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Figure 8.1
Structure of the crop factor method: meteorology and properties of the
reference crop determine reference evapotranspiration. Crop type and stage deter-
mine optimal evapotranspiration for that crop. (Adapted from Allen et al.,
1998.
)
In fact, Eq. (
8.1
) serves as a
deinition
of the crop factor, but no claim is made yet
E
E
about its value:
K
≡ , which implies that the crop factor is speciic for a speciic
c
ref
choice of the reference evapotranspiration. The idea is that all variation of
E
with crop
type or management practice can be contained in the crop factor
K
c
(see
Figure 8.1
).
One should make a clear distinction between the
development
of the crop factor
method and the
use
of it:
•
The
development
of the crop factor method entails the performance of a large num-
ber of ield experiments, for different crops (and possibly different management
practices). In those experiments the actual evapotranspiration
E
needs to be mea-
sured, alongside the input variables that are needed to compute the reference evapo-
transpiration
E
ref
. From
E
and
E
ref
the crop factor can be computed (which will vary
through the growing season). This inally yields a tabulated collection of crop factors
for various crops, for various crop growth stages (initial, lowering, maturity, full
senescence).
The
•
use
of the crop factor method (e.g., to determine water requirements for a crop)
entails the calculation of the reference evapotranspiration from observed mete-
orological data and the selection of the appropriate crop factor from the tabulated
collection.
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