios were effect ive at Good 1 and and Lubbock than at the other three
sites. Goodland tends to have more natural precipitation eventsland more
days with trace amounts. This suggests that, for many scenarios, Goodland
receives more additional precipitation, which, in turn, leads tola greater
increase in yield. Sandy soil and the low initial soil water combine to
allow Lubbock to respond positively to even small increments of wrecipita-
tion. It is the only site at which, for several scenarios, the p,ercentage
increase in yield exceeds the percentage increase in precipitati~n. There is
no evidence suggesting that the timing of precipitation at Lubbod:k is more
favorable than at the other sites.

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2/ Four scenarios are not included in the discussion. ScenariosllA and 4A
provided little additional precipitation, and the yield increases were wen
below 6 percent. Scenario 5C showed essentially the same results as
Scenario 58. Scenario 9 provided little additional yield (excepi at Lubbock),
and the mean normalized values were nearly the same as those for Scenario 3B.
All 15 scenarios are included in Appendix B.

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