Agriculture Reference
In-Depth Information
Biomass yield as function of T
Agronomic yield as function of ET
Cumulative T or ET
fIguRe 19.1
Generalized relation between production of biomass and transpiration and
between agronomic yield and evapotranspiration. (Adapted from R.S. Loomis, 1983, in:
H.M. Taylor, W.R. Jordan, and T.R. Sinclair, Eds.,
Limitations to Efficient Water Use in
Crop Production
. Madison, WI: American Society of Agronomy, Crop Science Society of
America, and Soil Science Society of America, pp. 345
-
380.
the climate, but yield will increase proportionately with increasing evapotranspira-
tion. Since water use efficiency was defined as the amount of harvestable crop prod-
uct divided by the volume of water used by evapotranspiration, it is apparent from
Figure 19.1 that the highest water use efficiency will always be at the highest yield.
This is because the proportion of the evapotranspiration that makes up the threshold
amount becomes smaller as the amount of evapotranspiration increases. Of course,
there is a maximum amount of water that can be utilized as evapotranspiration, and
this will ultimately determine the maximum yield for a given crop grown under a
given environment.
The generalized relations shown in Figure 19.1 are useful in indicating ways to
improve the efficient use of water in agriculture and serve as the foundation for dis-
cussions in this chapter. In theory, the efficient use of water can be enhanced several
ways. First, the slope of the line showing the amount of biomass produced as a func-
tion of transpiration can be increased. This is essentially increasing the photosyn-
thetic rate. Second, the amount of evapotranspiration can be increased, and this will
increase the agronomic yield and water use efficiency. Third, management practices
that decrease the evaporation portion of evapotranspiration will increase water use
efficiency. Fourth, since the agronomic yield of a crop is the harvestable product that
is usually only a portion of the aboveground biomass, an increase in the harvestable
portion will increase water use efficiency. The harvestable product divided by the
total aboveground biomass is defined as the harvest index. Fifth, evapotranspira-
tion includes only evaporation and transpiration of water from a defined area dur-
ing the growing season and is only a part of the field water supply. Management
practices that increase the proportion of the total field water supply that is used for
