Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 337

where the subscript 'w' denotes water limitation. f θ is identical to [()]

θ − used in

1

f 4

the Jarvis-Stewart method (see Eq. ( 9.28 )).
In the end, the A-g s approach contains the same stomatal responses to environmen-
tal factors as the Jarvis-Stewart approach, but in the A-g s method they are parameter-
ized in a way that is more closely related to the physiology of plants. The radiation,
temperature and the external CO 2 concentration enter the model through the model
for the net assimilation. The vapour pressure deicit determines the ratio of internal
to external CO 2 concentration. Soil moisture - empirically - affects the reduction of
the gross assimilation. Owing to the close relation between the A-g s method and the
physiology of plants, synergistic interactions between different responses are implic-
itly incorporated in A-g s models, rather than that all responses act independently, as
in the Jarvis-Stewart approach. Furthermore, some of the parameters in A-g s models
are related to widespread characteristics of the photosynthesis process and thus vary
little between plant species (Jacobs, 1994 ).
Up to this point only the stomatal conductance was dealt with. To scale this to a
canopy conductance one would need to integrate over all layers of a canopy where
all environmental factors close to the leaf may vary (radiation, temperature, humidity
and external CO 2 concentration). Ronda et al. ( 2001 ) take into account the vertical
exponential decay of PAR through the canopy, but use single values for leaf tempera-
ture and vapour pressure deicit. More elaborate methods can be found in, for exam-
ple, Goudriaan ( 1977 ) and Sellers (1984).

Question 9.11: Assimilation increases with increasing internal CO 2 concentration and
light input.
a) Determine the initial slope of the CO 2 response curve (Eq. ( 9.30 )).
b) Determine the initial slope of the light response curve (Eq. ( 9.31 )).

Question 9.12: Given the following observations at leaf level: I PA R = 300 W m -2 ,
D = 1.5 kPa, q ce = 5.77 · 10 -4 kg kg -1 (corresponding to 380 ppmv). Leaf temperature
is 298 K. Do the following calculations for a C 3 plant (see Table 9.5 for plant param-
eters).
a) Calculate the internal CO 2 concentration.
b) Calculate the CO 2 -limited net assimilation A n,c .
c) Calculate the radiation-limited gross assimilation A g,cl .
d) Calculate the stomatal conductances for CO 2 and water vapour.
e) Calculate the stomatal resistance for CO 2 and water vapour.

Question 9.13: Given a vegetation with LAI equal to 2. Assume that half of the leaf
area is exposed to I PAR =300 W m -2 and the other half to I PAR =100 W m -2 . The other
environmental conditions are identical to those given in Question 9.12 .
a) Calculate the stomatal conductance for water vapour for the leaf area exposed to
I PAR =100 W m -2 .
b) Calculate the canopy conductance for water vapour for this canopy.