Geology Reference
In-Depth Information
cal upper limit - unity - independent of the exact
particle-size distribution, although this may be a
qualified statement, because the two ACIAR sites
with sandy soils also happen to be the ones with
very gentle slopes.
The results obtained from the ASIALAND
sites show that databases from USLE-type of
experiments can be readily re-analysed to esti-
mate necessary parameters for the physically-
based erosion model GUEST. In other words,
the erosion model GUEST can be tested, vali-
dated and used at no greater costs than validat-
ing the USLE. In fact, validation and use of the
GUEST approach is actually easier because no
long-term experimentation (perhaps >20 years)
is required, as is the case for the USLE (Edwards,
1987).
(e.g. Proffitt
et al
., 1993b; Misra & Rose, 1995,
1996). These controlled studies supported the
theory used in program GUEST, and its recogni-
tion of the pertinence of soil structural stability
and soil strength as two factors affecting soil
erodibility. In GUEST, the effect of the size dis-
tribution of aggregates and particles on sedi-
ment concentration is separated out through
recognition in the theory of the role of the soil's
settling velocity characteristics (or depositabil-
ity). Although the erodibility factor
b
evaluated
in the Type B version of GUEST is designed to
represent properly the effect of soil strength in
reducing sediment concentration, its magni-
tude also incorporates the role of any other
factors or processes which contribute to the
net erosion of soil. Because soil erodibility can
be significantly affected by soil management
(Rose, 1995), and even wetting and drying (Misra
& Teixera, 2001), the ability to predict the
magnitude of
b
is currently limited to general
soil characteristics of the type illustrated in
Fig. 11.8. However, the ACIAR project results
(in particular) provide advice on the role of soil
management in affecting
b
, and support the
move towards minimizing tillage in agricul-
tural land management (Goddard
et al
., 2007).
The main purpose and requested aim of this
chapter is to describe the GUEST program which
(in its simpler Type-B form) enables efficient deri-
vation of the physically-based erosion parameter
b
from experimental data on soil and water loss from
an essentially plane bare soil plot in any single ero-
sion event. Of course, data from repeated events
can add information on time-related change in
erodibility, but application of GUEST is not lim-
ited to longer-term datasets, as is recommended
for the USLE methodology (Edwards, 1987). In
comparison to WEPP, GUEST is also not limited
to information only available in certain regions.
Information on the erodibility of soil in a bare
condition is important in soil loss prediction and
decision-making on the suitability of land for
uses which expose it, and in the choice from a
range of possible soil-conserving management
systems. However, given the many factors
involved in the design and adoption of land-using
11.8 Concluding Comments
Probably the largest body of data on soil erosion
has been that collected using the USLE method-
ology, thus lacking measurement of runoff rates.
In the ASIALAND section of this chapter it is
demonstrated that the GUEST methodology can
be applied to data collected for analysis using the
USLE methodology. Thus there is the opportu-
nity, in future, to analyse other USLE datasets to
yield values of
b
which would be expected to pro-
vide further evidence of how soil erodibility is
affected by soil characteristics and soil manage-
ment (e.g. Fig. 11.8). The advantage of using the
erodibility parameter
b
described in the GUEST
methodology is that it has an approximate physi-
cal basis in fundamental soil erosion theory. In
comparison, the soil erodibility
K
in the USLE
equation is simply a proportionality factor intro-
duced to ensure agreement between measured
soil loss and the adopted product-type arrange-
ment of other factors which do not include runoff
or runoff rate.
Field applications of the GUEST program,
such as those recorded in this chapter for the
ACIAR and
ASIALAND
projects, benefited
from a more fundamental controlled erosion
program using simulated rainfall equipment
