Agriculture Reference
In-Depth Information
contact. The rate of water loss (
Q
) from the permeameter is measured at steady
state infiltration. The value of
K
is calculated from an approximate formula that
allows for the lateral suction effect on infiltration, that is,
Q
/
r
0
2
K
(A8.2)
1
4/
r
0
The value of
ranges from 0.0.05 to 0.5 cm
1
for clay loams to sands. The disc
permeameter is used to obtain the relationship between
K
and
p
for the initial
part of the curves shown in figure B6.7.1.
K
values measured close to saturation
can vary 10- to 100-fold within an area of 1 ha because of soil variability. A pres-
sure head of
20 mm (
0.2 kPa) is recommended for measuring
K
s
, because at
this small head, the effect of very large pores is eliminated and the measured
K
s
is less variable spatially. Even so, at least 10 measurements should be made per
hectare to obtain a mean
K
s
of acceptable precision.
Appendix 9
Table A9.1
Calculation of Stored Soil Water at
FC
(mm/m depth of soil)
for a Soil Profile with Varying Texture with Depth
Depth at
Which
Effective
Stored Water
is
Measured
Depth
per Depth
at
FC
a
Interval,
z
Interval,
z
Soil Texture
(m)
(m
3
/m
3
)
(m)
(mm)
90
b
Sandy loam
0.2
0.30
0.3
50
b
Sandy loam
0.4
0.25
0.2
84
b
Clay loam
0.6
0.42
0.2
84
b
Clay loam
0.8
0.42
0.2
42
b
Clay
1.0
0.42
0.1
Total effective
Total profile
depth of measurement
stored water
1 m
S
350 mm
a
Measured by neutron probe or TDR (section 6.4.3)
b
z
1000
Volume of Irrigation Water Applied
by Different Irrigation Methods
Appendix 10
The soil is a sandy clay loam for which the
DAW
is 0.05 m
3
/m
3
or 50 mm/m
depth. The desired
SWD
range is between
80 and
130 mm/m depth of soil.
The data refer to water required per ha and per vine, for
RDI
by different irriga-
tion methods, on vines planted at different densities.
(a) For
overhead sprinklers
,
flood
, or
furrow
methods, the volume is calculated
per ha. It is assumed that the vine roots extend to the midrow and midvine po-
