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R
q
drain
m
k
t
D
k
b
aquitard
1/2
L
Figure 9.8
Hydrological scheme for the Hooghoudt drainage equation.
conductivity above the drain level is 0.8 m d
-1
, below the drain level 1.2 m d
-1
. What is
the design drain spacing
L
?
9.1.6 Solute Transport
SWAP focuses on the transport of salts, pesticides and other solutes that can be
described with relatively simple kinetics: convection, diffusion, dispersion, root
uptake, Freundlich adsorption and irst-order decomposition. The physical back-
grounds of these processes are described in
Chapter 5
.
In the case of more advanced pesticide transport, such as volatilization and kinetic
adsorption, SWAP can be used in combination with the model PEARL (Pesticide
Emission Assessment at Regional and Local scales; Leistra et al.,
2000
; Tiktak et al.,
2000
). For detailed nutrient transport (nitrogen and phosphorus), SWAP can be used
in combination with the model ANIMO (Agricultural Nutrient Model; Rijtema et al.,
1997
; Kroes and Roelsma,
1997
).
As discussed in
Chapter 5
, we may derive a general transport equation for dynamic,
one-dimensional, convective-dispersive mass transport, including nonlinear adsorp-
tion, linear decay and proportional root uptake in the vadose zone:
∂
∂
c
z
∂
θ
D
(
)
=−
∂
()
∂
∂+
θρ
c
Q
qc
z
(
)
−
b
+
−
µθ ρ
c
+
QKSc
(9.9)
b
r
∂
t
∂
z
where
c
is the solute concentration in the soil water (kg m
-3
),
ρ
b
is the dry soil bulk
density (kg m
-3
),
Q
is the solute amount adsorbed (kg kg
-1
),
D
is the effective diffu-
sion coeficient (m
2
d
-1
),
μ
is the decomposition parameter (d
-1
),
K
r
is the preference
factor for solute uptake by plant roots (-) and
S
is the water uptake by roots (d
-1
).
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