Geoscience Reference
In-Depth Information
(Ben-Hur and Lado, 2008). Consequently, our under-
standing of dryland seal formation is not as yet com-
plete, but we have considerable evidence of the effects
of seals once they have formed. A range of views about
the relative importance of compaction, inwashing, pore
blockage, structural rearrangement of clay particles and
other mechanisms that may be involved in seal formation
was reviewed by Assouline (2004). One problematic is-
sue is distinguishing between structures produced
during
rain from those that result
after
rain, as clays settle to the
surface and the soil dries out. In laboratory experimental
work, the soil trays are often allowed to dry prior to sam-
pling the seal and it is not clear how the seal properties
change through drying. The lack of correspondence of ex-
perimental conditions with those in the field needs to be
borne in mind at all times. For example, Tanaka, Yokoi
and Kyuma (1992) made a study of soil surface sealing
using simulated rain at an intensity of nearly 270 mm/h,
but it is not clear how their findings relate to field condi-
tions in drylands, where much of the rain arrives at rates
of
successively higher levels of kinetic energy or rain rate in
storms incrementally increase the particle packing den-
sity, perhaps over a period of years. Much would depend
upon the rate at which the surface was being lowered by
erosion, since in the case of such a surface, the seal would
need to be renewed as the erosion progressed.
The soil materials involved in seal formation actually
comprise a physicochemical system. The interaction of
soil particles with water is partly governed by physical
factors such as drop or interparticle collisions and partly
by electrochemical processes such as clay hydration, clay
dispersion, slaking and osmosis. In these latter processes,
both the chemistry of the soil and of the rainwater are sig-
nificant. The soil characteristics that have been considered
as potentially significant include texture, organic matter
content, sodicity and clay mineralogy. Again it is not fully
established just how particular soil characteristics affect
seal formation. Clay particles, for example, disperse in
water when water molecules move between clay platelets
and separate them to the extent that the Van der Waal's
forces between the platelets are no longer sufficiently
strong to hold the soil aggregates together. Dispersed clays
are then able to be rearranged structurally or to move into
the deeper soil pore spaces where pore clogging may re-
sult. Owing to their crystalline microstructures, smectitic
soils (containing smectite or montmorillonite clays) tend
to be readily dispersible and are susceptible to seal forma-
tion, while kaolinitic soils are much less dispersible and
are less prone to sealing (Lado and Ben-Hur, 2004). Soils
with abundant clays may exhibit aggregate stability that
is too high to permit seal development, while soils with
<
<
5 mm/h (Dunkerley, 2008a).
7.5.2
Factors known to be significant in the
formation of raindrop impact seals
Field and laboratory studies have shown that seal forma-
tion and seal properties can be affected by a range of local
factors. Though there is much to be resolved in terms of
fine detail, the factors fall into two categories:
1. Properties of the rainfall.
10% clay may release too few particles for widespread
pore clogging. Thus, soils with perhaps 20-30% clay-size
particles may be the most prone to seal formation (Singer
and Shainberg, 2004).
2. Properties of the soil materials.
Seal formation involves the structural rearrangement
of soil materials (such as aggregate breakdown, reorgan-
isation of particles and pore spaces, inwashing of fines,
plugging of pore spaces and so on), all of which represent
work done by the energy of the raindrops. Therefore, the
kinetic energy of the rain has been identified as a param-
eter likely to be of importance. However, it is difficult to
know how best to describe a complex phenomenon such
as rainfall, and other parameters including rain rate (inten-
sity), total depth of rain and total kinetic energy delivered
have been explored. The complex details are not necessary
here, but it can be noted that for unsealed experimental
soils exposed to rain, lower kinetic energy of incident rain
seems to be associated with less densely compacted and
more permeable seals. However, in drylands, once a seal
is formed, the behaviour of subsequent rain would appear
7.5.3
Depositional seals
Depositional seals are laid down where water slows and
transported silts and clays settle to the soil surface form-
ing thin, dense, bedded sediment layers. Valentin and
Bresson (1992) distinguished depositional seals of two
kinds, deposited either from flowing or still water, but
noted that in fact most depositional seals involve com-
ponents of both. The thickness of depositional seals can
grow to many millimetres if there are successive episodes
of deposition. Curling and cracking of these seals is com-
monly observed as they dry, because the uppermost layers,
exposed to sun and wind, dry and shrink while the soil be-
neath is still moist and deformable. Often the cracking
