Agriculture Reference
In-Depth Information
Example 9.7
A subsurface drainage should be provided through an agricultural farm of several
kilometers long. Average expected recharge to the water table in the area is about
3 mm/d. It is required to maintain the water table not closer than 1.2 m from the
soil surface. The value of hydraulic conductivity up to 2.0 m is 40 mm/d, and below
that is 42 mm/d. Relatively, impervious layer exists 8 m below the soil surface.
Determine the drain spacing.
Solution
q
=
3mm/d
1.2 m
2m
0.8 m
2r
0
d
e
D
=
6m
S
We know, Hooghoudt's drainage equation is
4
k
a
h
2
q
8
k
b
d
e
h
q
S
2
=
+
Given,
q
=
3 mm/d
=
0.003 m/d
k
a
=
40 mm/d
=
0.04 m/d
k
b
=
42 mm/d
=
0.042 m/d
D
6m
Assuming drain diameter
=
=
20 cm. Thus,
r
0
=
20/2
=
10 cm
=
0.1 m
=
−
=
Placing the drain at 2.0 m below the surface,
h
2.0
1.2
0.8 m
Determination of equivalent depth (
d
e
):
We know,
S
8ln
S
π
π
r
0
d
e
=
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