Geoscience Reference
In-Depth Information
resistance, the boundary-layer conductance, that scales linearly with the area of
leaf, when calculating the mean boundary-layer resistance it is necessary to first
calculate the area-weighted average of the
reciprocal
of the boundary resistance
for each leaf, and then to take the
reciprocal
of that mean.
The numerical values of shelter factors are poorly defined and are likely to vary
greatly with canopy structure but they are believed to be of the order 2 to 3. The
fact that shelter factors are so large and so poorly defined compromises the value
of early wind tunnel research into the boundary-layer resistance for individual
leaves to some extent. However, basic knowledge of the order of magnitude of
boundary-layer resistances found in those studies, and the understanding they
gave of the difference between boundary-layer resistance for momentum transfer
and for other transfers, is important when writing equations describing the whole-
canopy aerodynamic resistance, as discussed in the next chapter.
Stomatal resistance
At each level in the canopy, contributions to the overall canopy exchange of sensible
heat flux originate from the exposed surface of the vegetation elements present at
that level, and arise because of the difference between the surface temperature of
the vegetation and the temperature of the canopy air stream. As just discussed, the
magnitude of the contributions is controlled by the mean boundary-layer resistance
of the vegetation elements at each level.
Plant cells are about 90% water and would quickly desiccate and die if exposed
to an unsaturated atmosphere. For this reason, plants seek to retain water content
using surface layers that are resistant to water loss. Most leaves, for example, have
a waxy cuticle that inhibits the loss of gases such as water vapor and carbon
dioxide from their surface. But, if plants are to grow, they need to allow the cells
within leaves not only to absorb photosynthetically active radiation but also to
have access to the CO
2
present in the atmosphere. They do this by gas exchange,
with internal cells gaining access to CO
2
through small pores in the leaf surfaces
called '
stomata
' which can be opened in environmental conditions favorable for
photosynthesis.
The same stomata that allow carbon dioxide to enter leaves also allow water
vapor evaporated from the moist cell walls inside the leaf to escape to the
atmosphere. Consequently, if the outside surface of vegetation canopy is dry (not
wet as after rainfall), the primary source of water leaving plants is from cells inside
leaves. This water vapor diffusing by molecular diffusion from sub-stomatal
cavities through stomata to the leaf surface and the need to diffuse through narrow
stomata inhibits the rate of water vapor flow, depending on the extent to which the
plant stomata are open. This inhibition on flow is represented in equations and
models by a resistance, the stomatal resistance,
r
ST
, to vapor flow from inside to
just outside the leaf, see Fig. 21.5. Thus, the stomatal resistance per unit area of leaf
is used in much the same way that boundary-layer resistance is used to represent
