Geoscience Reference
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(a)
500
Q
*−
G
L
v
E
H
400
300
200
100
0
-100
(b)
0.8
rad.
aero.
0.6
0.4
0.2
0.0
(c)
12
8
4
0
(d)
20
15
10
5
0
3
6
9
12
15
18
21
24
Time (hour UTC)
Figure 7.4
Diurnal cycle of latent and sensible heat lux
(a)
as determined with the
Penman
-
Monteith method with
z
0
= 0.03 m and
r
c
= 30 s m
-
1.
(b)
Relative contri-
bution of radiation and aerodynamic term to total evapotranspiration, for conditions
when (
Q
*
- G
) > 0 only Diurnal cycle of driving variables vapour pressure deicit
(c)
,
air temperature
(d)
and wind speed
(e).
(Data from Haarweg Meteorological station,
May 23, 2007)
descriptive equation. Only if it is used with observations that are related to the surface
and situation for which one would like to compute the evapotranspiration will it give
the correct answer.
Another note regarding the use of the Penman-Monteith equation for computa-
tions of evapotransporation is related to the surface temperature. Although it is no
longer visible in the expression (through Penman's linearization) it still plays a role.
The surface temperature is a complex resultant of the various heating processes (for
daytime: incoming radiation) and cooling processes (for daytime: upwelling radi-
ation, soil heat lux and turbulent luxes of heat and water vapour). As the surface
temperature affects many of these processes (
L
↑
,
G, H
and
L
v
E
), it will adjust itself
until the terms of the energy balance actually balance (see also
Figure 7.3
). When one
prescribes Q*
-
G
in the computation of evapotranspiration, one implicitly assumes
to know the resulting surface temperature beforehand. This is true when observed
values for
Q*
and
G
are used, or when
Q*
and
G
have a relatively ixed relationship
to incoming radiation (as is the case for well-watered vegetated surfaces).
Although the Penman-Monteith equation is designed to describe transpiration,
it can also describe the evaporation of water from the surface of vegetation (e.g.,
after rain or dewfall). In that case there is no stomatal control and the canopy
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