Geoscience Reference
In-Depth Information
22
Whole-Canopy
Interactions
Introduction
Real vegetation canopies are extended three-dimensional entities. Even in the
case of agricultural crops in the middle of their growth season (arguably the
most uniform vegetation cover), assuming the whole-canopy interaction can
be adequately represented as if it occurred at a single effective source/sink level as
in the Penman-Monteith equation requires some faith, because observed profiles
of atmospheric variables change substantially with height, see Fig. 22.1.
However, as already discussed in Chapter 21 (see Fig. 21.6 and associated text),
the alternative approach of using numerical models to simulate multi-level
exchanges is impractical, given that such models require detailed site- and time-
specific knowledge of canopy structure that is rarely available. Multi-layer canopy
models also usually make the problematic assumption that K Theory applies
within plant canopies, as discussed in Chapter 19 (see Fig. 19.3). But, given the
necessity to resort to the single source/sink assumption, how can the knowledge
of in-canopy processes discussed in the last chapter be used to best advantage in a
big leaf model? This is one of the topics considered in this chapter.
When the big leaf representation of the plant-atmosphere interactions and
Penman-Monteith equation are adopted, surface exchanges depend on the values
of weather variables at some reference level in the ABL. However, the discussion of
ABL development given in Chapter 18 indicates that atmospheric variables in the
ABL (including those at the reference level) are themselves partly determined by
surface energy inputs. Thus, because the air in the ABL is not totally 'free' but rather
often partly 'contained' by an inversion layer, feedback processes can come in to
play, with surface exchanges not only determined by, but also in part determining,
the value of near surface weather variables. The effect of ABL feedbacks on area-
average surface exchange is also discussed in this chapter.
