Geoscience Reference
In-Depth Information
However, there are problems with the Penman
equation which make it less than perfect as an
estimation technique. The assumption is made that
the soil heat flux is unimportant in the evaporation
energy budget. This is often the case but is an
acknowledged simplification that may lead to some
overall error, especially when the time step is less
than one day. It is normal practice to use Penman
estimates at the daily time step; however, in some
modelling studies they are used at hourly time steps.
One major problem with the Penman equation
relates to its applicability in a range of situations
and in particular in the role of advection, as
discussed on p. 38. This is where there are other
energy sources available for evaporation that cannot
be assessed from net radiation. Calder (1990) shows
the results from different studies in the UK uplands
where evaporation rates vastly exceed the estimates
provided by the Penman equation. The cause of this
discrepancy is the extra energy provided by cyclonic
storms coming onto Britain from the Atlantic
Ocean, something that is poorly accounted for in the
Penman equation. The part of the Penman equation
dealing with the ability of the atmosphere to absorb
the water vapour (sensible heat transfer function)
does account for some advection but not if it is a
major energy source driving evaporation and it is
highly sensitive to the aerodynamic resistance term.
This does not render the Penman approach invalid;
rather, in applying it the user must be sure that net
radiation is the main source of energy available for
evaporation or the aerodynamic resistance term is
well understood.
2.52
y = -0.0024x
2.501
2.50
2.48
2.46
2.44
2.42
2.40
-10
0
10
20
30
40
50
Temperature (˚C)
Figure 3.7
The relationship between temperature and
latent heat of vaporisation.
1.35
y = -0.0042x
1.2905
1.3
1.25
1.2
1.15
1.1
-10
0
10
20
30
40
50
Temperature (˚C)
Figure 3.8
The relationship between air temperature
and the density of air.
Every other term in the equation is either a constant,
a simple relationship from another variable or can
be measured once. Of these four variables net
radiation is the hardest to obtain from meteor-
ological stations as net radiometers are not common.
There are methods of estimating net radiation from
measurements of incoming solar radiation, surface
albedo
(or reflectivity) and day length (see Oke,
1987).
The modified Penman equation provides estimates
of potential evaporation at a surface for time
intervals much less than the monthly value from
Thornthwaite. This makes it extremely useful to
hydrology and it is probably the most widely used
method for estimating potential evaporation values.
Simplifications to Penman
There have been several attempts made to simplify
the Penman equation for widespread use. Slatyer
and McIlroy (1961) separated out the evaporation
caused by sensible heat and advection from that
caused by radiative energy. Priestly and Taylor
(1972) derived a simplified Penman formula for
use in the large-scale estimation of evaporation, in
the order of 'several hundred kilometres' where it
can be argued that large-scale advection is not
