WMOD00570

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

precipitation, the greater the yield. Lubbock and San Angelo show the
strongest relationship, while the weakest were at Concordia and Oklahoma
City. Goodland is intermediate, but its behavior is closer to that of Concordia
and Oklahoma City.

The reason for the different responses to total precipitation lies in the
soil characteristics and climatologies of the five sites. The model uses two
parameters to describe the evaporative properties of the soil. The first
parameter characterizes the stage of evaporation when the soil is sufficiently
wet for the water to be transported to the surface at a rate at least equal
to the evaporation potential. The second parameter relates to the evaporation
stage in which the surface soil water content has decreased below a threshold
value, so that the soil evaporation depends on the flux of water through the
upper layer of soil to the evaporating site near the surface (Maas a.nd Arkin,
1978). For both parameters, the values at Lubbock and San Angelo are similar
and the values at Concordia, Goodland, and Oklahoma City are similar. The
slopes of the regression lines in figure 2 are about the same for Lubbock and
San Angelo and for Concordia, Goodland, and Oklahoma City. This suggests
that the incremental response in yield to an increment of precipitation is
determined, at least in part, by soil characteristics.

~

Equally important are the cl imat ic characteri st ics of the five sites'. Both
Concordia and Oklahoma City have moist subhumid climates with mean precipi-
tation for the study period in excess of 450 mm. Lubbock and San Angelo are
semiarid, with mean seasonal precipitation well below 300 mm. Goodland is
intermediate, with an average of just over 300 mm of precipitation for the
study period. The relatively abundant precipitation at the two easternmost
sites sometimes results in a water surplus, that is, water in excess of that
needed to saturate the soil. When water surplus is present, it has the
effect of reducing the statistical correlation between precipitation and
yield, even though the model ignores the presence of surplus water. . This is
illustrated in figure 3. Oklahoma City had water surplus sometime during the
season in 16 of the 21 years. When linear regression is applied using total
observed precipitation, the coefficient of correlation is 0.22 (not significant).
When the water surplus is subtracted from the total precipitation, giving the
effective amount available to the plant, the correlation betweenyield and
precipitation increases to 0.58 (significant at the I-percent level) and the
slope of the regression line increases.

A second climatic factor affecting the relationship between total Rrecipita-
tion and yield is temperature. Both Lubbock and San Angelo are warm. through-
out the growing season, rarely experiencing mean temperatures lower than
25 oC. On the other hand, mean temperatures at Goodland and Concordia are
frequently below 25 oC. The effect of temperature stress in depressing yield
is illustrated in figure 4. The regression line for the entire 17 years of
data at Concordia shows that, in general, the greater the precipitation, the
greater the yield. When only the six driest years are considered, the
regression line shows a much stronger relationship between precipitation and
yield. On the other hand, when just the six wettest years are consiaered,
the regression line has a negative slope in spite of the relatively large
amount of water available to the plant. Four of these six years had pro-
nounced temperature stress that more than offset the otherwise positive
effect of abundant precipitation.

~

12