Geoscience Reference
In-Depth Information
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Level 1
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Figure 21.6
Resistance scheme used to describe whole canopy surface energy flux exchanges in (a) multi-layer computer
simulation models, and (b) single source or 'big leaf ' models.
A second school of thought (Monteith, 1965) preferred the much simpler
big
leaf
approach to describe plant canopy exchange with the overlying atmosphere
(see Fig. 21.6b). The big leaf approach had its origin in the Penman-Monteith
equation and essentially assumes that the exchange of the whole canopy can be
adequately represented by assuming that all the radiation capture and partitioning
of energy into latent and sensible heat can be described as if it occurred at a single
level, the effective source-sink height. At this level, the whole-canopy, parallel-
average values of stomatal resistance and boundary-layer resistance control the
exchange between the hypothetical big leaf and the surrounding air, these
resistances being appropriately scaled-down from the resistance for individual
leaves by dividing by the leaf area index (
LAI
) of the whole canopy. The
aerodynamic resistance for latent and sensible heat is then used to represent the
turbulent transfer of energy fluxes upward into the atmosphere. In a simple big-
leaf model, the canopy-average boundary-layer resistance and the aerodynamic
resistance act in series for both the latent and the sensible heat transfer, and are
often combined as a single aerodynamic resistance.
The relative merits of multi-layer computer modeling of whole-canopy
exchanges versus the simpler big leaf approach were debated throughout the late
1960s and early 1970s. However, during the 1970s, the big leaf approach gained
preference over multi-layer computer modeling, mainly because it was realized
that multi-layer canopy models require a level of detail in the specification of can-
opy properties and canopy structure that limit their use to research sites where
such detailed knowledge might be available. It was also realized that when repre-
senting and modeling whole-canopy interactions, detailed representation of
within-canopy exchanges is less important numerically than adequately represent-
ing the major controls of stomatal resistance and bulk aerodynamic transfer
between the canopy and the overlying air. The big leaf approach is now almost
