Geology Reference
In-Depth Information
substantial decline in
b
with time, as at Imbil and
Kemaman. Thus soil erodibility, at least as indi-
cated by
b
, appears not to be a general soil-type
constant, although soil type has a major role in the
likely range of values of
b
.
11.6.2
General discussion of ACIAR project
erodibility results
Table 11.3 summarises the plot characteristics
and soil erodibility values for the five ACIAR
sites. For both clay-textured soils at Los Banos
and VISCA, the calculated value of
b
fluctuated
during each year, especially in response to culti-
vation, which was a necessary practice in order to
keep the bare soil plots essentially weed-free
(Paningbatan
et al
., 1995). Thus, as shown in
Table 11.3, soil erodibility is not a constant for a
particular soil. Rather, at least in the agricultural
context represented by the program sites, erodibil-
ity can be significantly affected by any land man-
agement activity, including cultivation, and the
effects of rainfall in structural breakdown and
compaction also appear to play an additional role.
The value of
b
obtained using the Type B version
of GUEST analysis consistently used in the
ACIAR projects includes, but does not explicitly
recognize, the contribution that rainfall impact
can make to sediment contribution.
Results showed that the value of
b
calculated
from bare plot data using GUEST varied in a
manner which depended upon soil type, cultiva-
tion, and time since last cultivation (cultivation
commonly being the method used for weed con-
trol), and evidently upon soil strength, consolida-
tion, selective loss of finer fractions, and related
factors. The types of cultivation varied consider-
ably between sites. The bare soil at Kemaman
was not cultivated, leading to a decline in
b
with
time as initially loose soil was lost, followed by
soil consolidation. These strong effects of initial
cultivation followed by consolidation probably
blur the dependence of soil erodibility on soil
type at this site.
The ability to evaluate soil erodibility
b
on an
event-by-event basis allows investigation of the
effects of time, soil management and other factors.
For cultivated plots, values of
b
were commonly in
the range 0.7-1.0, tending to be higher after culti-
vation or weeding. Consolidation (and associated
soil strength increase), together with lack of rill
development led to lower values of
b
. Soils which
were not cultivated to control weeds showed a
11.6.3
Cover effects in soil conservation
The aerial cover dominantly provided by leaves
of vegetation can offer some protection against
rainfall impact, although damaging drops larger
than raindrops can be formed. However, much
more effective protection of soil against erosion is
provided by surface contact cover; this cover con-
sists of mulch, plant parts, and so on, which are
in such intimate contact with the soil surface
as to impede overland flow, as well providing
protection against raindrop impact. Figure 11.5,
based on data from the Los Banos site (Paningbatan
et al
., 1995), illustrates the commonly-found dra-
matic effect of increasing surface contact cover in
reducing sediment concentration and so soil loss.
The relationship between the contact cover frac-
tion (
C
f
) and sediment concentration
c
relative to
that from bare soil (
c
b
) has often been found to be
of the general functional form:
cc
/
=-
exp(
k
C
)
(11.21)
b
f
In Fig. 11.5 the coefficient
k
has the approximate
value of 10, this rapid exponential decline indi-
cating the strong effectiveness of surface contact
cover in this experiment, which was provided by
crop residue and the trimmings of contour-
planted leguminous hedgerows.
11.6.4 Use of ACIAR project data
in developing a three-parameter
hydrological model
These ACIAR projects provided a vast body of
data on runoff rate, possibly the largest body
of data on plot runoff rate measured at one-minute
time intervals with infiltration-excess overland
flow. This led to a search for the most appropriate
and efficient form of infiltration equation to inter-
pret the data. At all sites, apparent infiltration
