LI-COR LI-6400, Manual

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Using an Energy Balance
17-2 Using the LI-6400
17
Sometimes it is not convenient or even possible to measure leaf temperature
with thermocouple. LI-6400 chambers that do not measure leaf temperature
directly are the 6400-05 Conifer Chamber, the 6400-07 Needle Chamber, and
the 6400-08 Clear Bottomed Chamber. The method of getting leaf tempera-
ture in these cases is to use an energy balance. This chapter explains the the-
ory and the practice of using energy balance.
The Theory
The energy balance of a leaf in the LI-6400 cuvette has three major compo-
nents: net radiation
R (W m
-2
), sensible heat flux Q (W m
-2
), and latent heat
flux L (W m
-2
).
(17-1)
We consider two components to net radiation, solar and thermal
(
). The solar part we will estimate by converting the
incoming PAR reading in the chamber R
Q
(µmol m
-2
s
-1
) to total visible and
near IR energy using an empirical (measurable with a spectroradiometer)
conversion factor
k , and using the leaf absorptivity
a
averaged over the spec-
trum of the light source.
(17-2)
The net thermal term is the difference in black body radiation balance, deter-
mined by the chamber wall temperature
T
w
and the leaf temperature T
l
.
(17-3)
where
e
is the thermal emissivity of the leaf, and
s
is the Stefan-Boltzmann
constant. The 2Õs in (17-3) account for both sides of the leaf. Latent heat flux
L is the transpiration rate E (mol m
-2
s
-1
) converted to W m
-2
:
0
Q L
R
+ + =
R R
solar
R
thermal
+ =
R
solar
a
kR
Q
=
R
thermal
2
es
T
w
273 +
( )
4
2
es
T
l
273 +
( )
4
Ð =