Geoscience Reference
In-Depth Information
9.2.4 Canopy Resistance
In the previous section, the canopy resistance was left unspeciied. Some basic char-
acteristics of the stomatal resistance (the resistance for individual stomata) have been
dealt with in
Chapter 6
. Furthermore, the concept of a canopy resistance has been
introduced in
Chapter 7
.
The main requirements for a model of the canopy resistance is that it should react
to external factors in a way similar to the reactions of the stomatal resistance (see
Section 6.4.3
). Furthermore, one would expect that the modelled canopy resistance
decreases when the amount of leaf area increases, as more leaf area provides more
parallel pathways for water vapour transport. However, because the microclimate
(temperature, radiation and humidity) varies vertically within a canopy and the vari-
ation of the orientation of leaves exposes them to different amounts of radiation, the
dependence on leaf area may not be a simple one (Jarvis and McNaughton,
1986
;
Baldocchi et al.,
1991
; Ronda et al.,
2001
).
Whereas in the meteorological context it is logical to express the stomatal control
on transpiration in terms of a resistance, the plant-physiology literature more often
uses the reciprocal of the resistance: the conductance. Here we use both terminolo-
gies interchangeably. We discuss two approaches to model the canopy conductance:
the empirical Jarvis-Stewart approach and the plant-physiology-based
A
-
g
s
(or Ball-
Berry) approach.
Jarvis-Stewart Approach
The general structure of the so-called Jarvis-Stewart approach (Jarvis,
1976
and
Stewart,
1988
) is based on a minimum stomatal resistance (i.e., the stomatal resis-
tance under optimal conditions) that is modiied by a number of empirical response
functions plus a scaling from a single square meter of leaf to a canopy:
r
()
()
r
=
s,min
fK fD f
()(
↓
)
T
f
θ
(9.24)
c
1
2
q
,a
3
a
4
LAI
where
r
s,min
is the minimum
stomatal
resistance; LAI is the leaf area index (surface
area of leaves per surface of ground); and
f
1
,
f
2
,
f
3
and
f
4
are response functions
for the inluence of global radiation, vapour deicit
D
q
,a
(here in terms of speciic
humidity: (
q
sat
(
T
a
) -
q
a
)), air temperature
T
a
and soil moisture (
θ
is soil moisture
content averaged over the part of the soil column where roots are present, not to
be confused with potential temperature). The proportionality of
r
c
to LAI
-1
can
be understood when considering
r
c
as the replacement resistance for a number of
parellel resistances (where the number of resistances is LAI, and the magnitude of
those resistances is
r
s,min
).
A wide range of formulations exists for the response functions, differing both
in the exact shape and in the parameter values involved. As an example, we here
Search WWH ::
Custom Search