Agriculture Reference
In-Depth Information
betweenfruitsetandveraison,whenthethresholddeicitforirrigatingisequated
to
RAW
plus some fraction of
DAW
. Take the example given in table 4.3. For the
0to40-cmdepth(Ahorizon),thethresholdcouldbesetat
RAW
(=36mm)plus
one half of
DAW
(=18mm),givingatotalof54mm.hisamountis68%ofthe
PAW
to 40-cm depth. A smaller deficit is advocated from veraison to harvest so
that shoot growth is controlled, but sugar accumulation in the fruit is not inhib-
ited. Methods of applying the water are discussed later in “Irrigation Methods.”
Sustained deicit irrigation (SDI) is another approach to deicit irriga-
tion that has been applied in inland regions of southeastern Australia, where
indroughtyearsirrigationwatermaybeinshortsupply(see“ManagingSoil
Water,”chapter6).hisapproachisanalogoustothe“volumebalance”concept,
as practiced in California's Central Valley region. There the threshold deficit is
setbytheleafwaterpotential(discussedlater),and,whenthethresholdpoten-
tialisreached,thevine'sweeklywatersupplyisreducedto60%to70%ofits
full demand until after harvest. The vines' full demand is assessed from
PET
data.
Methods for monitoring vine water stress through soil or plant measurements
are discussed in the following paragraphs.
MonitoringSoilWater
As explained in table 4.2, one way of monitoring soil water is to measure matric
suction with tensiometers or gypsum blocks. The trigger for irrigation to start is
set between 60 and 400 kPa matric suction, depending on the soil's texture and
the degree of stress to be imposed.
Another way is to measure the volumetric water content
θ
. For
θ
measure-
ments, the preferred instruments are those that allow repeated nondestructive
sampling,suchascapacitanceprobes(basedonfrequencydomainrelectometry)
or probes based on time domain relectometry (TDR). Box 4.9 describes this
equipmentinmoredetail.Neutronprobesaregenerallynotusednowforhealth
and safety reasons.
The values obtained for all these measurements depend on soil type, depth,
and distance from the vine. Because all vineyards show some soil variation, there
should be at least one measuring site per soil type with at least two depths of
measurement—oneinthezoneofmaximumrootdensity(20to30-cmdepth)
andtheothernearthebaseofthe“managed”rootzone(50to60cm).Moresites
may be needed in undulating land where soil moisture is likely to vary between
the top and bottom of slopes, but a compromise must be made between the cost
of more sensors and the value of the extra information obtained. Tensiometers,
gypsum blocks, or access tubes for a capacitance probe should be placed in the
rowsandbetweenvinesatadistanceofatleast10to15cm(sandysoil)or20to
25cm(claysoil)fromanemitterorminisprinkler.
