EVAPORATION - Fundamentals of Hydrology - page 37

which actually occurs (i.e. if there is not much
available water it will be less than potential). When
conditions are very wet (e.g. during a rainfall event)
E t will equal PE , otherwise it will be less than PE .
In hydrology we are most interested in E o and E t but
normally require PE to calculate the E t value.
All of these definitions have been concerned with
'evaporation over a surface'. In hydrology the surface
is either water (river, lake, ponds, etc.) or the land.
The evaporation above a land surface occurs in
two ways - either as actual evaporation from the
soil matrix or transpiration from plants. The
combination of these two is often referred to as
evapotranspiration , although the term actual
evaporation is essentially the same (hence the t sub-
script in E t ). Transpiration from a plant occurs as
part of photosynthesis and respiration. The rate
of transpiration is controlled by the opening or
closing of stomata in the leaf. Transpiration can
be ascertained at the individual plant level by
instruments measuring the flow of water up the
trunk or stem of a plant. Different species of plants
transpire at different rates but the fundamental
controls are the available water in the soil, the
plant's ability to transfer water from the soil to its
leaves and the ability of the atmosphere to absorb
the transpired water.
Evaporation is sometimes erroneously described
as the only loss within the water balance equation.
The water balance equation is a mathematical
description of the hydrological cycle and by
definition there are no losses and gains within this
cycle. What is meant by 'loss' is that evaporation is
lost from the earth's surface, where hydrologists
are mostly concerned with the water being. To a
meteorologist, concerned with the atmosphere,
evaporation can be seen as a gain. Evaporation
although not a loss, can be viewed as the opposite
of precipitation, particularly in the case of dewfall,
a form of precipitation. In this case the dewfall
(or negative evaporation) is a gain to the earth's
surface.

EVAPORATION AS A PROCESS

It has already been said that evaporation requires an
energy source and an available water supply to
transform liquid water into water vapour. There is
one more precondition: that the atmosphere be dry
enough to receive any water vapour produced. These
are the three fundamental parts to an understanding
of the evaporation process. This was first understood
by Dalton (1766-1844), an English physicist who
linked wind speed and the dryness of the air to the
evaporation rate.

Available energy

The main source of energy for evaporation is from
the sun. This is not necessarily in the form of direct
radiation, it is often absorbed by a surface and then
re-radiated at a different wavelength. The normal
term used to describe the amount of energy received
at a surface is net radiation ( Q *), measured using
a net radiometer. Net radiation is a sum of all the
different heat fluxes found at a surface and can be
described by equation 3.1.
Q * = Q S ± Q L ± Q G (3.1)

where Q S is the sensible heat flux; Q L is the latent

heat flux and Q G is the soil heat flux.
Sensible heat is that which can be sensed by
instruments. This is most easily understood as the
heat we feel as warmth. The sensible heat flux is the
rate of flow of that sensible heat.
Latent heat is the heat either absorbed or released
during a phase change from ice to liquid water, or
liquid water to water vapour. When water moves
from liquid to gas this is a negative flux (i.e. energy
is absorbed) whereas the opposite phase change (gas
to liquid) produces a positive heat flux.
The soil heat flux is heat released from the soil
having been previously stored within the soil. This
is frequently ignored as it tends to zero over a 24-
hour period and is a relatively minor contributor to
net radiation.
Incoming solar radiation is filtered by the
atmosphere so that not all the wavelengths of the