Geoscience Reference
In-Depth Information
tendency for the diurnal cycle in soil heat flux to average out over the day. Because
of the change in average air temperature between summer and winter, there is also
a seasonal cycle of soil heat flux upon which the diurnal cycle is superimposed.
This longer cycle also tends to average out over the year. It is more general to speak
of a
substratum heat flux
into the underlying medium rather than soil heat flux
because in some cases, e.g., paddy fields, the underlying medium may be water.
Soil heat flux is discussed in greater detail in Chapter 6.
Physical energy storage,
S
t
Some energy is stored within the sample volume because of the thermal capacity
of its contents. The amount of energy stored will change with time as the tempera-
ture of the air or vegetation changes or if the humidity of the air changes. In prac-
tice, this storage term is often neglected for short crops. However, the change in
physical energy storage can become significant in comparison with the latent and
sensible heat fluxes in the case of tall (forest) vegetation, because there is more
biomass and more air in the deeper sample volume.
The amount of energy stored per unit time per unit area in the interfacial layer
between the level
z
1
in the soil and the reference level
z
2
in the atmosphere is cal-
culated by:
z
z
2
⎡
⎤
2
∂⎛
⎞
∂
⎡
⎤
∑
(
)
S
∫
∫
cT dz
q dz
=
ρ
+
ρ λ
⎢
⎥
⎜
⎟
(4.1)
⎢
⎥
t
i
i
i
a
∂
t
⎝
⎠
∂
t
⎣
⎦
⎣
⎦
i
z
z
1
1
[change in temperature]
[change in humidity]
In Equation (4.1), the first term is the energy associated with temperature changes
and the index
i
corresponds to contributions that arise from (i) the layer of soil and
roots that is above
z
1
but below the soil surface; (ii) the vegetation (including
trunks, branches, and leaves) between the soil surface and
z
2
; and (iii) the air
which permeates the vegetation and lies above it up to the level
z
2
. The second
term is the change in latent heat energy associated with changes in the humidity of
the air that permeates the vegetation or lies just above it up to the level
z
2
.
When attempts are made to estimate physical energy storage for forest stands it
is usually considered sufficient to measure changes in the temperature and humid-
ity at a few sample levels in the air in and above the canopy, and to measure changes
in temperature in a sample of trunks and large branches at depths considered
characteristic.
Biochemical energy storage,
P
The photosynthesis and respiration of any vegetation present in the volume sam-
ple involves the capture or release of energy. In practice, the amount of energy
