Environmental Engineering Reference
In-Depth Information
0
0
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-2
-1
0
0
0.1
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0.4
0.5
pressure head, h (m)
water content,
θ
Fig. 18.9
Numerically calculated pressure heads (left) and water contents (right) versus depth for
infiltration in a homogeneous sand assuming a water flux
q
of 2
10
−
6
m/s. The solid line is for
×
t
=
0, while the distributions at other times (1, 5, 10, 20, 30, 40, 50, 60, 70 h) are indicated by
dashes of decreasing length
Having a constant water content between 0 and 3 m would greatly facilitate any
sampling effort when the water content has to be measured in a deep homoge-
neous field soil profile. As water infiltrated, a less negative pressure head developed,
reaching a constant value of
0.46 m from the soil surface to close to the water
table. In other words, with a pressure head gradient d
h
/d
z
−
0, the hydraulic gra-
dient d(
h
+
z
)/d
z
becomes one, which corresponds to a unit gradient. The water flux
is then determined by gravity only, often referred to as gravity flow. In that case,
the unsaturated hydraulic conductivity equals the applied water flux and the water
content in the profile is equal to the water content of hydraulic conductivity at
that flux.
Results for the homogeneous silt soil using the same water flux of 2
≈
10
−
6
m/s
are shown in Fig.
18.10
. The flux is now approximately five times lower than the
saturated hydraulic conductivity. Compared to the sand, the degree of saturation
degree will now be much higher. In contrast to the sand, the initial water content
profile is now much smoother, consistent with the reduced nonlinearity of the soil
water retention curve. The higher initial (at
t
×
0) degree of saturation for the silt
means that a smaller volume of pore space has to be filled with water during the
infiltration process as compared to the sand. As a result, the infiltration front moves
faster downward in the silt than in the sand.
=
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