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10000
Global radiation
50-200 Wm
−2
200-400Wm
−2
400-600Wm
−2
600-1000 Wm
−2
1000
100
10
5
0
10
15
20
Vapour deficit(gkg
-1
)
Figure 7.5
Observed canopy resistance of a Douglas ir stand in the Netherlands, as a
function of vapour deicit (
q
sat
(
T
)
- q
, comparable to the vapour pressure deicit
e
sa
t
(
T
)
- e
) and global radiation. (Data from Bosveld and Bouten,
2001
)
deicit of the air than low vegetation (see
Chapters 6
and
9
). This difference is particu-
larly visible at midday when VPD is highest. Furthermore, broadleaf trees tend to show
resistances that are somewhat lower than those of needleleaf tree (see, e.g., Dolman,
1986). Models that describe the variation of the canopy resistance due to variations in
external (meteorological) and internal (plant) factors are dealt with in
Chapter 9
.
Question 7.5:
In
Chapter 6
the
stomatal
resistance was discussed, whereas in this chap-
ter the related
canopy
resistance is used.
a) If a vegetation has a leaf area index (LAI) larger than 1, will the canopy resistance
be higher or lower than the stomatal resistance?
b) The stomatal resistance depends on the amount of photosynthetically active radia-
tion (PAR; see
Figure 6.15
). If two types of vegetation have the same LAI, but the
irst has one layer of leaves, and the second has a number of layers, shading each
other, which of the vegetation types will have the highest canopy resistance?
Question 7.6:
Consider the transpiration from a short grass vegetation. Use the Pen-
man-Monteith equation in your answers. Given are the following observations:
Quantity
Value
Unit
Q*
250
W m
-2
G
22
W m
-2
T
a
(2 m)
15
°C
e
a
(2 m)
14
hPa
r
a
50
s m
-1
r
c
60
s m
-1
P
1013
hPa
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