Agriculture Reference
In-Depth Information
Box 4.7
(continued)
Figure B4.7.2
A Class
A evaporation pan. The pan
diameter is 1.2 m and the
depth is 254 mm. The water
level is kept 54 mm from the
top rim.
Evenwhenthereisanadequatesupplyofsoilwater,
ET
from a vineyard does
not necessarily occur at the potential rate because the canopy may not be fully
developed. Research in California has shown that water use by well-watered vines
islinearlyrelatedtothepercentageofthesoilsurfaceshadedbythecanopy(at
midday),achievingamaximumrateatabout75%shading.Toaccountforthis
effect,
PET
is adjusted with a crop coefficient
C
c
to give an adjusted
ET
as follows
=
×
Adjusted
ET
PETC
c
(B4.7.1)
Similarly,if
E
p
data only are available, the adjusted
ET
is obtained using a
crop factor
C
f
accordingtotheequation
Adjusted
ET EC
p
=×
(B4.7.2)
Table B4.7.1 gives an example of crop coefficients and crop factors for two
vineyardsinsouthernAustralia.Notethat
C
f
values are consistently less than
C
c
because
E
p
is normally greater than
PET
.Notealsothatifdeicitirrigationis
f
(continued)
