Agriculture Reference
In-Depth Information
evaporation, water loss from the soil's surface and transpiration, water loss from the
plant leaves. These latter two, along with the former three, determine how much
water is available for crop and animal production. It is primarily these five that
account for variations in crop and animal production in various parts of the world.
The Thornthwaite climate classification takes into account evaporation from soil
surface and transpiration through leaves of plants. These two components are hard to
measure independently, and so agriculturalists generally combine the two into the
amount of water lost from soil surface and plants and term this combined water loss
as evapotranspiration. This is the amount of water needed to produce a crop. Thus,
one needs to know both the rainfall and the evapotranspiration to know how much
water is available for growing a crop or for watering animals.
Measurement of evapotranspiration is extremely difficult in that, although moisture
in the air can be measured, it is difficult to differentiate between the moisture already in
air and that which is added by the plant and that evaporated from the soil surface. In
addition the water vapor arising from evapotranspiration is rapidly diluted in the atmos-
phere. In addition other factors such as dew formation can cause additional difficulties
in determining evapotranspiration.
A way around some of these problems is the open-pan evaporation method of esti-
mating evapotranspiration. Here the amount of water lost from a standardized open pan
of water is related to evapotranspiration by various conversion factors.
Figures 8.12 and 8.13 show graphs of rainfall verses evapotranspiration for a year
in a dry and a wet climate. Wet climates are those where the rainfall exceeds evapotran-
spiration, in dry or desert climates rainfall is less than evapotranspiration. These two
factors, that is, rainfall and evapotranspiration, can both be expressed as centimeters
(cm) or millimeters (mm) of water and graphed together on a month by month bases.
In this way shortfalls in soil water can be determined and irrigation plans developed
Figure
8.12.
Rainfall (RF) verses evapotranspiration (ET) for a dry climate; D represents times of
water deficit.
