Geology Reference
In-Depth Information
Table 11.3
Plot characteristics and soil erodibility for five ACIAR sites in southeast Asia and Australia
(after Yu
et al
., 1999).
Site
Country
Location
Soil texture
Slope (%)
No. of events
Soil erodibility
β
Goomboorian
Australia
26°04
′
S, 152°48
′
E
Sand
5
79
1.049 ± 0.009
Kemaman
Malaysia
4°18
′
N, 103°19
′
E
Sandy loam
17
50
0.319 ± 0.018
Los Banos
Philippines
14°6
′
N, 121°12
′
E
Clay
26
19
0.879 ± 0.058
VISCA
Philippines
10°45
′
N, 124°49
′
E
Clay
50
4
0.890 ± 0.128
Khon Kaen
Thailand
16°30
′
N, 102°50
′
E
Loamy sand
3.6
22
0.928 ± 0.245
In another 14 events rilling may have occurred, but
was not obvious following the event, providing
some uncertainty in flow geometry, and hence in
b
. Figure 11.4 shows some decline in
b
as stream-
power of the flow increased, a trend in
b
also
observed for a site in Thailand (Yu
et al
., 1999).
Values of
b
exceeding unity at lower values of
streampower may indicate a contribution of rain-
fall impact to soil erosion. A detailed investigation
was made of the effects of weeding on
b
. This
showed that if weeding was carried out whilst the
soil was wet, then sediment concentration, soil
loss and
b
were all reduced for a storm event fol-
lowing weeding (which disturbed the soil surface).
The hand cultivation of moist soil produced a
rough, cloddy and well aggregated soil condition.
However, this trend was reversed (i.e.
b
increased)
if weeding was carried out when the soil was drier,
and soil attached to the removed weeds was scat-
tered onto the plot.
the soil surface with stone and gravel fractions
developed. This decline was more rapid for the
7 and 12 m length plots than the 22 m plots, which
developed deep rills. For the 22 m long plots,
b
fell
from 1.24 following cultivation to 0.32 some 28
months and 19 erosion events later, hence the
large standard deviation in a mean value of 0.67
(see Table 11.3). Whether or not the runoff rate
was sufficient to remove stone armouring from
the surface seemed to affect the value of
b
, small
events which left armour in place yielding lower
values of
b
(Ciesiolka
et al
., 1995). Thus at this
site, the dynamics of armouring, combined with
general soil consolidation through time, played
important roles in modifying the erodibility of
the soil.
In contrast, at Goomboorian the sandy loam
soil had no stone or gravel component. Preparation
for planting the pineapple crop was preceded by
intense cultivation, but since weeds were control-
led chemically, there was no further cultivation.
Over the three-year period of experimentation,
the value of
b
fell steadily from about 1.1 to 1.0,
probably due to soil consolidation, with some
minor fluctuation which may be due to pulsing of
sediment through the 36-m long ridge/furrow
system plots (Rose
et al
., 1997). Since the furrow
slope was only 5%, the reason for
b
being greater
than 1.0 initially could well be due to a rainfall-
driven contribution to sediment concentration.
The gradual limited decline in
b
with time was
accompanied by an increase in soil strength as
measured by torvane, presumably associated with
gradual soil compaction or consolidation follow-
ing the initial cultivation.
Imbil and Goomboorian, Australia
: Similar
experiments to those at other ACIAR sites were
carried out on commercial pineapple farms at
Imbil and Goomboorian on soils with different
parent material and slopes (Tables 11.1 and 11.2).
At Imbil, with a gravelly, weak-crumbed loam to
clay loam soil, measurements were made on plot
lengths of 7, 12 and 22 m (Ciesiolka
et al
., 1995).
Initially, when soil strength was low following
cultivation, values of
b
were high and similar for
all three plot lengths. However, as events contin-
ued over the period 1989 to 1991, values of
b
declined following the removal of the finer soil
fractions, and as consolidation and armouring of
