Agriculture Reference
In-Depth Information
Crop Factors and Crop Coefficients for Vines at Different Growth Stages
on a Sandy Loam Soil in a Temperate Climate
Table 6.4
Crop factor
C
f
a
Crop coefficient
C
c
b
RDI
c
RDI
c
Stage of Growth
No Stress
No Stress
Bud burst
0.1
0.1
0.2
0.15
Flowering
0.25
0.25
0.4
0.3
Veraison
0.5
0.25
0.7
0.5
Harvest
0.5
0.25
0.75
0.55
Post-harvest
0.25
0.15
0.75
0.55
a
The crop factors are for healthy vines, on a single trellis up to 2 m high, with a maximum canopy
coverage of 70-80% of the ground, and a mown, inter-row cover crop (Goodwin 1995).
b
The crop coefficients are for similarly managed vines, except that the inter-rows are bare. A decrease
in either
C
f
or
C
c
postharvest depends on whether the vine canopy continues to actively
photosynthesize.
c
Assuming that the
SWD
is to be maintained between the no-stress and lower stress limits (
SWD
between
50 and
80 mm/m depth in this soil)
Although a weekly time period is used, rainfall and evaporation should be
measured daily. A
SWD
develops only when
E
a
P
, and the solution to these
equations is negative.
SWD
values calculated in this way are used to determine
whether the soil water content lies in the desired range. In California's Central
Valley, where the weather during the growing season is very reliable, irrigation
can be scheduled in advance by substituting historical average weather data in
these equations. This approach is called the “volume balance approach” (Williams
Box 6.11
Measurement of Crop Coefficients and Crop Factors
E
a
for vines can be measured using a
weighing lysimeter
, similar to that
installed at the Kearney Agriculture Center at Fresno, California. This lysimeter,
which is 2 m wide
2 m deep, is planted with two vines, but is
located in a vineyard so that the ambient conditions for these vines are no different
from other vines. The lysimeter, which sits on a weighing mechanism so that
E
a
can be measured from its change in weight, is automatically drip-irrigated when
ET
4 m long
2 mm. Thus, the actual evaporation rate from well-watered vines can be
measured as the grapevine develops, and the ratio of
E
a
to
E
p
established to obtain
the crop coefficient
C
c
. In California,
E
p
values calculated from a Penman-type
equation are available from a network of 90 weather stations that comprise the
California Irrigation Management System (Pitts et al. 1995).
Weighing lysimeters of this size are not commonly available. However,
Williams (1999) has found that vine water use at Fresno is linearly related to the
percentage of soil surface shaded by the vine canopy, once this is
15%. This
property, measured using a digital camera, can be used as a surrogate variable for
E
a
. An advantage of this approach is that differences in shaded area due to
different trellis systems can be measured, and values of
E
a
/
E
p
C
c
for particular
vine-trellis systems calculated. Where pan evaporation
E
o
is measured, values of
C
f
are calculated the same way.
RDI
can also be applied to the vines in the lysimeter
to enable
C
c
and
C
f
values for vines under some stress to be estimated.
