Environmental Engineering Reference
In-Depth Information
Table 3.1. Average hydraulic conductivity (
K
s
), saturated water content (
θ
s
), residual water content (
θ
r
), and
van Genuchten
α
and
n
for generic soil texture classes as determined by Carsel and Parrish (
1988
).
Soil texture
K
s
(cm/hr)
θ
s
θ
r
α
(cm
-1
)
n
Clay
0.20
0.38
0.068
0.008
1.09
Clay loam
0.26
0.41
0.095
0.019
1.31
Loam
1.04
0.43
0.078
0.036
1.56
Loamy sand
14. 59
0.41
0.057
0.124
2.28
Silt
0.25
0.46
0.034
0.016
1.37
Silt loam
0.45
0.45
0.067
0.020
1.41
Silty clay
0.02
0.36
0.070
0.005
1.09
Silty clay loam
0.07
0.43
0.089
0.010
1.23
Sand
29.70
0.43
0.045
0.145
2.68
Sandy clay
0.12
0.38
0.100
0.027
1.23
Sandy clay loam
1.31
0.39
0.100
0.059
1.48
Sandy loam
4.42
0.41
0.065
0.075
1.89
boundary condition is assigned to the bottom
of the column. Flow out of the bottom of the
column represents recharge unless the depth
of the bottom column is above the water table,
in which case the flow represents drainage.
Multiple soil types can be represented in the
column to simulate water movement through
layered media.
The physically realistic approach for rep-
resenting flow through the unsaturated zone
offered by the Richards equation provides a use-
ful tool for identifying controls on groundwater
recharge and examining the effects that climate
and land-use change may have on recharge.
Richards-equation models can also account for
flow upward from the water table to the soil
zone. Numerical solution of Equation (
3.4
) can
be computationally intensive, however, because
of the nonlinear nature of the water-retention
and hydraulic conductivity terms. A number of
studies have used a Richards equation-based
model for estimating recharge or drainage,
including Fayer
et al
. (
1996
), Keese
et al
. (
2005
),
Smerdon
et al
. (
2008
), and Webb
et al
. (
2008
).
The Richards equation can be simplified by
application of the kinematic wave approxima-
tion, in which drainage within the unsaturated
zone is assumed to be driven solely by gravity
(Colbeck,
1972
):
1(
−
α
h
)
n1
−
[1(
+
α
h
) ]
n m 2
−
}
Kh
()
=
(3.6)
r
[1
+
(
α
h
) ]
n m/2
where θ is volumetric water content,
h
is pres-
sure head, θ
s
is saturated water content, θ
r
is
residual water content, α and
n
are referred
to as the van Genuchten parameters, and
m
is
taken equal to 1 - 1/
n
. On the basis of thousands
of published measurements, Carsel and Parrish
(
1988
) determined average values of saturated
water content, residual water content, α, and
n
for 12 soil textural classifications (
Table 3.1
).
Models such as VS2DI and HYDRUS offer users
the option of using these values. Pedotransfer
functions, such as ROSETTA (Schaap
et al
.,
2001
), have also been developed for predicting
van Genuchten parameters on the basis of soil
texture.
A typical model scenario consists of a col-
umn of soil, with the top representing land
surface and the bottom representing a fixed
depth that could be above, equal to, or below
the water-table depth. The column is divided
into a number of computational cells or layers.
Input data commonly consist of measured daily
precipitation and potential evapotranspiration;
the top boundary is treated as an infiltration
or an evaporation boundary depending on
whether precipitation occurs during the cur-
rent day. A fixed pressure-head or free-drainage
∂ ∂ =∂
θ
/
t
KK
( )/
θ
∂ +
z
Q
'
(3.7)
sr
