Geoscience Reference
In-Depth Information
7
Combination Methods for Turbulent Fluxes
In
Chapter 3
a number of methods were presented that can be used to determine the
atmospheric luxes of the surface energy and water balance: sensible and latent heat
lux. Those methods were based solely on the use of data regarding wind, temperature
and humidity: either the luctuating parts of the signal (eddy-covariance method) or
the mean values: vertical gradients or differences (similarity theory).
In this chapter we not only use our knowledge on the turbulent luxes, as dealt
with in
Chapter 3
, but also combine it with the energy balance equation (
Chapters 1
and
2
) and information on the vegetation (
Chapter 6
). First the Bowen ratio method
is discussed in
Section 7.1
. Next the Penman-Monteith equation that describes the
transpiration from vegetation is dealt with in
Section 7.2
. Finally, simpliied estimates
for evapo(-transpi)ration are given in
Section 7.3
and dewfall (inverse evaporation) is
discussed in
Section 7.4
.
The term “combination methods” in the title of this chapter has two different con-
notations:
•
In general, the term “combination methods” refers to methods that combine the
energy balance equation with information on turbulent transfer (all methods in this
chapter).
In a more restricted sense, the term “combination equation” refers to the Penman method
•
(and derived methods) that combines the effects of two factors that determine evapora-
tion (see Farahani et al.,
2007
). These are available energy (represented by the “radiation
term”) and atmospheric demand (ability of the atmosphere to remove water vapour, rep-
resented by the “aerodynamic term”). In this restricted sense, the term refers only to the
methods discussed in
Section 7.2
.
7.1 Bowen Ratio Method
7.1.1 Sensible and Latent Heat Flux
The Bowen ratio is the - dimensionless - ratio of sensible heat lux and latent heat
lux. If we apply this to the surface luxes, the
surface
Bowen ratio is deined as:
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