Agriculture Reference
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occur within both regions. Thus, a common set of model parameters is
utilized for both regions. Such an assumption has been made for equilib-
rium (linear, nonlinear, and ion exchange) as well as kinetic reversible and
irreversible reactions. Therefore, this model disregards the heterogeneous
nature of various types of sites on matrix surfaces. This is not surprising
since soils are not homogeneous systems but, rather, are a complex mixture
of solids of clay minerals, several oxides/hydroxides, and organic matter
with varying surface properties.
8.6 Multiflow Domain Approaches
Following the basic structure of the mobile-immobile approach, other
approaches were developed to describe the observed deviation of nonreac-
tive solute transport based on the classical convection-dispersion equation
for one-region or -domain. Common among such models is the assumption
that the soil consists of several flow regions or domains where each flow
domain is characterized by different a different water flux, soil water con-
tent and dispersion coefficients. This is in contrast with the mobile-immobile
approach where a stagnant (no-flow) domain or region is assumed. Without
loss of generality, a convective-dispersion flow equation for nonreactive sol-
utes in domain
i
may be expressed as:
2
∂
∂
C
t
C
x
∂
∂
C
x
∂
∂
i
i
i
=
D
−
v
(8.69)
i
i
2
where
D
i
,
Θ
i
,
and
v
i
are the dispersion coefficient, soil water content, and
pore-water velocity associated with each domain
I
, where
l
l
∑
∑
Θ=
Θ
and
q
=
q
(8.70)
i
i
i
=
1
i
=
1
and the pore-water velocity
v
for domain
i
is
q
i
v
=
Θ
(8.71)
i
i
The two-domain transport model is the simplest case where one divides
soil water into two regions based on their flow velocities. Both water regions
have a non-zero flow rate. Without loss of generality, we denoted the fast flow
region as
A
and slow flow region as
B
. The soil system was characterized by
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