TARGAS-1 Operation Manual V. 1.02 87 support@ppsystems.com
M
= molecular weight of air
C
= specific heat at constant pressure
r
= boundary layer resistance to water vapor transfer, empirically determined for each cuvette by
the pseudo-leaf (filter paper) method. 0.93 converts it to that for heat transfer.
= Stefan Boltzmann constant
T
= cuvette air temperature
H is calculated from the photon flux incident on the cuvette (Q), taking into account the ratio of infra-red to
visible radiation and typical reflection/absorption factors by the leaf:
H = Q × Trans
Where Trans = 0.14 the ratio of infrared to visible radiation, and converting photon flux to energy units
To simply computation, the following approximation is made:
×
((
T
+ 273)
)
4.639 +
(
0.5834 × T
)
(A.7) Then, the leaf temperature is:
T
=
(
T
+ t
)
Saturation Vapor Pressure
Derive saturation vapor pressure at leaf temperature (e
) from T
(A.8) From Buck, 1981 (using e
w1
and f
w1
) we calculate:
e
= 6.1365 × exp
T
×
(
17.502
)
T
+ 240.97
Where e
= saturated water vapor pressure inside leaf at T
Stomatal Conductance
Calculate stomatal conductance (g
s
)
(A.9) From von Caemmerer & Farquhar, 1981 (Eq B14), total leaf conductance to H
2
O transfer is
calculated as:
g
=
E ×
(
P‐(e
+ e
)/2
)
(
e
‐e
)
(A.10) Since 1/g
total
= r
s
+ r
b
,
(A.11) stomatal resistance can be calculated as:
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