Agriculture Reference
In-Depth Information
will be, then
v
=−
K
ψ
z
(
2
.
9
)
and for steady-state flow of water through the soil,
v
=−
K
p
ψ
2
−
ψ
1
z
2
−
z
1
=−
K
c
ψ
3
−
ψ
2
(
2
.
10
)
z
3
−
z
2
where subscripts 1, 2 and 3 refer to the indicated depths in Figure 2.1 and
K
p
and
K
c
are the conductivities of the puddled and compacted layers, respectively.
Rearranging Equation (2.10) and substituting
L
p
and
L
c
for the depths of the
puddled and compacted layers gives
ψ
1
−
ψ
3
L
p
/K
p
+
L
c
/K
c
v
=
(
2
.
11
)
Equation (2.11) shows how the flow increases with increasing depth of the flood-
water and decreases with increasing impermeability of the compacted layer.
The effect of percolation on transport of solutes through the soil is quantified
as follows. If there is a concentration gradient of a solute through the soil, from
Equation (2.4) the net flux due to mass flow and diffusion is
=−
D
d
C
F
d
z
+
vC
(
2
.
12
)
Mass flow and diffusion act together and cannot be separated. However an idea
of their relative contributions to the net flux can be obtained by estimating the
distance the solute would be transported if each process acted independently. If
in time
t
mass flow transports the solute a distance
z
1
=
vt
(
2
.
13
)
the mean distance moved by diffusion would be
z
2
=
√
Dt
(
2
.
14
)
and the ratio of the two would be
z
2
=
v
t
z
1
(
2
.
15
)
D
This equation indicates that, for a constant flow rate and diffusion coefficient, the
distance transported by mass flow will exceed that by diffusion after a certain time
has elapsed, i.e. mass flow eventually becomes more important than diffusion.
However note that
z
2
is only the mean distance moved; some of the solute will
have diffused beyond this.
Equation (2.15) can be used to calculate the relative importance of mass flow
and diffusion under conditions in ricefields. From the discussion above, rates of
