Environmental Engineering Reference
In-Depth Information
Table 6.2 Hydraulic properties of generic sediments from Carsel and Parrish (
1988
) and Loheide
et al
.
(
2005
).
θ
s
,
θ
r
,
n
, and
α
are parameters of the van Genuchten (
1980
) equation (
Equation 5.2
).
K
s
is satu-
rated hydraulic conductivity. Estimates of
S
y
are from: A) Equation (
6.5
) after Duke (
1972
) with depth to
water table,
H
d
, of 1 m; B) Equation (
6.5
) with
H
d
= 2 m; C) Equation (
6.8
) after Nachabe (
2002
) with
H
d
= 1 m and drainage time, Δ
t
= 12 hours; D) Equation (
6.8
) with
H
d
= 2 m and Δ
t
= 12 hours; and E)
Loheide
et al
. (
2005
).
Sediment
K
s
S
y
α
Tex tur e
n
m
-1
m/d
θ
s
-
θ
r
A
B
C
D
E
θ
s
θ
r
Sand
0.43
0.045
2.68
14. 5
7.1
0.39
0.38
0.38
0.35
0.35
0.32
Loamy sand
0.41
0.057
2.28
12 .4
3.5
0.35
0.34
0.35
0.31
0.31
0.26
Sandy loam
0.41
0.065
1.89
7. 5
1.1
0.35
0.29
0.31
0.27
0.27
0.17
Loam
0.43
0.078
1.56
3.6
0.25
0.35
0.19
0.23
0.19
0.20
0.08
Silt
0.46
0.034
1.37
1.6
0.06
0.43
0 .11
0.15
0 .11
0 .13
0.03
Silt loam
0.45
0.067
1.41
2
0 .11
0.38
0.12
0.17
0.12
0.15
0.04
Sandy clay
loam
0.39
0.1
1.48
5.9
0.31
0.29
0.17
0.20
0.16
0.17
0.07
Clay loam
0.41
0.095
1.31
1.9
0.06
0.32
0.08
0 .11
0.08
0.10
0.02
Silty clay
loam
0.43
0.089
1.23
1
0.02
0.34
0.04
0.06
0.04
0.06
0.01
Sandy clay
loam
0.38
0.1
1.23
2.7
0.03
0.28
0.07
0.09
0.07
0.07
0.02
and yield. The height of the column is also
important. A column that is shorter than
the height of the capillary fringe is likely to
produce a value of
S
y
that is much less than
that obtained from a column that is greater
in height than that of the capillary fringe
(Prill
et al
.,
1965
). Romano and Santini (
2002
)
described similar laboratory techniques for
approximating field capacity. For practical
purposes,
S
r
and field capacity are often con-
sidered equivalent.
Many variants of Equation (
6.5
) exist,
whereby
S
r
or field capacity is estimated
from information on water-retention curves.
Jamison and Kroth (
1958
) proposed setting field
capacity equal to volumetric water content at
a pressure head of -33 kPa; this approach has
gained widespread acceptance within the agri-
cultural community. Cassel and Nielsen (
1986
)
pointed out that field capacity for coarse-
textured soils is usually better represented
by water content at -10 kPa, whereas water
content at a pressure head of much less than
-33 kPa is appropriate for fine-textured soils.
If the water-retention curve is measured (or
approximated by pedotransfer functions as
described in
Section 5.2.3
), water content at
any prescribed pressure head can be used to
obtain an estimate of
S
r
.
Field methods for determining
S
y
a q u i f e r
t e s t s
Traditional aquifer tests provide
in-situ
m e a s u r e -
ments of
S
y
a nd of t he r a q u i f e r pr op e r t ie s t h at a r e
integrated over fairly large areas. Observation
wells are located at various distances from the
pumping well. Water-level drawdown vs. time
data from observation wells are matched with
theoretical type curves developed by Boulton
(
1963
), Prickett (
1965
), Neuman (
1972
), and
Moench (
1994
,
1995
,
1996
) to generate esti-
mates of
S
y
and transmissivity. The methods
have some drawbacks. Interpretation of results
is nonunique (Freeze and Cherry,
1979
), and
the validity of assumptions inherent in the
techniques is difficult to verify. Expense is also
a key concern; installation of pumping and
observation wells may be cost prohibitive for
many recharge studies. Aquifer tests require
