Calculation of Fluxes
FIGURE 6.2. Conductors in parallel: vapor diffusion through the surface of a leaf.
resistance. If the total resistance were 10 m2 slmol, the coat resistance
were 6 m 2 slmol, Tb = 37" C and T, = 0" C, the temperature change
from the skin surface to the outer edge of the coat would be (6110) x
(37 - 0) = 22.2" C.
6.5 Resistors in Parallel
Now consider the loss of water from a leaf surface. Water can be lost both
through the stomata and directly through the cuticle of the leaf. These
represent parallel pathways for vapor loss, as shown in Fig. 6.2. Here C,,
and C,, represent water vapor concentration just inside and outside the
leaf epidermis. The conductances are for cuticle and stomata. From the
rules for parallel resistors in electronics, it is known that the combined
resistance of resistors in a parallel circuit is the reciprocal of the sum of
the reciprocals of the component resistors. The conductances simply add.
The total conductance for the leaf is therefore
The total, or equivalent resistance is:
The vapor concentration difference across the two resistances is the same,
and the vapor flux through each is proportional to the conductance of each
resistor.
6.6 Calculation of Fluxes
In order to use Eq. (6.7) to calculate the rate of vapor exchange or latent
heat exchange between plant canopies and the atmosphere, or between
FIGURE 6.2. Conductors in parallel: vapor diffusion through the surface of a leaf.
resistance. If the total resistance were 10 m2 slmol, the coat resistance
were 6 m 2 slmol, Tb = 37" C and T, = 0" C, the temperature change
from the skin surface to the outer edge of the coat would be (6110) x
(37 - 0) = 22.2" C.
6.5 Resistors in Parallel
Now consider the loss of water from a leaf surface. Water can be lost both
through the stomata and directly through the cuticle of the leaf. These
represent parallel pathways for vapor loss, as shown in Fig. 6.2. Here C,,
and C,, represent water vapor concentration just inside and outside the
leaf epidermis. The conductances are for cuticle and stomata. From the
rules for parallel resistors in electronics, it is known that the combined
resistance of resistors in a parallel circuit is the reciprocal of the sum of
the reciprocals of the component resistors. The conductances simply add.
The total conductance for the leaf is therefore
The total, or equivalent resistance is:
The vapor concentration difference across the two resistances is the same,
and the vapor flux through each is proportional to the conductance of each
resistor.
6.6 Calculation of Fluxes
In order to use Eq. (6.7) to calculate the rate of vapor exchange or latent
heat exchange between plant canopies and the atmosphere, or between