Geoscience Reference
In-Depth Information
Soil aggregate
Stable
Soil Matrix
Soil water
Swollen soil aggregates
and soil structure
breakdown
Chemical
Dispersion
Surface water
Suspended particle
Filtration
Clogging of pores by
sediment particles
Aggregate breakdown
and compaction
Physical
Dispersion
Figure 11.6
A schematic representation of the different mechanisms of seal development (Romkens, Prasad and Whisler,
1990).
break up. Le Bissonnais (1996) has reviewed the various
mechanisms of aggregate break-up - slaking, breakdown
by differential swelling, mechanical breakdown by rain-
drop impact and physicochemical dispersion (Mualem,
Assouline and Rohdenburg, 1990; Romkens, Prasad and
Whisler, 1990) - and suggested that as well as the different
effects of soil size and chemistry, the rate of wetting is also
important. Thus, larger storm events are likely to increase
the rate of aggregate breakdown and thus produce more
sealed surfaces. Abu-Awwad (1997) noted that crusts on
silty, calcareous soils with low organic matter components
in Jordan tended to form after any wetting of the soil, so
that the impact of wetting rate will depend on the initial
soil conditions. However, Luk and Cai (1990) demon-
strated using micromorphological analyses that seal for-
mation could be dynamic within a single event, so that the
effects on infiltration could both increase and decrease.
Mualem, Assouline and Rohdenburg (1990) suggest that
these mechanisms, so that the controls on their formation
and destruction during a single event is also likely to be
highly variable. Aggregate stability is reduced in dryland
soils because of the relative paucity of organic matter,
and the spatial pattern of organic matter - e.g. clustered
beneath shrubs or grass clumps - is likely to provide a
feedback in terms of infiltration and vegetation patterns
(Dunne, Zhang and Aubry, 1991; Boix-Fayos
et al.
, 1998).
There is also a feedback between sediment contained in
the infiltrating water and crust formation. Abu-Sharar and
Salameh (1995) measured reductions of 80-85 % in the in-
filtration rate when the water contained as little as 0.07 %
fine clay, as compared to infiltration from clear water.
Thus, as a storm event continues and more fines are moved
in the flow, there is likely to be a dynamic feedback with
runoff, infiltration and erosion rates.
The effects of vegetation in drylands are frequently
overlooked, yet have a significant impact (Wainwright,
