Soil and Hillslope (Eco)Hydrology - Environmental Modelling: Finding Simplicity in Complexity - page 163

the bare areas in between affecting water transfers at the
hillslope scale.
Puigdef abregas (2005) reviews the multiple lines of evi-
dence that show how such resource concentration might
occur in drylands. At the same time, he makes clear why
dryland soils should be considered biophysical entities.
(Peatlands might be thought of as a special case: peat
is a wholly biological material whose physical proper-
ties depend almost entirely on the plants of which it is
composed and on the degree to which they have been
decomposed and compacted by overlying layers of peat.)
Puigdef abregas (2005) notes that when plants are present
they tend to shield dryland soils from radiation and rain-
drop impact and thus hinder the development of surface
seals which can reduce infiltration. Plants add organic
matter to the soil which enhances the biological activity
of the soil, which, in turn, leads to the building of stable
soil aggregates. These stable aggregates are associated with
an increase in water-storage capacity compared to soil in
adjacent nonvegetated areas, an increase in hydraulic
conductivity ( K ), and a decrease in soil erodibility. The
presence of plants also leads to an increase in nutrient
inputs to the soil via (i) dry deposition on the plant
canopy and subsequent wash into the soil and (ii) tighter
nutrient cycles. Finally, Puigdef abregas (2005) notes that
plants affect the sediment balance in drylands. Plants filter
and collect air-borne dust, and the soil beneath them is
a net recipient of sediments mobilized during rainsplash.
With the latter, vegetated patches behave somewhat like
absorbing barriers in a Markov chain. Sediment can be
splashed into them from surrounding bare areas but less
or none is returned because the plants protect the soil
from rain splash; usually, the kinetic energy of incom-
ing rain drops is dissipated by the plant canopy so that
water arriving at the soil surface beneath plants is 'energy
depleted' (e.g. Wainwright et al ., 1999). All of these pro-
cesses are examples of selective autonomous processes (see
Section 10.1) that help (i) maintain conditions favourable
for plant growth and, therefore, (ii) reinforce pattern.
As noted above, patch dynamics in drylands will depend
on local flows but also on the larger pattern of patches. In
terms of the former, the upslope part of a vegetation patch
may receive more water flowing from a bare area than
the downslope part (Dunkerley, 1997). This difference
may mean that plant growth at the upslope boundary
is enhanced, while at the downslope boundary plants
become stressed and start to die, which in turn may lead
to upslope patch migration. It was also noted above that
water flow into a patch will not just depend on the size of a
bare area upslope but also on the size, density and spatial

Figure 10.2 Contour-parallel stripes in a peatland in NW
Scotland (near Inverewe) (see scale bar for size). The dark areas
are linear pools. The areas between are ridges of peat. Image
obtained from Google Earth (copyright Getmapping plc;
copyright 2010 Tele Atlas). See screenshot in file.

(1997) have suggested that patterning has been present in
some areas for at least centuries.
Given the absence of external controls, the patterns
that are found in drylands and peatlands are almost
certainly the result of biophysical processes within the
soils. But, how do these biophysical processes lead to
pattern formation? It is easy to see how small variations
in some key property, such as plant biomass, give rise
to patchiness if there is a strong local positive feedback
occurring whereby the small differences are amplified.
For example, a slightly higher plant cover in one part of
a dryland hillslope may lead through various processes
to a net transfer of resources to that place such that
its plant biomass increases and that of adjacent 'donor'
areas decreases. Limits will, of course, act on the con-
centration of resources and act as a brake on the growth
of vegetated patches. There may be a finite quantity of
resource that limits the overall size of vegetated patches;
a vegetated patch may not be able to grow because it
has sequestered or accumulated all of a resource in its
immediate neighbourhood. However, if it can gain more
of that resource from areas outside of its neighbourhood
it may be able to expand further. The amount of resource
from further afield that can flow to a particular patch
will depend on larger scale patterns across the hillslope.
For example, a vegetation patch in a midslope position
may receive different amounts of water during a rain-
storm (from direct rainfall, net transfers of water from
adjacent bare areas, and from areas upslope of the patch's
immediate neighbourhood) than a patch near the base
of the slope, with the disposition and size of patches and