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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