Geoscience Reference
In-Depth Information
the potential rates of evaporation for well-watered grass and moist bare soil might
be related to that from open water using multiplicative factors.
By the mid-1970s this by then long-established way of thinking about the evaporation
process determined the United Nations' Food and Agriculture Organization (FAO)
recommended method for estimating the water requirements for irrigated crops
(Doorenbos and Pruitt, 1977), i.e., vegetation for which stomatal resistance is not
subject to water stress. FAO followed Penman's approach by first defining a potential
rate called '
reference crop evapotranspiration
',
ET
RC
, which was defined to be the evapo-
transpiration rate for short green grass plentifully supplied with water. This rate, it was
recommended, was to be estimated by one of several alternative equations depending
on available weather data. Evapotranspiration from any other well-watered crop,
ET
c
,
was then assumed to be calculated using a crop specific coefficient,
K
c
, thus:
ET
=
K ET
(23.12)
C
C
RC
FAO provided a table of
K
c
values for a range of well-watered vegetation stands the
values of which (although not individually traceable by reference) are assumed
to have been derived from field studies where the well-watered crop evapotran-
spiration rate
ET
c
and the weather variables needed to calculate
ET
0
were also
measured, so that
K
c
could be derived. Many years of application followed and
refinements to this approach were introduced, including experiments to evaluate
and/or validate
K
c
for different crops in different climates (e.g., Howell et al., 2002;
Inman-Bamber and McGlinchey, 2003; Barton and Meyer, 2008).
However, the agricultural community's adoption of the original Penman (1948)
approach as recommended by FAO failed to recognize important subsequent advances
in the specification of evapotranspiration. Penman (1963) himself determined that the
'two stage process' using the 'factor' approach wasn't needed, and computed 'potential
evaporation' from any natural surface using an aerodynamic term and an energy term
specific to that surface. Shortly afterwards, Monteith generalized Penman's 'one step'
approach and derived the Penman-Monteith equation (introduced in Chapter 21).
Notwithstanding the publication and widespread adoption of the Penman-Monteith
equation, agriculturalists continued to use the original two-step approach. But there
were signs that the approach might be problematic when
K
c
values derived in one
place were used for the same crop in another place with different weather conditions.
The reason for this was demonstrated by Wallace (1995) who showed that crop
coefficients are inherently a complex mixture of both the physiology of the crop they
represent and the climate within which
K
c
values are derived and/or used. They also
depend on the method used to calculate reference crop evaporation.
Penman-Monteith equation estimation of
E
RC
With this last point in mind, and recognizing the greater realism of the
Penman-Monteith equation, FAO subsequently modified their guidelines (Allen
et al.
, 1998) by adopting the Penman-Monteith equation to calculate reference
