Agriculture Reference
In-Depth Information
Box 7.6
Calculating Optimal Spacings for Different Drain Depths in Clay Soils
The
K
s
values for clay subsoils lie between 1 and 5 mm/day. Suppose a pipe
drain system is designed to remove 10 mm of surplus water per day (
J
w
). Also
suppose the height of the water table midway between two drains is not to be
1
m below the soil surface. This means that the maximum
m
at the surface, in the
absence of evaporation, will be
10 kPa, which should provide an adequate air-
filled porosity in the soil. Referring to figure 7.6, we have
d
1.0
We can then solve equation 7.3 for different drain depths
d
to obtain the
optimum drain spacing
L
for soils of known
K
s
value, as shown in table B7.6.1.
In practice, drains are placed at wider spacings because of the very high cost
associated with spacings as close as those listed in table B7.6.1. Deep ripping
before planting will increase
K
s
and hence permit wider, more affordable drain
spacings.
h
Table B7.6.1
Optimal Drain Spacings for Different Drain Depths
Soil
K
s
Value
Drain Depth
d
h
Value
Drain Spacing
L
(m/day)
(m)
(m)
(m)
0.001
2
1
0.63
1.5
0.5
0.32
0.005
2
1
1.4
1.5
0.5
0.71
where
K
s
is the saturated hydraulic conductivity of the clay layer and
J
w
is the de-
sired rate of drainage. Box 7.6 provides an example of the use of equation 7.3.
7.2.4.2
Draining Sands and Gravels with High
K
Values
If the hydraulic conductivity of the substrata is high (
1 m/day), but ground-
water is a problem, pumps can be used to control the water table. Pipe drains can
also be used at much wider spacings than are possible in clay soils. If the ground-
water is saline, as in much of the Griffith wine-producing area in Australia, the
pumped water cannot be reused but must be transferred to evaporation basins.
Biological Properties
7.3
A biologically healthy soil has the following properties:
• Diverse and active macro- and microorganisms engaged in the
decomposition of organic matter, which releases mineral nutrients and
provides the residues that improve soil structure
• Earthworms and other macrofauna that burrow through the soil and
improve infiltration, drainage, and aeration
• A low incidence of pests and diseases
