Geoscience Reference
In-Depth Information
1.3 Modes of Transport of Energy and Mass
In the presentation of the surface water and energy balances a number of transport
processes have been introduced, without detailing by what means the transport takes
place. For the transport of energy, three modes of transport are possible:
•
Radiation (transport by propagation of electromagnetic radiation; no matter is needed)
•
Conduction (transport of energy through matter, by molecular interactions; matter is
needed, but the matter does not move [macroscopically])
Advection
•
2
(transport of energy by the movement of energy-containing matter)
In the context of the surface energy balance, all three modes occur: net radiation is
radiative transport, the soil heat lux is based on conduction, and the sensible lux is
an example of advection (in the sense that turbulent transport involves the motion of
energy-containing air).
For the transport of matter (e.g., water or solutes) the two modes of transport are
•
Molecular diffusion
•
Advection
In the present context, mainly advection is important, but it can have a number of
different manifestations. In soil, water lows more or less smoothly, whereas in the
atmospheric surface layer transport of matter (e.g., water vapour or CO
2
) takes place
by turbulence, where air containing the given constituent is moved from one place
to another (see
Chapter 3
). Molecular diffusion plays a role in solute transport in the
soil, and in the atmosphere in thin layers adjacent to surfaces (e.g., leaves).
One of the main objectives of this topic is to quantify the various luxes in water
balance and energy balance. A generally used method to describe the lux of a quan-
tity is based on an analogy with transport by diffusion on the molecular scale, that is,
Fick's law for diffusion of matter and Fourier's law for heat diffusion:
=−
∂
∂
C
x
a
Fk
(1.6)
a
a
where
F
a
is the lux density of quantity
a
in the
x-
direction,
C
a
is the concentration
of
a
and
k
a
is the molecular diffusion coeficient for quantity
a
(
k
a
has units m
2
s
-1
).
Essential in the case of molecular transport is that
k
a
is known and depends only on
the luid or solid under consideration and on the state of that luid or solid (temper-
ature, pressure). For many combinations of transported quantities (e.g., momentum,
heat, water vapour) and luids (e.g., air or water),
k
a
is known and tabulated.
2
Here the word 'advection' is used in the sense of large-scale (relative to the molecular scale) motion of matter.
Sometimes the term convection is used for this as well, but this may cause confusion with thermal convection, and
in some applications convection is the sum of advection and diffusion.
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