Geoscience Reference
In-Depth Information
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.
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 rain during the night, yet how
quickly it dries out during the day. Similar patterns occur
seasonally. Potential evapotranspiration reaches a peak
during the summer months and declines markedly during
the winter (
Figure 5.18
).
The magnitude of the peak
depends upon geographical location and climatic factors,
especially cloud.
Energy inputs: wind
The second important factor is the wind. The wind
enables the water molecules to be removed from the
ground surface by a process known as
eddy diffusion
.This
maintains the vapour pressure gradient above the surface.
Wind speed is obviously one of the variables determining
the efficiency of the wind in removing the water vapour,
but it is not the only one. The rate of mixing is also
important, and that depends upon the turbulence of the
air and the rate of change of wind speed with height.
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.
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.
Energy inputs: the vapour pressure gradient
Third, evapotranspiration is related to the gradient of
vapour pressure between the surface and the air.
Unfortunately the vapour pressure gradient has proved
very difficult to measure precisely in the layer immediately
above the surface, so wherever possible, methods of
calculating PE use measurements of vapour pressure at
one level only.
Actual evapotranspiration
Actual evapotranspiration equals PE only if there is a
constant and adequate supply of water to meet the
atmospheric demand. Such a situation exists over moist,
vegetated surfaces and it is also approximated over water
surfaces such as the open sea or large lakes, but most
land surfaces experience significant periods when water
supply is limited. As a result, AE falls below PE. We can
get some idea of the importance of surface conditions by
considering evapotranspiration in a variety of situations.
Let us start by examining evapotranspiration from an
open water surface.
Potential evapotranspiration
Energy inputs: the sun
The main variable determining potential evapotranspira-
tion 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 consider-
ably at any single place. Daily variations in radiation
Evaporation from water surfaces
Because there is an unlimited supply of water to maintain
evaporation, and because there is no vegetation to
complicate the process, the surface of oceans or large
lakes provides the simplest situation in which to study
evapotranspiration. Under these conditions, transpiration
