Agriculture Reference
In-Depth Information
Typical Values for RAW and DAW in Soils of Different Texture
Table 6.3
Loamy Sand to
Sandy Loam
Sandy Clay Loam
to Clay Loam
(mm/m Depth of Soil)
Silty Clay to Clay
RAW
DAW
RAW
50
30
80
70
50
120
60
70
130
DAW
kPa in clay soils. When these limits are reached, irrigation should be applied. The
difficulty is to know how much water to apply to return the bulk of the root zone
to
40 to
60 kPa, an issue discussed in the following section.
Using Soil Water Deficits to Manage Irrigation
The
6.5.2
m
range between no stress (
40 kPa) and the lowest stress limit (
400 kPa)
defines the desirable deficit range. Because of the relationship between
m
,
these stress limits can be expressed in terms of
SWD
. The control of
SWD
in a
desired range by irrigation is called
regulated deficit irrigation
(
RDI
).
To implement
RDI
, the value of
and
should be measured at regular intervals
down to at least 1 m, so that the value of
S
, in mm of water per m depth of soil,
can be calculated (section 6.4.3). At least one site per vineyard block should be
chosen, in a way that samples any variation in profile soil water content caused
by soil type and topography. By regular measurement of
S
, the development of a
SWD
can be monitored. The
SWD
is then related to the soil's
RAW
and
DAW
,
and a decision can be made on when to irrigate and how much water to apply.
Examples of
RAW
and
DAW
values for soils of different texture are given in table
6.3. An example of
RDI
, based on this approach, is given in box 6.10.
Using Crop Factors or Crop Coefficients to Manage Irrigation
Rather than directly measuring the
SWD
, we can calculate the expected
SWD
from weather data. In most vineyards, the soil will be at
FC
at bud burst. The
subsequent development of an
SWD
depends on the balance between rainfall and
evapotranspiration, and the rooting volume of the vines.
Rainfall is easily measured (box 6.5), and
ET
can be estimated from an evap-
oration pan (
E
o
) or calculated from weather data (
E
p
). However, the
actual evap-
oration rate
(
E
a
) from a vineyard depends on the vine spacing and canopy devel-
opment, and the degree of soil water stress imposed on the vines. If
E
o
values are
available, an empirical
crop factor C
f
is used to calculate
E
a
(mm/day), according
to the equation
E
a
E
o
C
f
(6.12)
An example of crop factors for a vineyard in southern Australia is given in
table 6.4. These factors are for vines that are well watered or under
RDI
. When
the vines are experiencing some stress (under
RDI
), the transpiration rate is low-
ered and the crop factors are therefore less than for fully irrigated vines. If
E
p
is
6.5.3
