Agriculture Reference
In-Depth Information
Box 4.8 Calculating a Soil Water Deficit
he
SWD
is estimated relative to the soil profile water content at
FC
(sometimes
calledthe“reillpoint”).Normally,inwinterrainfallregions,thesoilregains
FC
by
bud burst as a result of winter rain, or, when this rain is insufficient, supplementary
irrigationmaybeused.hewatercontent(inmillimeters)inthefullproileat
FC
is best calculated from the average of
θ
measurements made toward the end
ofwinter(seeequationB4.2.3,box4.2).Alternatively,
PAW
, estimated from the
textures of soil layers as shown in table 4.4, can be used as an estimate of the refill
point.
At the refill point, the
SWD
istakenaszeroandadeicitdevelopsasthesoil
water content falls below this value. Above the refill point, the soil holds surplus
water that can drain away within one to two days and is not considered to be
available to plants. By convention, the
SWD
is plotted as a negative value, and any
waterheldabovethereillpointisplottedasapositivevalue(seeigure4.17).
160
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
140
120
100
80
60
40
20
0
−
20
Summer
Summer
Summer
−
40
Winter
Winter
−
60
−
80
−
100
Figure 4.17
heseasonalchangeinsoilwaterdeicit(
SWD
)overthreeyearsforSauvignon
Blancwithoutirrigationinacool-climatevineyardinVictoria,Australia.Rainfall(solidlines);
SWD
(dashedline).Whenthe
SWD
ispositive,drainagecanoccur.DatesareinU.S.format.
(OriginaldatacourtesyofDr.LilangaBalachandra,UniversityofMelbourne,Australia.)
thescion-rootstockcombination)andthestyleofwinetobemade.Forexample,
ShirazisavigorousvarietyoftenrequiringamoresevereRDIregimetooptimize
qualitythanMerlot,whichismoresensitivetowaterstress.CabernetSauvignon
is intermediate. Furthermore, rootstocks confer differential tolerance to water
stress on the scion varieties, as discussed in “Rootstocks” in chapter 5.
The preferred RDI method uses a threshold deficit, which is the deficit that,
when exceeded, signals the need for irrigation. In Australia, RDI is usually applied
