Environmental Engineering Reference
In-Depth Information
Figure 5.14
Schematic diagram showing exchanges of water
and gases by transpiration in plants.
Evapotranspiration is governed mainly by atmospheric conditions. Energy is needed to
power the process, and wind is necessary to mix the water molecules with the air and
transport them away from the surface. In addition, the state of the surface plays an
important part, for evaporation can continue only so long as there is a vapour pressure
gradient between the ground and the air. Thus as the soil dries out the rate of
evapotranspiration declines. Lack of moisture at the surface often acts as a limiting factor
on the process.
We can therefore distinguish between two aspects of evapotranspiration.
Potential
evapotranspiration
(PE) is a measure of the ability of the atmosphere to remove water
from the surface, assuming no limitation of water supply.
Actual evapotranspiration
(AE)
is the amount of water that is actually removed. Except where the surface is continuously
moist, AE is lower than PE.
POTENTIAL EVAPOTRANSPIRATION
ENERGY INPUTS: THE SUN
The main variable determining potential evapotranspiration is the input of energy from
the sun, and it has been estimated that this accounts for about 80 per cent of the variation
in PE. The amount of radiant energy available for evapotranspiration depends upon a
number of factors, including latitude (and hence the angle of the sun's rays), day length,
cloudiness and the amount of atmospheric pollution. Thus PE is at a maximum under the
clear skies and hot days of tropical oceans, and at a minimum in the cold, cloudy polar
regions. In the short term, however, rates of potential evapotranspiration may vary
considerably at any single place. Daily variations in radiation inputs cause marked
fluctuations in PE, so that very little evapotranspiration occurs at night. Even subjectively
we can get some idea of this by noting how long the ground stays wet after a shower of
