Agriculture Reference
In-Depth Information
loam or clay, the suction pulling water sideways increases relative to the constant
forceofgravitysothatthewettedzoneismorelikealattenedonionbulb(igure
igure4.26B).Also,becauseiniltrationratesareslow,thepreferredemitterdis-
chargerateis0.8to2L/hourandtheemitterscanbeplacedfartherapart.
Table 4.6 shows the amount of water needed to deliver a specified RDI, depending
on soil texture and emitter spacing. In practice, if winegrowers are advised to control
thewatercontentofthetop0.5mofsoilonly,pulsedapplicationofwater(30minutes
onand30minutesof)maybenecessarytopreventdeeperwettinginsandyproiles.
Dependingonsoildepthandpermeability,subsurfacedripirrigation(SSDI)
linesareplaced0.20to0.25mbelowgroundalongavinerow(igure4.27).his
method is favored in clay soils where suction is the dominant force drawing water
throughthesoilanddrainagebelowtherootzoneisminimal;itisnotassuitable
for sandy soils in which gravitational movement of water is much greater. Because
soilevaporationtendstoconcentratedissolvedsalts,SSDIisagoodoptionwhen
reclaimedwaterisused(see“ReclaimedWater,”chapter2).
Directly pumped river water may contain colloidal sediment that can cause block-
ages, and some groundwaters may contain sufficiently high concentrations of soluble
Fe,which,onexposuretoair,precipitatesasaninsolubleFehydroxide(ferrihydrite).In
these cases, valves installed in individual drip lines can be monitored to check for block-
ages. The problem of Fe precipitates can be alleviated by passing through the delivery
line a solution of commercially available humic acid, which chelates and dissolves the
Fecompound(see“AvailabilityoftheMicronutrientsandTraceElements,”chapter3).
Table 4.7 summarizes the advantages and disadvantages of drip irrigation
relative to other systems.
Table 4.6
Examples of Water Requirements for One Application of RDI on Two Soils of
Different Texture
Total
water
required
(kL/ha)
b
Waterrequiredif
only one third of
the area is watered
(kL/ha)
Soil
texture
and depth
Vine
spacing
(m)
Irrigation
RDI
a
Loamy
sand,
0.6 m
1.5
×
2.5
4L/hremitters
at 0.6 m
spacing
(4444per
hectare)
38
375
125
Clay
loam,
0.6 m
1.5
×
3
2L/hrat1
m spacing
(2222per
hectare)
54
543
181
Note.
RDI = regulated deficit irrigation.
a
AssumetheRDIwaterrequiredisequaltoreadilyavailablewaterplusonehalfofthedeicitavailablewater,derivedfrom
table 4.4; figures rounded to the nearest mm.
b
1mm=1L/m
2
and 1 ha = 10,000 m
2
.
