Geoscience Reference
In-Depth Information
z
= -20 cm,
k
i
-½
j
= (0.016+0.0019) / 2 = 0.0090 cm d
-1
. Applying Darcy's law, the lux
at
t
= 150 d and
z
= -20 cm amounts:
h
(
−−−
15
)
h
(
25
)
j
=− −
q
k
(
20
)
−−
k
(
20
)
i
∆
z
(4.49)
−−−
( )
−
156
112
=−
0 0090
.
00
.
090
=+
0 0306
.
cm d
−
1
10
Be mindful of a proper use of the signs! The lux
q
i
j
has a positive sign, which means
that the low is directed upwards. In the same way we can calculate the other mean
values for
k
and the soil water luxes
q
i
+1
j
= +0.0156 cm d
-1
,
q
i
j
+1
= +0.1166 cm d
-1
,
and
q
i
+1
j
+1
= +0.0167 cm d
-1
. The upward luxes gradually increase in time.
Finally we can derive the extraction
S
with the water balance (
Figure 4.32
):
j
j
+
1
j
j
+
1
q
+
q
qq
+
∆
∆
z
t
(
)
+
()
−
( )
j
j
+
1
S
=
i
+
1
i
+
1
−
i
i
θ
h
θ
h
(4.50)
i
i
2
2
which gives:
0 1166
.
+
0 0167
.
0 0306
.
+
0 0156
.
325175
10 0
.
−
.
S
=
−
+
2
2
.
(4.51)
md
−
=
0 043
.
+
0 150
.
=
0 193
.
So in this example the root layer at -30 <
z
< -20 cm contributes 1.93 mm d
-1
to the
plant transpiration.
Figure 4.33
shows the measured root water uptake pattern of a red cabbage crop
growing on sticky clay in the presence of a 90-cm deep groundwater table (Feddes,
1971
). At the top of the proile the magnitude of the root extraction rate is generally
small due to a smaller root density and lower water contents. Downward the extrac-
tion rate increases to a certain maximum and next decreases to zero at the root zone
bottom. As the soil dries in the growing season, the zone of maximum root water
uptake moves from shallow to greater depths. Later in the season water uptake from
the upper layers becomes relatively less important. Most of the water is absorbed
from the zone with higher water contents near the water table (
Figure 4.33
). As dis-
cussed in
Section 6.2
, the main factors that affect root water uptake are potential tran-
spiration, root density distribution (which may change in time), soil water pressure
head proile, soil hydraulic conductivity and plant wilting point.
4.14 Summary
Soil water low forms the basis for analysis of root water extraction, surface runoff,
groundwater recharge and solute transport. Reservoir models as Warrilow may
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