WMOD00570

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

about the same amount of rain, but 1969 received most of its rain at the
start of grain filling - at a good time. The rains in 1955 occurred late in
the grain-filling period - a relatively poor time.

At San Angelo, both 1948 and 1963 (figs. 8a and 8b) received the same total
rain, but 1963 received about 20 mm less growi ng-season rain. In stage 6,
1963 had about 30 mm less rain than 1948, yet its yield was markedly higher
(6950 kg/ha vs. 6214 kg/ha for 1948). This is because the 1963 rains were
clustered about the beginning of grain filling, while the 1948 rains came
very close to- maturity.

Figures 9a and 9b exempl ify the interact ion between t imi ng and amoun't of
precipitation. In 1966 and in 1976, Concordia received rainfall around the
beginning of grain filling. However, in 1966, the rainfall was much greater,
resulting in a yield 2200 kg/ha higher (4347 kg/ha vs. 3135 kg/ha).

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Several pairs of precipitation enhancement scenarios that had similar increases
in precipitation but different yields for a given year were examined. The
results confirm those from examination of natural-conditions pairs, .i .e.,
that timing of the additional precipitation usually accounted for the differ-
ence in yield. .

CONCLUS IONS

This study used a daily growth and development model of a grain sorghum plant
to examine the effects of amount and timing of enhanced precipitation on crop
yield. The model was exercised both with observed daily values of precipita-
tion and temperature and with precipitation values increased according to 15
enhancement scenarios at each of five sites in the central and southern High
Plains (17 to 32 years of record per site). The results indicate that, in
most years, the dominant factor affecting sorghum yield is the total. amount
of precipitation during the period at least 2 months prior to planting
through to crop maturity. In general, the greater the precipitation, the
greater the yield. The precipitation enhancement scenarios also show that
the greater the increase in precipitation, the greater the increase in
yi e 1 d.

The relationship between precipitation pattern and yield was examined to
assess the effectiveness of additional preclpitation at particular times
during the study period. Years with precipitation near the start of grain
filling (early in stage 6) usually have yields greater than would b~ expected
from their total precipitation. In general, for a given amount of precipita-
tion, the closer a precipitation event is to the start of grain filling, the
greater the effect on yield. Precipitation at this time builds up soil water
(which in turn reduces water stress) during the period when evapotranspiration
is at a maximum and when most of the photosynthate goes to grain rather than
to leaves or other plant parts. Conversely, the absence~of_ precipitation
near the start of grain filling usually results in lower than expected yield.
In some years, timing of precipitation is the key factor determining yield,
and optimal timing can result in a near-normal yield in a year with below-
normal precipitation. Above-normal precipitation coupled with rain near the
start of grain filling usually results in the highest yields.

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