Geoscience Reference
In-Depth Information
Extra Crop Coeficients for Nonstandard Conditions
The method of Allen et al. (
1998
) also provides a way to deal with nonstandard or
suboptimal conditions. For example, in the case of insuficient soil moisture, the crop
factor (
K
c
or
K
cb
) is multiplied with a water stress coeficient (equal to 1 for suficient
soil moisture, and less than 1, depending on the degree of water shortage). Nonstan-
dard conditions that are dealt with are, for example, situations in which the vegetation
cover is less than would occur under optimal conditions, or situations in which the
management is different than standard.
Question 8.4:
Figure 8.3
refers to a crop that is irrigated in regular intervals.
a) How can you identify from
Figure 8.3
the moments in which irrigation is applied?
b) Explain the rapid increase and decrease in time of
K
e
.
c) Why does the relative importance of
K
e
in the total crop factor decrease during the
'crop development' stage?
d) Why are there no peaks of
K
e
in the 'late season' growing stage?
8.1.5 Crop Factor Method: Makkink Equation for
E
ref
Instead of the Penman-Monteith equation, other methods can be used to compute
the reference evapotranspiration as well. In the Netherlands the Makkink equa-
tion is used to determine
E
ref
. The use of a less sophisticated method to determine
E
ref
for temperate conditions is warranted given the fact that - especially for the
growing season - evapotranspiration is largely driven by radiation. DeBruin and
Stricker (
2000
) showed that during the growing season the Makkink method is
equivalent to the Penman-Monteith equation (where the skill depends on which
variables have been measured and which have been approximated empirically)
and compares well with observed evapotranspiration from well-watered grass (the
reference crop).
Furthermore, the advantage of the Makkink method is that only observations of
global radiation and temperature are needed. Observations of wind speed and humid-
ity, which may be more easily disturbed by the exact location of the weather station,
are not needed. The limited amount of input data also makes the Makkink method
suitable to derive reference evapotranspiration from satellite data (Schüttemeyer
et al.,
2007
; DeBruin et al.,
2010
).
In
Appendix E
the crop factors for use with the Makkink equation for conditions in
the Netherlands are given for a variety of vegetations.
Question 8.5:
In the Netherlands the Royal Netherlands Meteorological Institute
(KNMI) reports the Makkink reference evapotranspiration. Below are given the clima-
tological monthly averages (period 1971-2000) for station De Bilt (station 280) (centre
of the Netherlands) in mm/month. In addition, the precipitation (
P
) is also given.
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