Geoscience Reference
In-Depth Information
Question 8.2:
Explain why a crop factor (or set of crop factors) is linked to a speciic
deinition of the reference evaporation (in other words: why do crop factors for a spe-
ciic crop and growing stage differ between different deinitions of the reference evapo-
transpiration)?
Question 8.3:
For the determination of crop factors the actual evapotranspiration of a
crop should be measured, alongside the input variables needed for the calculation of the
reference evapotranspiration. Under what conditions (i.e., above what type of surface)
should those input variables for
E
ref
be measured?
8.1.4 Crop Factor Method: Penman-Monteith
Equation for E
ref
The FAO (Food and Agriculture Organization of the United Nations) has published
two manuals that propose the crop factor method for practical application. In Dooren-
bos and Pruitt (
1977
) the Penman equation was used to determine the reference evapo-
transpiration. But later it was recognized that the Penman-Monteith equation was
better suited to determine
E
ref
(Allen et al.,
1998
). Here we discuss the latter method.
Note that the method has been developed to be used mainly for estimations of evapo-
transpiration on daily to decadal (10-daily) basis.
Reference Evapotranspiration: A Hypothetical Crop
First the evapotranspiration of a reference crop
E
ref
(mm d
-1
) is calculated accord-
ing to the Penman-Monteith equation. This hypothetical reference crop is deined
as a full cover crop with height
h
c
= 0.12 m, a ixed canopy resistance
r
c
= 70 s m
-1
and an albedo
r
= 0.23. If the diurnal cycle of
E
ref
needs to be resolved canopy resis-
tances of 50 s m
-1
and 200 s m
-1
are to be used for daytime and nighttime, respec-
tively. The roughness lengths for momentum and heat are 0.012 m and 0.0012 m
respectively, and the displacement height is 0.08 m. This hypothetical crop closely
resembles an extensive surface of green grass of uniform height, actively grow-
ing, completely shading the ground and with adequate water. Because generally
not all data required to compute
E
ref
with the original Penman-Monteith equation
is available, Allen et al. (
1998
) list a range of empirical methods to deal with lack
of data:
•
The aerodynamic resistance is calculated from observed wind speed and the roughness
length of the reference crop, but without stability corrections. This could be warranted by
the fact that under well-watered conditions the stability correction is relatively small.
2
For daily and 10-daily calculations, the soil heat lux is generally neglected. For calcula-
•
tions with time steps of less than one day empirical relationships with net radiation are
2
But the daily mean aerodynamic resistance is not necessarily identical to the aerodynamic resistance calculated
with the mean wind speed and mean (near neutral) stability (see
Chapter 3
, on the Schmidt paradox).
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