Geoscience Reference
In-Depth Information
3
EVAPORATION
LEARNING OBJECTIVES
When you have finished reading this chapter you should have:
An understanding of the process of evaporation and what controls its rate.
A knowledge of the techniques for measuring evaporation directly.
A knowledge of the techniques used to estimate evaporation.
Evaporation is the transferral of liquid water into a
gaseous state and its diffusion into the atmosphere.
In order for this to occur there must be liquid water
present and available energy from the sun or
atmosphere. The importance of evaporation within
the hydrological cycle depends very much on the
amount of water present and the available energy,
two factors determined by a region's climate. During
winter months in humid-temperate climates evap-
oration may be a minor component of the hydro-
logical cycle as there is very little available energy
to drive the evaporative process. This alters during
summer when there is abundant available energy
and evaporation has the potential to become a major
part of the water balance. The potential may be
limited by the availability of liquid water during
the dry months. This can be seen in extremely hot,
arid climates where there is often plenty of available
energy to drive evaporation but very little water to
be evaporated. As a consequence the actual amount
of evaporation is small.
It is the presence or lack of water at the surface
that provides the major semantic distinction in
definitions of the evaporative process.
Open water
evaporation
(often denoted as
E
o
) is the evaporation
that occurs above a body of water such as a lake,
stream or the oceans. Figure 1.6 shows that at the
global scale this is the largest source of evaporation,
in particular from the oceans.
Potential evap-
oration
(
PE
) is that which occurs over the land's
surface, or would occur if the water supply were
unrestricted. This occurs when a soil is wet and what
evaporation is able to happen occurs without a lack
of water supply.
Actual evaporation
(
E
t
) is that
