1169
Technical Committee 106 /
Comité technique 106
increase mechanical resistance for plant roots to penetrate
through soil. As a result, more roots would concentrate (i.e.,
higher RAI) in shallower depths and the number of roots would
decrease with an increase in depth. The observed RAI profile in
this study is useful for interpreting suction induced in vegetated
soil with the tree. This is because RAI indicates surface area of
roots available for root-water uptake, which would affect the
magnitude of suction induced in the soil. Any relationship
between RAI and suctions induced in box T is discussed later.
0
0.5
1
0-10
20-30
40-50
60-70
80-90
100-110
120-130
140-150
160-170
180-190
200-210
220-230
240-250
Root area index (RAI)
Dpeth (mm)
Figure 2
.
Measured distribution of RAI along root depth of the tree in
box T
4 EFFECT OF TREE LEAF ON ENERGY DISTRIBUTION
Figure 3 compares the measured horizontal distributions of
radiant energy received on soil surface (
R
s
) for boxes B and T.
As expected, the soil surface for bare box B receive almost all
the energy (i.e.
R
i
≈
R
s
) uniformly along the width of the box.
On the contrary, the measured distribution of
R
s
is different in
box T. It can be seen that at the centre of the box, the measured
R
s
is minimum. This is because of the substantial interception of
incoming radiant energy by tree leaves. The maximum
percentage of energy interception (i.e. (
R
i
–
R
s
) /
R
i
) is estimated
to be about 50 %. Moreover, measured values of
R
s
are found to
increase when the distance is further away from the tree stem on
both sides of the box. This is because there are smaller number
of tree leaves, and hence amount of interception, away from the
tree stem (see Figure 1). At the two edges of the box, the
measured values of R
s
are found to be close to the applied
radiant energy of 2.1 MJ/m
2
(see Figure 3). This is expected
because energy measurements were made outside the canopy of
tree and thus no radiant energy can be intercepted. Similar
distribution of intercepted radiant energy was also found by
Buler and Mika (2009) for an apple tree of which the canopy
was also spindle
–
shaped. It is obvious that energy distribution
is strongly dependent upon the canopy shape of a tree species.
For a given apple tree, Buler and Mika (2009) showed that
distributions of intercepted energy depended on the shape of
canopy (i.e., V
–
shape and Hybrid
–
shape). Measured
interception of radiant energy in this study is used to interpret
plant-induced soil suction later since the magnitude of energy
intercepted by tree leaves would affect amount of tree
transpiration and hence root-water uptake.
Although the amount of radiant energy interception was not
determined for box G due to the full coverage of grass surface,
it could be estimated based on LAI of grass (i.e., 2.2) using
Beer-Lambart Law, which has been widely used for estimating
intercepted radiant energy for various plant species including
grass (
Kiniry et al. 2007
). The law
states that amount of light
intercepted through grass leaf increases with an increase in leaf
area exponentially (Monsi and Saeki 1953). For a given LAI
and assuming that the extinction coefficient (i.e., a measure of
the absorption of light) to be 1.1 based on the thickness of grass
shoot (
Kiniry et al. 2007)
, the percentage of radiant energy
interception by grass is estimated to be more than 90%. In other
words, a large portion of radiant energy would be intercepted by
grass shoot for transpiration, whereas only a limited amount of
it would fall on bare soil surface (i.e., less than 10 %) for
evaporation. This implies that for a given incoming radiant
energy in the atmospheric-controlled room, suction induced in
box G would be attributed to grass transpiration mainly.
0.0
0.1
0.1
0.2
0.2
0.3
150 125 100 75 50 25 0 25 50 75 100 125 150
Radiant energy recievedat soil/grass surface,
R
s
(MJ/m
2
)
Distanceaway from the tree stem (mm)
B
T (LAI = 4.6)
Diameter of tree canopy
Tree stem
Applied radiant energy
R
i
of 2.1 MJ/m
2
Figure 3. Measured horizontal distributions of radiant energy received at
soil surface in boxes B and T
5 COMPARISON OF SUCTION PROFILES WITH AND
WITHOUT VEGETATION
Figure 4 compares measured distributions of induced suction
along depth in all three test boxes, B, G and T during two weeks
of testing. The measured root depths of grass and tree are
depicted for reference. The measured initial distribution of
suction in bare box B is found to be nearly uniform, suggesting
that the hydraulic gradient is about one. In other words, water
flow in these two boxes was mainly driven by the gravity,
seeping towards bottom drainage holes. When grass was
present, suctions recorded at depths within the root zone in box
G are slightly higher than those in deeper depths because of
root-water uptake. For box T, the measured initial suctions were
higher than those in the other two boxes, particularly at 30 and
140 mm depths. As compared to bare box B, the observed
higher suctions retained in the two vegetated boxes G and T
were likely attributed to the changes of water retention ability of
soils, due to the presence of plant roots (Scanlan and Hinz 2010).
0
50
100
150
200
250
300
0
20
40
60
80
Depth (mm)
Suction (kPa)
B_Initial
B_14 days
G_initial
G_14 days
T_Initial
Tree_14 days
Depth of grass root
Depth of tree root
Figure 4. Comparisons of measured suction profiles between bare and
vegetated test boxes before and after two weeks of the monitoring
period
