Combination Methods for Turbulent Fluxes - Transport in the Atmosphere-Vegetation-Soil Continuum - page 263

Figure 7.3 Partitioning of available energy between latent heat lux and sensible heat
lux as a function of canopy resistance (a) and aerodynamic resistance (b) . Fluxes
determined with the Penman-Monteith equation, with a ixed available energy of 500
W m -2 , T a = 20 °C, RH = 60%. In (a) r a is set to 50 s m -1 . Because the surface tem-
perature increases when transpiration decreases, the longwave emission will increase
with increasing r c (the effect of this is shown in thin lines). The effects of changes in
stability on r a have not been taken into account.
To summarize: in the case of a decrease in r a , the type of energy that is most easily
available is extracted extra from the surface. If the surface is wet (or at least has a
low r c ), latent heat is most readily available, so evapotranspiration will beneit from a
lower r a , at the expense of the sensible heat lux. But for higher r c the situation will be
reversed, as sensible heat is more easily available than latent heat (release of water to
the atmosphere is hampered by stomatal closure).
The partitioning between latent and sensible heat lux usually varies through the
day because available energy and latent heat lux are out of phase ( Figure 7.4a ). The
latent heat lux generally peaks a few hours later than the available energy and hence
the sensible heat lux peaks before the available energy. This observation can be
explained using the Penman-Monteith equation as follows. Whereas the radiation
term in the Penman-Monteith equation is in phase with the available energy (pro-
vided r a and r c are constant), the aerodynamic term generally has its maximum later.
This delay in the peak of the aerodynamic term is caused mainly by the fact that the
vapour pressure deicit peaks late in the afternoon, which is closely linked to the fact
that the maximum temperature usually occurs late in the afternoon (and hence the
maximum in e sat ( T a ); Figure 7.4c , d ). In some cases VPD also increases by a decrease
in the vapour pressure due to entrainment of dry air (see Section 7.3.1 ). The phase
shift between radiation term and aerodynamic term also implies that the relative con-
tribution of both terms to the total evapotranspiration varies through the day, where
the contribution of the aerodynamic term gradually increases ( Figure 7.4b ).
Regarding the predictive powers of the Penman-Monteith equation the same dis-
claimer holds as for the Penman equation: the Penman-Monteith equations is only a