Agriculture Reference
In-Depth Information
■
The relationship between
m
is called the
soil water retention curve
. This
relationship determines how much water is released from the soil pores under a
negative pressure (or suction) or how much drains under the influence of gravity.
The
and
value to which a soil drains, 48 hours after being thoroughly wetted, de-
fines its
field capacity
(
FC
), when
m
is approximately
10 kPa. The
value at
which plants lose turgor and wilt is called the
permanent wilting point
(
PWP
),
when
m
is approximately
1500 kPa. The amount of water held between
FC
and
PWP
is the
available water capacity
(
AWC
), which is the maximum amount
of plant-available water per unit depth of soil. The pore-size distribution of a non-
swelling soil can be derived from the soil water retention curve.
■
The processes of rain falling, infiltration, runoff (by surface and subsurface lateral
flow), evaporation, and drainage are part of the
hydrologic cycle
in a vineyard. In-
filtrated water is redistributed according to gradients in
in the soil profile. The
water balance of a vineyard over time may be written as
P
E
a
R
s
R
ss
D
S
where
P
is precipitation,
E
a
is the actual evapotranspiration from soil and vines,
R
s
and
R
ss
are surface and subsurface runoff, respectively,
D
is deep drainage be-
low the root zone, and
S
is the change in soil water storage. All these terms are
measured in mm, which is a volume of water per unit area. When runoff and
drainage are negligible and
E
a
P (as in summer),
S
is negative and a
soil wa-
ter deficit
(
SWD
) develops.
■
When vines are well supplied with water, they transpire water (evaporation through
the leaves) at the
potential rate E
p
, which is determined by the net solar radiation
absorbed and the weather conditions. As soil water becomes limiting, the vine be-
gins to suffer some water stress and the transpiration rate falls. Soil water held at
values above
40 to
60 kPa, depending on soil texture, is called
readily avail-
able water
(
RAW
), whereas water held from
40 to
60 kPa down to
100 to
400 kPa is called
deficit available water
(
DAW
).
■
RAW
and
DAW
should be calculated for the effective rooting depth of the vines,
which in an irrigated vineyard may be 60-70 cm. Managing the soil water within
these ranges is the basis of
regulated deficit irrigation
(
RDI
).
RDI
is important for
controlling “excess vigor” in irrigated vines, especially in warm to hot regions.
Par-
tial root zone drying
(
PRD
) is another technique for saving water, without sacri-
ficing yield or grape quality. Dryland vines usually have more extensive root sys-
tems than irrigated vines.
■
RDI
can be applied by monitoring soil
values or by calculating the
SWD
, ei-
using a range of instruments or by estimating
E
a
.
Where
E
p
is known,
E
a
is the product of
E
p
and
C
c
, a crop coefficient, which de-
pends on the stage of canopy development and the degree of water stress to be
imposed. If only pan evaporation
E
o
is measured,
E
a
is the product of
E
o
and a
crop factor,
C
f
.
■
Irrigation by
overhead sprinklers
or
traveling irrigators
and by
flood
or
furrow
wets all
or most of the soil, whereas
drippers, microjets,
and
minisprinklers
allow targeted ap-
plication and conserve water. Drip irrigation is most suited to
RDI
or
PRD
, but the
water should be filtered to avoid blockage of emitters. Because the wet soil zone is
at a near-constant water content, water of higher salinity can be used, but adequate
winter rain is necessary to leach any accumulated salts at the edge of the wetted
zone. The efficiency of water use is higher for drip systems than for other systems.
ther by direct measurement of
