The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 6

Figure 1.5 Control volume for energy: sensible heat (left, dark grey arrows) and
latent heat (right, light grey arrows). Energy is exchanged between sensible and
latent heat if water changes phase (here only phase changes between liquid and gas
phase are considered: L v C l↔g ,). Storage of energy in the form of chemical energy in
the plants (due to assimilation) as well as some other terms (see text) have been dis-
carded. The direction of the arrows holds for typical daytime conditions.

The control volumes for sensible and latent heat are depicted in Figure 1.5 . The
complete energy balance equation corresponding to this igure is as follows:

QHLE GA AS *−− −++ = + + +

v

∆∆∆∆ S S S (1.3)

h lat ha hv hs lat

where Q* is the net radiation (see Chapter 2 ), H is the sensible heat lux, G is the soil
heat lux at the bottom of the control volume and L v E is the latent heat lux (where
L v is the latent heat of vaporization). If the inputs and outputs do not balance, heat
can be stored in the air (∆ S ha ), in the vegetation (∆ S hv , i.e., the vegetation becomes
warmer) and in the soil (∆ S hs ). Finally, A h and A lat are the net advections of sensible
and latent heat.
The latent heat lux plays a special role in the energy balance in the sense that it
transports energy through the transport of water vapour. The actual energy consump-
tion related to evaporation is contained in the term L v C l↔g , but because this term is
internal to the control volume, it does not appear in the energy balance. The energy
related to evaporation leaves the control volume as latent heat. In the case that there
is a change in the water vapour content in the control volume (i.e., ∆ S lat ≠ 0 ) or non-
zero advection of water vapour, the transport of latent heat out of the control volume
( L v E) may be unequal to the energy related to the phase change within the volume.
Note that the opposite phase change can happen as well: when dew is formed water is
transformed from the gas phase to the liquid phase and energy is released.
Some terms have not been accounted for in Eq. ( 1.3 ). Part of the solar radiation that
hits the vegetation will be used for photosynthesis. This leads to a conversion of radi-
ative energy to the storage of chemical energy in the plant material. This storage term