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1.0
0.8
Drought during late
development stage
0.6
DM
a
DM
p
0.4
Drought during early
development stage
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
T
a
T
p
Figure 6.19
Generalized relation between relative yield and relative evapotranspira-
tion, indicating the effect of stress timing (Kirkham,
2005
).
radiation and momentum is absorbed; see
Figure 6.20
). For annual plants and decidu-
ous trees and shrubs, the location of the active level, and the scalars for which it serves
as the source or sink (heat, water vapour or CO
2
), changes through the year owing to
changes in the height of the vegetation and changes in the presence of leaves.
In contrast to the low over a lat surface where sources and sinks are located only
at the surface, in a canopy sources and sinks of radiation, momentum, heat, water
vapour and various scalars can be located throughout the canopy. This implies that an
extra term needs to be added to the budget equations for momentum and scalars (here
exempliied by temperature; compare to Eq. (
3.16
)):
∂
∂
=−
∂
u
t
uw
z
′′
+
S
u
∂
∂
∂
=−
∂
θ
w
′′
θ
θ
(6.37)
+
S
t
∂
z
where
S
u
and
S
θ
are the source/sink of momentum and heat respectively (e.g., drag on
leaves, heating of the air by contact with hot leaves; in the case of CO
2
photosynthesis
in the leaves would serve as a sink). Note that in Eq. (
6.37
) for simplicity only verti-
cal gradients occur whereas in reality the conditions within a canopy will be highly
nonhomogeneous in the horizontal as well.
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