Desert soils - Arid Zone Geomorphology - page 105

to 12.6 cm 3 /kg. Large pores contribute to the permeability
of soils and a decline in their abundance following com-
paction under raindrop impact explains some of the effect
of soil seals in reducing soil water uptake rates, which is
quantified shortly.
A second layer has also been described, lying above
the washed-in layer. This is a surficial layer of silt parti-
cles, 0.1-0.5 mm thick, which McIntyre (1958a, 1958b)
reported to be lacking in void spaces. He termed this the
'skin seal' and showed that a 0.1 mm seal reduced infil-
tration by as much as a 1 mm washed-in layer. The low
permeability of the skin seal would restrict the reciprocal
escape of air from the soil pore spaces that must occur
if water is to enter them. Consequently, infiltrating water
can result in compression of the air trapped within the
pore spaces, and once the soil is damp and deformable,
the air pressure deforms the pore spaces until they are
roughly spherical, since this minimises the pressure on
the walls of the void. The spherical pore spaces created
by air pressure within the soil are termed 'vesicles'. In
this way a vesicular horizon, containing abundant closely
packed voids, can form beneath the surface, where the sur-
face has low permeability to air and water. These features
are considered in more detail later.
It is thus clear that raindrop impact can result in several
kinds of layers close to the soil surface that may exhibit
lack of void spaces, low permeability, high bulk density
or spherical void spaces that are not interconnected, and
which therefore do not contribute to permeability. Vesicles
remain air-filled even when the soil is saturated, because
the soil suction force retains water in the soil matrix.
Thus, every potential infiltration pathway that contains a
vesicle is excluded from the infiltration process. A square
metre of the soil surface that is underlain by vesicles cov-
ering 75% of the area therefore actually only presents
0.25 m 2 through which infiltration can take place. With
reference to raindrop impact seals, the particular layers
that are formed will vary with the rate of surface erosion,
the texture and chemical composition of the soil materi-
als, the energy delivered by the rain and so on. Much of
what is known in relation to these effects has been de-
rived from experimentation using artificial soil mixtures
and artificial rain, and cannot be taken to represent ex-
actly the processes occurring in nature. Moreover, soil
seals are of particular significance in agricultural soils of
the humid zone. Following ploughing, these soils can be
left bare and exposed to striking rain just as dryland soils
are, and much of what has been learned about seal be-
haviour relates to agricultural soils, not dryland soils. For
example, the wetting rate is known to influence aggregate
breakdown in tilled soils (see Fan et al. , 2008), but this

A feature commonly reported from soils subjected to
prolonged raindrop impact is a layer up to a few mm thick,
just below the soil surface, where porosity is reduced in
comparison with the deeper layers and where correspond-
ingly the bulk density is higher. It has been suggested
that this layer results from the 'inwashing' of fine parti-
cles from the surface, the fine particles lodging in void
spaces and obstructing them. Physical compaction might
also be involved (Moss, 1991). This has been termed the
'washed-in layer' or the 'compacted layer'. Raindrop im-
pact at the soil surface is considered to be the process
that releases fine particles by the breakdown of soil ag-
gregates. Experimental studies, including the analysis of
density using X-ray imaging, have shown that bulk den-
sity in sealed soils typically exhibits a maximum at the soil
surface, declines steeply through the underlying 3-5 mm
of soil and then remains at a constant value, unless other
processes intervene (e.g. Bresson, Moran and Assouline,
2004). The surface bulk density can be 0.1-0.3 g/cm 3
(roughly 25-30%) greater than that of the bulk soil below
a depth of 5 mm. Again using simulated rain and X-ray
density analysis, Fohrer et al. (1999) reported peak bulk
densities of about 1.84 g/cm 3 at a depth of about 1 mm in
sealed soils. The bulk soil below, unaffected by raindrop
compaction effects, had a bulk density about 1.45 g/cm 3 ,
nearly 0.4 g/cm 3 lower than the densest part of the seal.
Others have reported little evidence of significant washed-
in fines following experimentation with simulated rain and
microscopic examination of the resulting soils. For exam-
ple, Tarchitzky et al. (1984) suggested that early obstruc-
tion of near-surface pores as soil aggregates were broken
down would of itself reduce the likelihood of further in-
washing. It seems likely that soil and eroded sediment
textural properties determine whether or not inwashing
is important. In particular, the relationship between pore
sizes among the soil matrix grains and the proportions of
silt and clay in materials delivered with infiltrating water
is likely to determine the probability that the clogging of
near-surface pores would occur.
If the near-surface soil materials are compacted during
raindrop impact seal development, then it would be antici-
pated that the proportion of void space would decline. This
has been confirmed experimentally in tests on soils sealed
under simulated rain at 65 mm/h, using mercury injection
porosimetry to assess voids in the range 0.005-100

m
diameter (Vazquez et al. , 2008). Results showed that after
260 mm of rain had been applied, the total pore space in
initially unsealed soil (315.6 cm 3 /kg) declined by nearly
9% to 288.0 cm 3 /kg. Perhaps more significantly, the dis-
tribution of pore sizes also changed, with a 32% decline in
the space occupied by large pores (50-100

µ