Geoscience Reference
In-Depth Information
a) Which salinity concentration do you expect in the centre of the root zone?
b) Which salinity concentration do you expect at the bottom of the root zone?
Consider now a triangular root density proile, with the maximum root density at the
soil surface. We may assume that the soil water extraction by roots is proportional to the
root density.
c) Which salinity concentration do you expect in the centre of the root zone?
d) Which salinity concentration do you expect at the bottom of the root zone?
To quantify the loss of crop yield due to salinization, scientists have long searched for
a proper way of averaging the strongly nonlinear salinity proile (Jury et al.,
1991
).
Raats (
1975
) developed a very useful approach
w
ith plausible, generally valid assump-
tions. He deined the weighted average salinity
C
of the root zone as the average salin-
ity of the soil water extracted by the plant:
0
∫
SzCz z
() ()
d
l
=
−
D
C
(5.28)
r
0
∫
Sz z
()
d
−
D
r
where the denominator is equal to the plant transpiration
T
(m d
-1
). In steady-state con-
ditions,
Cz
l
()
=
00
Cq
/()
qz
and
S
=−dd
q
/
z
. Thus Eq. (
5.28
) may be written as:
=
−
qC
T
d
d
q
z
d
z
q
=
−
qC
T
d
q
q
=
−
qC
T
1
00
0
q
−
∫
∫
0
C
00
00
ln
(5.29)
L
D
q L
r
0
f
f
Finally, using Eq. (
5.25
) and
S D
r
=
T
, we may write Eq. (
5.29
) as (Jury et al.,
1991
):
C
C
1
1
=
−
ln
(5.30)
1
L
L
0
f
f
Note that the average salinity of Eq. (
5.30
) is independent of the shape of the water
uptake distribution and depends only on the leaching fraction
L
f
and
C
0
! This average
concentration might be used in combination with the Maas and Hoffman criteria to
estimate crop yield loss (
Section 6.2.4
).
Question 5.13:
Which average root water uptake salinity
C
do you calculate for the
ield in
Question 5.12
with the uniform root density? And which average root water
uptake salinity for the same ield with the triangular root density?
5.8 Pesticide Pollution of Groundwater
To be effective, pesticides should be mobile enough to reach their target organism
and persistent enough to eliminate this organism. However, persistency and mobility
are unfavourable solute properties from an environmental point of view (
Section 5.6
).
Search WWH ::
Custom Search