Geoscience Reference
In-Depth Information
electromagnetic spectrum are received at the earth's
surface. Incoming radiation that reaches the surface
is often referred to as short-wave radiation: visible
light plus some bands of the infrared. This is not
strictly true as clouds and water vapour in the
atmosphere, plus trees and tall buildings above the
surface, emit longer-wave radiation which also
reaches the surface.
Outgoing radiation can be either reflected short-
wave radiation or energy radiated back by the earth's
surface. In the latter case this is normally in the
infrared band and longer wavelengths and is referred
to as long-wave radiation. This is a major source of
energy for evaporation.
There are two other forms of available energy that
under certain circumstances may be important
sources in the evaporation process. The first is heat
stored in buildings from an anthropogenic source
(e.g. domestic heating). This energy source is often
fuelled from organic sources and may be a significant
addition to the heat budget in an urban environ-
ment, particularly in the winter months. The second
additional source is
advective energy
. This is
energy that originates from elsewhere (another
region that may be hundreds or thousands of kilo-
metres away) and has been transported to the
evaporative surface (frequently in the form of latent
heat) where it becomes available energy in the form
of sensible heat. The best example of this is latent
energy that arrives in cyclonic storm systems. In
Chapter 1 it was explained that evaporating and
condensing water is a major means of redistributing
energy around the globe. The evaporation of water
that contributes to cyclonic storms normally takes
place over an ocean, whereas the condensation may
occur a considerable distance away. At the time of
evaporation, thermal energy (i.e. sensible heat) is
transferred into latent energy that is then carried
by the water vapour to the place of condensation
where it is released as sensible heat once more. This
're-release' is often referred to as advective energy
and may be a large energy source to drive further
evaporation.
Water supply
Available water supply can be from water directly
on the surface in a lake, river or pond. In this case
it is open water evaporation (
E
o
). When the water is
lying within soil the water supply becomes more
complex. Soil water may evaporate directly,
although it is normally only from the near surface.
As the water is removed from the surface it sets up
a soil moisture gradient that will draw water from
deeper in the soil towards the surface, but it must
overcome the force of gravity and the withholding
force exerted by soil capillaries (see Chapter 4). In
addition to this the water may be brought to the
surface by plants using osmosis in their rooting
system. The way that soil moisture controls the
transformation from potential evaporation to actual
evaporation is complex and will be discussed further
later in this chapter.
The receiving atmosphere
Once the available water has been transformed into
water vapour, using whatever energy source is
available, it then must be absorbed into the atmos-
phere surrounding the surface. This process of
diffusion
requires that the atmosphere is not already
saturated with water vapour and that there is
enough buoyancy to move the water vapour away
from the surface. These two elements can be assessed
in terms of the
vapour pressure deficit
and
atmospheric mixing.
Boyle's law
tells us that the total amount of
water vapour that may be held by a parcel of air is
temperature and pressure dependent. The corollary
of this is that for a certain temperature and air
pressure it is possible to specify the maximum
amount of water vapour that may be held by the
parcel of air. We use this relationship to describe the
relative humidity
of the atmosphere (i.e. how close
to fully saturated the atmosphere is). Another
method of looking at the amount of water vapour
in a parcel of air is to describe the
vapour pressure
and hence the
saturation vapour pressure
. The differ-
ence between the actual vapour pressure and the
