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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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