Agriculture Reference
In-Depth Information
Irrigation Requirement for
RDI
by Overhead Sprinkler, Flood, or Furrow
Table A10.1
Volume per m
2
Depth of Soil
Depth of
Volume of Water
DAW
Drying
Irrigation Water
of Soil Surface
per ha
(ML)
a
(mm/m Depth)
(m)
(mm)
(L)
50
0.4
20
20
0.2
0.6
30
30
0.3
0.8
40
40
0.4
a
Megaliters
sitions. When the lower limit of
SWD
is reached, the amount of water needed to
replenish the
DAW
for different rooting depths (hence different depths of soil dry-
ing) is as given in table A10.1.
(b) For irrigation by
microjets
or
microsprinklers
, the water is applied within
the row, and the wetted soil area is restricted (ca. 1 m
2
). The volume of water re-
quired depends on the number of jets, which is determined by the planting den-
sity (vine by row spacing) and the rooting depth. The example is for
RDI
on a
soil of
DAW
0.05 m
3
/m
3
(50 mm/m depth). The irrigation requirements are
listed in table A10.2.
Note that microjets and minisprinklers conserve more water per ha than over-
head sprinklers, flood, and furrow.
(c) For
drip irrigation
(section 6.6.1), the spread of water under a dripper de-
pends on how dry the soil is and how the soil's hydraulic conductivity
K
changes
with
. Initially, when the soil is dry, matric forces pull the water laterally as well
as vertically. However, as the soil gets wet, gravity becomes the dominant force.
For short wetting times and soils of relatively low
K
, the wetting zone under a
dripper will be approximately hemispherical. For longer times, the zone becomes
more elongated downward. In soils of high
K
value (sands and sandy loams), the
wetting zone may become approximately cylindrical (with the long axis vertical).
Irrigation Requirement for
RDI
by Microjet or Minisprinkler
Table A10.2
Volume of Water (ML)
Volume of Water per Vine
(L) for a Rooting Depth of
per ha for a Rooting
Depth of
Vine by
Row Spacing
No. of
Vines
Wetted Area
per Vine
m)
(m
2
)
(m
per ha
0.4 m
0.6 m
0.8 m
0.4 m
0.6 m
0.8 m
Dense
1
2
5,000
1
20
30
40
0.1
0.15
0.2
Less dense
a
1.5
3
2,222
1
20
30
40
0.044
0.067
0.089
a
If the number of jets were increased to 2 per vine, the quantities of water required per ha would be similar to that
required in the denser planting.
