Geoscience Reference
In-Depth Information
because run-off shed from the rocky hill slopes is absorbed by the aureole of sandy
colluvial-alluvial sediment at the base of the granite hills, so that the soil moisture
is enough to sustain the growth of perennial trees and shrubs. Another less obvious
factor is subsurface lateral flow of water through the soil concentrated at the foot of the
hill, leading to
eluviation
, or mechanical leaching, of the finer clay and silt particles
from the weathered rock and soil and their deposition as
illuvial
clay particles several
hundred metres further out in the plain at shallow depths. The higher clay content in
these soils enhances their capacity to store water for plant growth. (The importance
of
eluvial
and
illuvial
processes in soil formation is discussed in
Chapter 15
.)
Run-off occurs during rain once the infiltration capacity of the soil is exceeded,
so any excess rain is shed as surface run-off or overland flow. In the case of deep
permeable soils, there may be little or no run-off unless rainfall intensities are very
high. Run-off may at first be confined to shallow depressions, but if rainfall persists
or is intense, the entire surface may become submerged beneath a sheet-flood. With
the additional turbulence imparted by falling rain, run-off can be an efficient agent of
surface erosion, the resulting slopewash deposits often showing weak, sub-horizontal
planar bedding structures. The rate of erosion will vary with slope length, gradient
and curvature, as well as with the nature of the surface (rocky, bare, vegetated) and
type of soil. As a rule, sandy soils are more prone to erode under slopewash than are
more cohesive clay soils.
In many instances, surface run-off is channelled into shallow linear depressions,
or rills. Such rills have been termed first-order channels. Where two rills join, the
channel below the confluence is considered a second-order channel and so on, until
one is dealing with large drainage basins in which the axial channel may be a sixth-
order channel or greater. In practice, it is a moot point as to what type of channel
should be included, especially in the case of highly ephemeral or intermittent streams.
Nevertheless, there are some well-established empirical relationships between stream
order and other drainage basin attributes, such as sediment yield and discharge.
Another measure is drainage density, or the basin area divided by the total stream
length. High drainage densities are common in areas of severe gully or 'badland'
erosion. Drainage density is high where soil permeability is low, as with many clay
soils.
This description of raindrop impact, infiltration, run-off and rill erosion is highly
generalised. The reality is more complex. Within desert landscapes, even subtle dif-
ferences in microtopography and in plant type and distribution can exert a profound
influence on local rates of infiltration (Dunkerley,
2000
). Run-off itself is not a uni-
form process, because within a thin sheet of flowing water, there are concentrated
threads of deeper and faster flow, which play an important role in sediment trans-
port (Dunkerley,
2004
). In addition, the definition of what a rainfall event is will
vary with how the intervals between events are defined, so modelling the impact
