Geoscience Reference
In-Depth Information
energy between sensible heat and evaporation, and that for 'saturated surfaces'
a reasonable estimate of evaporation can be made from:
Δ
l
E
=
1.26
A
(22.32)
Δ+
g
However, Fig. 22.12 also shows that as surface resistance rises and water
availability at the surface decreases, evaporation rate necessarily falls and
ultimately becomes zero.
Important points in this chapter
Use of in-canopy results
: results from in-canopy studies of individual leaf
exchange and near-canopy turbulent transfer can improve representation of
whole canopy exchanges by single source, 'big leaf ' Penman-Monteith models.
●
LAI dependency of aerodynamic properties
: taking 0.2 as the average drag
coefficient for an individual leaf inside a canopy, Shaw and Periera (1982)
used a second order closure model of canopy exchange to give simulation
of zero plane displacement and aerodynamic roughness consistent with
field observations.
●
Excess resistance
: in one-dimensional models the fact that the exchange
coefficient for individual leaves is significantly less for momentum than for
other exchanges is often accommodated by assuming other exchanges act at
a source/sink deeper in the canopy and have 10 times less aerodynamic
roughness length than momentum.
●
Enhanced efficiency of near surface turbulence
: enhanced efficiency of
turbulent transfer near vegetation canopies has been accommodated in K
Theory by a height dependent re-definition of similarity relations and
typically reduces aerodynamic resistance for short crops by
●
∼
10% but for tall
(forest) crops by about a factor of two.
●
Rutter model
: the most successful representation of evaporation from
canopies wet or partially wet during and after rain is using the Rutter model
or derivatives thereof, which make a running balance of rain water stored on
vegetation and assume evaporation of intercepted water is proportional to
the fractional fill of a canopy water store.
Equilibrium evaporation
: when incoming radiant energy is incident on a
water surface evaporating into an isolated volume of air, the energy is shared
as latent and sensible heat inputs to the air such as to keep the air saturated
and evaporation occurs at a well-defined
equilibrium evaporation
rate given
by Equation (22.25).
●
Greater than equilibrium evaporation
: evaporation mainly occurs during
the day into an ABL partly (but not wholly) constrained by a stable inversion,
●
