Geoscience Reference
In-Depth Information
to recognise that this is not the detachment mechanism.
Third, even making these assumptions, the theoretical ca-
pacity is size-selective in that it is inversely related to the
particle diameter and thus there is no simple definition of
a single value for real soils, which are typically made up
of a wide range of particle sizes.
infiltration, runoff and erosion are becoming increasingly
recognised. Similarly, in erosion studies, new models are
being developed that can account for the variability of
erosion rates at different scales and pick out key con-
tinua and thresholds. Methodologically, the integration
of field observation, numerical modelling and theoretical
conceptualisation in an iterative process is fundamental in
overcoming limitations from just looking at small-scale
processes or from the landscape perspective. In this way,
we can hope that the coming decades will produce equally
impressive strides forward in the understanding of dryland
surface processes
and
landforms as a result of overland
flows and related erosion.
11.5.4
Patterns and scales of sediment transport
The relative effectiveness of splash, unconcentrated and
concentrated erosion on hillslopes is a product of the de-
tachment rates and distance moved in each case. In broad
terms, the rates of unconcentrated erosion are typically
an order of magnitude bigger than rates of splash, but in
turn are an order of magnitude lower than those in con-
centrated erosion. These differences can produce marked
changes in the rate of sediment movement at different
locations along a hillslope (Figure 11.13). An important
consequence of these differences is that erosion rates do
not scale linearly and that unconcentrated erosion scales
in a different way (increases then decreases along a slope
due to the balance in the increase in distance moved in re-
lation to reduced detachment as the flow depth increases)
from concentrated erosion (non-linear increase with slope
length). This observation has significant consequences for
the understanding of sediment fluxes at different spatial
scales and in the way sediment delivery at catchment scale
is represented (Parsons
et al.
, 2006a, 2006b).
References
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pressional storages: the effect of DEM spatial resolution.
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grain velocity and sediment concentration in overland-flow.
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11.6
Conclusions
Significant advances have been made over the last decades
in the understanding of the dynamics of runoff and ero-
sion processes in drylands. In particular, in recent years
there have been conceptual developments that have al-
lowed the complexity of the processes to be evaluated
in a way that accounts for their highly nonlinear charac-
teristics at point scale, but also the major nonlinearities
that occur when moving from point to slope and to land-
scape scales. Infiltration is a complex process and there
are numerous reasons why the standard models do not
work well in dryland contexts. There are many processes
at play, which means that simple approximations based
on grain size are not appropriate and many surface and
subsurface characteristics that need to be accounted for
in predicting infiltration rates and thus runoff. Not least,
over the last decade or so has been the importance of
the development of concepts of landscape connectivity
in understanding larger-scale runoff. The effects of veg-
etation in producing ecogeomorphic feedbacks between
