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water pressure head
h
(cm)
50
0
10
20
30
40
60
0
water
-10
-20
-30
-40
-50
z (cm)
Bottom sealed,
No flow
soil
Bottom open,
Flow downward
-60
Figure 4.17
Soil water pressure head in sealed and free draining soil column.
Constant head
Water
Water
L
1
k
1
L
2
k
2
Homogeneous
soil column with
equivalent
conductivity
Soil column
with
N
layers
k
eff
N
Σ
L
j
j
=1
k
N
-1
L
N
-1
k
N
L
N
q
q
Figure 4.18
Water low through a layered and a homogeneous saturated soil column,
both with the same soil water lux.
The ratio
L
j
/
k
j
can also be viewed as a
hydraulic resistance
. In fact, in Eq. (
4.14
)
we are adding the hydraulic resistances of soil layers in series to get the hydraulic
resistance of the entire proile. Rewriting Eq. (
4.14
) results for the effective hydraulic
conductivity in:
N
∑
L
j
eff
=
=
j
1
k
(4.15)
L
k
N
∑
j
j
=
1
j
With
k
eff
and Eq. (
4.14
) we can derive the soil water lux
q
. Subsequently, the pres-
sure drop across any homogeneous layer within the column may be calculated using
Darcy's law and the saturated hydraulic conductivity of the involved soil layer.
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