Agriculture Reference
In-Depth Information
evapotranspiration can greatly increase the efficient use of water under both irri-
gated and rain-fed conditions. The discussions that follow address advances that
have occurred for each of these approaches for improving water use by agriculture.
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Early work by Briggs and Shantz (1913) found striking differences between the
water requirements of different plants growing in the same environment and for
the same species growing in different environments. The millet, sorghum, and corn
groups were found to produce significantly more dry matter per unit of water use
than alfalfa and sweet clover. Also, the water requirement for the same species could
be two times or greater depending on the climate. A promising finding of Briggs
and Shantz was that there were measurable differences in the water requirements of
different varieties of the same crop species, suggesting the possibility of developing
through selection strains that were even more efficient in the use of water. However,
the optimistic view of Briggs and Shantz has not materialized.
Tanner and Sinclair (1983) stated that while evapotranspiration efficiency had
increased as a result of improved management, the increases had resulted mainly
from increased transpiration as a fraction of the evapotranspiration, and transpira-
tion efficiency had increased little, if at all. Richards et al. (1992) also concluded
that plant breeding has indirectly increased water use efficiency because yield has
increased with no additional water use. These increases have been substantial in
all the major food crops. Richards et al. stated that most of the increases, however,
resulted from improvements in harvest index values, and that there had been little
increase in water use efficiency for biomass. They further stated that improvements
in harvest index may be approaching theoretical limits in many crops and that if sig-
nificant further gains are to be realized there must be increases in transpiration effi-
ciency. There are many plant breeders and molecular geneticists working diligently
to improve transpiration efficiencies, and major advances could come at any time.
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As shown in Figure 19.1, the agronomic yield of a crop increases linearly with an
increase in evapotranspiration. Of course, this relationship assumes that water is the
most limiting factor. Evapotranspiration can be increased by a variety of manage-
ment practices. In areas where precipitation is not adequate to meet the climatic
demands of the crop and irrigation water is not available, the only way to increase
evapotranspiration is to make better use of the precipitation. There are three com-
ponents for successfully managing limited precipitation: (1) retaining the precipita-
tion on the land, (2) reducing evaporation, and (3) utilizing crops that have drought
tolerance and fit best with the rainfall pattern. Although these components have been
known for centuries, progress in adapting them to specific areas and situations has
been slow. However, technologies have emerged in recent years that have signifi-
cantly increased the proportion of annual precipitation used for evapotranspiration,
and these technologies, or the principles on which they are based, can be applied to
other regions and countries.
