LI-COR LI-6400 - Page 445

Using an Energy Balance
The Theory
Using the LI-6400 17-3
(17-4)
The sensible heat flux is a function of the leaf - air temperature difference, the
specific heat capacity of the air c
P
(J mol
-1
K
-1
), and the one-sided boundary
layer conductance of the leaf g
b
(mol m
-2
s
-1
)
(17-5)
The energy balance equation now becomes
(17-6)
If we let , and note that for small DT
(17-7)
then we can solve (17-6) for DT
(17-8)
where for most leaves, W m
-2
K
-1
, and c
P
= 28 J
mol
-1
K
-1
. While a and k can vary widely for different light sources
(Figure 17-2 on page 17-5), their product is fairly conservative (Table 17-1).
L 44100E=
Q 2c
p
g
b
T
l
T
a
Ð()=
akR
Q
2es T
w
273+()
4
2es T
l
273+()
4
Ð+
44100E 2c
p
g
b
T
l
T
a
Ð()+
=
DTT
l
T
a
к
T
l
273+()
4
T
a
273+()
4
4 T
a
273+()
3
DT+»
DT
akR
Q
2es T
w
273+()
4
T
a
273+()
4
Ð 44100EÐ+
2c
p
g
b
8es T
a
273+()
3
+
-----------------------------------------------------------------------------------------------------------------------------------=
e 0.95»s5.67 10
8Ð
´=