The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 9

Figure 1.7 Simpliied surface water balance (left) and energy balance (right). Very
light grey arrows indicate transport of water vapour, light grey arrows show liquid
water transport and dark grey arrows indicate energy transport.

all luxes are supposed to occur at the surface: they are called surface luxes . With
this redeinition of the luxes, the surface water balance and surface energy balance
become:

PIRED +−−−=0 (1.4)

QHLE G *−− −=

v

0 (1.5)

Although these balance equations are appealing in their simplicity, the compression of
the control volume to a surface may lead to problems in the interpretation of observed
luxes. In practice most luxes will be observed at some height above a canopy, or at
some depth below the surface (rather than at the hypothetical surface for which they
are supposed to be representative). The omission of storage and advection terms in
Eqs. ( 1.4 ) and ( 1.5 ) may cause a non-closure of the observed water balance or energy
balance (i.e., the terms do not add up to zero). To solve this problem one should revert
to the full Eqs. ( 1.2 ) and ( 1.3 ).
Note that the sign convention in Eqs. ( 1.4 ) and ( 1.5 ) is such that P, I and Q* are
considered as inputs, taken positive when directed towards the surface, whereas the
other terms are considered as outputs, being positive when directed away from the
surface. This sign convention is often used but arbitrary and other choices are used
as well.
A inal remark relates to the units of the transports in Eqs. ( 1.4 ) and ( 1.5 ). Rather
than delineating a real surface with a given extent, those equations are usually applied
to a unit surface area (e.g., one square meter). This implies that the units of the trans-
port terms in the water balance are either volume per unit area per unit time (volume
lux density) or mass per unit area per unit time (mass lux density). The terms in the
energy balance have units of energy per unit area per unit time (energy lux density).
Often, the terminology is used loosely: the term “density” is dropped and the word
“lux” is used where a “lux density” is meant.
To summarize, the various terms in the surface water balance and energy balance
are the subject of this topic. But the processes are not studied only in isolation, but the
interactions between them are at least as important.