Geoscience Reference
In-Depth Information
qz
()=−
0
q z
(5.22)
where
q
0
is a mean iniltration lux at the soil surface (m d
-1
) (negative, as the lux is
directed downward) and
z
is the soil depth, deined as positive upward and zero at soil
surface (m). When dispersion is neglected and solutes do not precipitate, dissolve or
enter plant roots, the steady-state solute lux should be constant with depth:
()=
() ()
Jz
qzCz
= constant
=
qC
00
(5.23)
l
where
C
0
is the average concentration in the iniltration water. Combination of Eqs.
(
5.22
) and (
5.23
) gives the solute concentration as function of depth:
qC
qz
qC
q
Cz
l
()
=
00
=
00
(5.24)
()
−
z
0
We may simplify this expression by deining the
leaching fraction L
f
:
drainage rate
irrigation rate
q D
q
+
L
=
=
0
r
(5.25)
f
0
where
D
r
is the rooting depth. Solving Eq. (
5.25
) for
S
and substituting it into Eq.
(
5.24
), gives the concentration proiles in terms of
L
f
:
C
Cz
()
=
0
(5.26)
l
z
D
1
+−
(
L
1
)
f
r
Figure 5.13
shows a plot of
C
l
/C
0
versus
z / D
r
for various values of the leaching frac-
tion. Note the rapid increase of salt concentrations near the bottom of the root zone.
The solute concentration at the bottom of the root zone equals:
C
L
CCD
=
()
=
0
(5.27)
max
l
r
f
Common values for the leaching fraction are 0.10-0.20. This yields salinity concen-
trations in the percolation water that are 5-10 times as large as the salinity concentra-
tion in the iniltration water.
Question 5.12:
Consider an irrigated ield with a crop completely covering the soil. The
irrigation is applied for such a long time and so often that the soil water luxes and salin-
ity concentrations in the root zone hardly change in time. Therefore we may consider a
steady-state situation. The average irrigation amount equals 6.0 mm/d, the average tran-
spiration amounts 5.4 mm d
-1
, and the salinity concentration
C
0
= 0.4 mg cm
-3
. Consider
a uniform root density and root water extraction pattern over the rooting depth.
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