Combination Methods for Turbulent Fluxes - Transport in the Atmosphere-Vegetation-Soil Continuum - page 272

7.3.2 Priestley-Taylor Equation
Priestley and Taylor ( 1972 ) were the irst to recognize from observations that the
actual evapotranspiration from well-watered surfaces (water or low vegetation) was
generally higher than the equilibrium evaporation:

s

( )

LE = α (7.18)

QG *

v PT

s + γ

where α PT is the Priestley-Taylor coeficient which is of the order of 1.26 for well-
watered surfaces. For a wet surface, where the Penman Eq. ( 7.13 ) would be a good
estimate of the actual evaporation, the Priestley-Taylor equation implies that the aero-
dynamic term of the Penman equation is proportional to the radiation term: the aero-
dynamic term equals 26% of the radiation term. Fixing the proportionality between
the two terms of course omits some variability that occurs in reality. But, as shown in
the previous section, feedbacks in the ABL make that variations in the aerodynamic
term will be damped. Furthermore, because the aerodynamic term is only of the order
of one quarter of the radiation term, deviations from this strict proportionality (due to
variations in aerodynamic resistance or VPD) do not inluence the total evaporation
greatly.
For vegetated surfaces, the situation is more complex, because then we need to
compare the Penman-Monteith estimate for evapotranspiration (Eq. ( 7.16 )) with the
Priestley-Taylor estimate (Eq. ( 7.18 )). In that case not only r a and VPD play a role,
but more importantly the canopy resistance.
Figure 7.8 shows the observed dependence of α PT on the canopy resistance for typ-
ical mid-latitude summer conditions over short vegetation. Indeed for low values of
the canopy resistance (wet or well-watered surfaces) the value of α PT is of the order
of 1.1 to 1.2, whereas it decreases with increasing r c .
Despite the fact that the Penman-Monteith equation is not a predictive equation it
can still be useful for sensitivity analyses, provided that the results are treated with
caution. In the present context one interesting feature that can be investigated with
the Penman-Monteith equation is the dependence of the Priestley-Taylor coefi-
cient on wind speed (or aerodynamic resistance). Therefore Figure 7.8 also shows
the modelled dependence of α PT on canopy resistance, for a number of values for
the aerodynamic resistance. The tendency of the modelled α PT vs. r c is the same
as for the observations. Furthermore, for low values of the canopy resistance the
evaporation (and hence α PT ) increases with decreasing aerodynamic resistance (i.e.,
with higher wind speed), as one would expect. But, for high values of r c the evapora-
tion decreases with decreasing r a . This behaviour is in accordance with the analysis
related to Figure 7.3b . The value of the canopy resistance at which α PT is independent







∂ LE

r

of the aerodynamic resistance i.e.,

v

= 0 happens to be the point where α PT = 1

 

∂

a