Geology Reference
In-Depth Information
ing maize. Soil-conserving treatments included
alley cropping between contour-planted hedgerows
of a leguminous shrub. Whether or not crop resi-
due and hedge trimmings were returned to the
cropped alley between hedgerows were sub-
treatments. Substantial reductions in loss of soil
and water due to adopted soil-conserving prac-
tices were recorded in all but limited extreme
events involving typhoons (Rose, 1995).
Acknowledging the importance of decreases
in soil loss due to the adoption of more soil-
conserving practices, the accompanying reduc-
tion in loss of chemical nutrients can also be
vital, especially for crop production in the longer
term (Hashim
et al
., 1997). The link between
soil erosion and nutrient loss is amplified by
nutrient enrichment of eroded soil, which is of
most importance in soils of lighter texture (e.g.
at the Khon Kaen and Goomboorian sites).
Using cost, production, and soil loss data from
the Los Banos site, an investigation was carried
out into the long-term effects of alternative land
management strategies on soil loss, crop yield,
and on-farm economics (Nelson
et al
., 1997). This
investigation, using simulation-based cost-benefit
methodology, was based on hedgerow intercrop-
ping data from the Los Banos site. The labour
input required for the establishment of this sys-
tem made it economically less attractive to poor
farmers with no access to formal credit markets.
Less labour-intensive soil conservation methods
investigated in ACIAR projects included the
return of maize crop residue into contour cultiva-
tion, and trash line cultivation. Adoption of
improved farming practices with the potential for
longer-term benefits is greatly inhibited by the
insecurity of land tenure, which is typical in
much of the Philippines' rural areas.
ment (IBSRAM) organised an international net-
work on the management of sloping lands for
sustainable agriculture in Asia (ASIALAND
Management of Sloping Lands). Previous soil ero-
sion studies in the ASIALAND network coun-
tries were based on USLE technology, so that
measurements included rainfall rate and event
runoff amount, but not runoff rate. The purpose
of the work described here was to develop and
test methods so that data from USLE-type experi-
ments could be used to evaluate soil erodibility
using the physically-based erosion model GUEST.
As described earlier, the use of models such as
GUEST requires data on either runoff rate or an
effective runoff rate (as distinct from runoff
amount). Hence in order to apply GUEST to the
data collected in ASIALAND projects, methods
were investigated by Yu and Rose (1999) on how
to infer runoff rates (or an effective runoff rate) for
erosion events in which runoff rate was not
measured.
When rainfall rate and total event runoff has
been measured (as in USLE-type experimenta-
tion), a complete hydrograph can be generated
using the simple but well-tested infiltration model
of Yu
et al
. (1997a,b) given in Equation (11.22) in
which
P
is rainfall rate and
I
m
is the only model
parameter.
I
m
represents the spatially-averaged
maximum rate of infiltration for the plot. Yu
(1997) developed a computer program GOSH
(Generation Of Synthetic Hydrograph) to facili-
tate this hydrograph generation process. Once the
hydrograph is generated, the erosion model can be
used in the same way as if the runoff rates had
been measured experimentally. Rather than using
the complete hydrograph, an effective event run-
off rate
Q
e
can be computed using Equation (11.18)
and then used in GUEST to calculate
c
−
t
(Equation
(11.17) ), and so
b
(Equation (11.11) ). Yu and Rose
(1999) showed that if only total runoff and rainfall
are measured, but peak rainfall rate is known,
then
Q
e
can be estimated using a scaling tech-
nique, again yielding
b
. Data were collected using
USLE-type methodology at the four experimental
sites in China, Malaysia and Thailand listed in
Table 11.4 and shown in Fig. 11.3. Using data from
11.7 Soil Erosion Research at
IBSRAM-ASIALAND Sites
Recognising the widespread and severe problem
of soil erosion in southeast Asia, in 1988 the
International Board for Soil Research and Manage-
