Environmental Engineering Reference
In-Depth Information
Figure 5.13
Generalized curves showing the relationship
between elevation and mean annual precipitation in different
climatic regimes
.
TRANSPIRATION
Transpiration is a related process involving water loss from plants. It occurs mainly by
day, when small pores, called
stomata
, on the leaves of the plants open up under the
influence of sunlight. They expose the moisture in the leaves to the atmosphere and, if the
vapour pressure of the air is less than that in the leaf cells, the water is transpired. As a
result of this transpiration, the leaf becomes dry and a moisture gradient is set up between
the leaf and the base of the plant. Moisture is drawn up through the plant and from the
soil into the roots (Figure 5.14).
For many plants, this is a passive process; it is controlled largely by atmospheric and
soil conditions, and the plant has little influence over it. Consequently transpiration
results in far more water passing through the plant than is needed for growth. Only 1 per
cent or so is used directly in the growth process. Nevertheless, the excess movement of
moisture through the plant is of great importance, for the water acts as a solvent,
transporting vital nutrients from the soil into the roots and carrying them through the cells
of the plant. Without this process, plants would die.
EVAPOTRANSPIRATION
In reality it is often difficult to distinguish between evaporation and transpiration.
Wherever vegetation is present, both processes tend to be operating together, so the two
are normally combined to give the composite term
evapotranspiration
.
