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capacity and the ability of the soil to support
plant life. If the soils are actively evolving then
the vegetation is also likely to be evolving.
There are a number of ways in which vegetation
can feed back to landform evolution, but the
most obvious is through the modification of the
vegetation cover factors in the erosion equation
(Evans & Willgoose, 2000). Several researchers
have used LEMS to model empirically the inter-
action between landforms and vegetation (e.g.
Collins et al ., 2004; Saco et al ., 2007), but a
fully-coupled physically-based soils and vegeta-
tion model that is suitable for use in LEMs is yet
to emerge.
0.008
24 m
16 m
8m
0.006
0.004
0.002
0.000
0
20
40
60
80
100
Time (years)
Fig. 18.9 The evolution of the erodibility ( K in
the erosion Equation (18.3) ) of a hillslope under the
action of armouring due to fluvial erosion using the
ARMOUR fluvial erosion and armouring model.
The distances are the surface erodibility at three
distances along the slope measured from the slope
divide. From Sharmeen & Willgoose (2007).
18.6
Future Trends in Landform
Evolution Modelling
No discussion of a new approach to erosion mod-
elling would be complete without some discus-
sion of the future directions of LEM development.
We have discussed some of the important science
issues in the previous section. Here we will focus
on issues related to their application to problems
of a more applied nature.
It is still early days in the development of
LEMs, particularly with respect to their applica-
tion to real problems. At an exploratory level it is
relatively easy for a researcher to write an LEM.
Accordingly there has been an explosion of
research-focused models in the last five years.
Many of them have strong underlying similari-
ties in the physics, but they typically reflect the
interests of the researcher so that there is much
that is unique in each particular model. Generally,
these research-focused LEMs do not have the
complete set of physics, support and analysis
tools, and documentation that are required by
the erosion practitioner. Nor, critically, do they
have the set of validation tests needed by the
practitioner to be able to defend the results of
their models. In some cases, like the Monte Carlo
aspects of valley development on landforms
(where comparing a landform in the field with a
single computer simulation is not possible even
in principle), we are only at the early stages of
of the hillslope. As a slope evolves the erosion
process strips out the finest fraction of the soil on
the surface. This process leaves behind a coarser
armour layer which is relatively more resistant to
erosion. As the surface erodes further, even more
fines are eroded and the surface continues to
coarsen. Thus the erodibility of the surface is inti-
mately tied to the cumulative erosion from the
surface (Evans, 1998). It is common, after land-
form construction, to observe a period of high
sediment transport off-site as fines are winnowed
from the landform surface. This process can be
modelled (Willgoose & Sharmeen, 2006; Sharmeen
& Willgoose, 2007), although until recently it was
believed to be too computationally intensive to
model this armouring process in conjunction with
landform evolution (Fig. 18.9). Recent advances
(Cohen et al ., 2009) have overcome this problem,
and combined landform evolution and soil pedo-
genesis models will soon start to emerge.
Coupled with the evolution of the soil sur-
face is the evolution through weathering of
material below the surface. The key importance
of the evolution of the subsurface materials is
that this grading drives the soil water-holding
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