Geoscience Reference
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saturated hydraulic conductivity. The top porous plate is just pressed against the soil
cylinder. The small open spaces at the top porous plate and tensiometers do not have
to be sealed, because they do not conduct water at
h
< 0. To the contrary, these spaces
are needed to provide access for air that needs to enter the sand column if it is to
desaturate when pressure heads are lowered and vice versa. If no air can enter or leave
the soil column, the water content cannot change with pressure head. The water inlux
is measured with the Mariot buret and the outlux with a graduated cylinder. The
pressure head in the soil column can simply be lowered by increasing the soil column
height with respect to the inlow and outlow levels.
Question 4.25:
During an experiment with the small tension setup depicted in
Figure 4.31
the following data were measured:
H
1
= 30 cm,
H
2
= 26 cm,
z
1
= 42 cm,
z
2
= 38 cm, inlux = outlux = 100 cm
3
d
-1
and cross section soil sample = 20 cm
2
.
a) Have the conditions of steady state and unit hydraulic gradient been achieved in the
experiment?
b) Which
k
(
h
) data pair results from this experiment?
c) Suppose we increase only the level of the soil column. How does this affect the pres-
sure head inside the soil column and the water lux?
4.13 Measurement of Root Water Uptake
In an unsaturated soil, water lows mainly in the vertical direction
z
. Root water
uptake patterns generally can be derived by applying the water balance equation to a
given volume of soil in combination with measurements of soil water pressure head
or soil water content.
Figure 4.32
shows the calculation procedure in case of tensi-
ometer measurements. Let us apply the method to a homogeneous, vegetated soil,
where at various depths during the growing season the soil water pressure head
h
is measured. The purpose of the tensiometer measurements is to determine the root
water extraction at different soil depth.
Table 4.4
lists for three depths (
z
= -15, -25
and -35 cm) at two times (
t
= 150 d and
t
= 160 d) the measured
h
values. The
soil water retention curve and hydraulic conductivity function of the soil were mea-
sured separately in the laboratory. The relevant section of these functions is listed in
Table 4.5
.
First calculate the water storage in the layer -30 <
z
< -20 cm at the beginning and
end of the time interval. We may assume that the water content at the centre of a depth
interval equals the average water content of that depth interval. Therefore, the water
amount at
t
= 150 d equals
θ
(
h
i
j
) Δ
z
= 0.325 × 10.0 = 3.25 cm, and at
t
= 160 d equals
0.175 × 10.0 = 1.75 cm.
Next calculate the incoming and outgoing soil water lux at
t
= 150 d and
t
= 160 d.
We assume that, at vertical spatial steps of 10 cm, the average unsaturated hydraulic
conductivity corresponds to the arithmetic average of
k
. Therefore at
t
= 150 d and
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