Geoscience Reference
In-Depth Information
vesicular horizons develop includes progressive changes
to infiltrability and surface water partitioning (Young
et al.
, 2004). The development and significance of stone
mantles are considered more fully in Chapter 9, and we
will not consider them further here.
a force on the water within the drop that causes the inter-
nal pressure to be positive. The pressure is only slightly
higher than atmospheric for large raindrops, but for very
small droplets the internal pressure can be 50% higher
than atmospheric pressure. Large droplets are deformed
to a flattened shape as they fall through the air and strike
the soil at a terminal velocity of about 30 km/h. The ki-
netic energy of the falling drop is then dissipated in various
ways, including splashing if there is water ponded on the
surface and the breakdown of susceptible soil aggregates.
When the soil surface is wet, water is thrown out laterally
from the point of impact, and the lateral jets, which can
have speeds 3-10 times greater than the fall speed of the
raindrops (e.g. Ghadiri and Payne, 1981), can exert con-
siderable tearing forces on any protruding soil particles,
the disruption caused being a function of factors includ-
ing the mechanical strength (especially tensile strength)
of the soil particle and the depth of any water ponded
on the soil surface (Hartley and Alonso, 1991). Splash
and surface shear are amplified by shallow surface pond-
ing (Ferreira and Singer, 1985), but shear forces decline
to low levels once the ponding reaches a depth equiv-
alent to about three diameters of the incident raindrops
(Hartley and Alonso, 1991). We will see later that algal fil-
aments and other organic structures that permeate the up-
permost soil layers can greatly increase the cohesiveness
of soil materials, so that they are resistant to disruption by
splashing rain.
A simple calculation suggests the enormous numbers
of drop collisions that occur on exposed soil surfaces dur-
ing rain. If we consider a rain event delivering 10 mm and
suppose for simplicity that all of the raindrops are spheres
uniformly 1.5 mm in diameter, then over each square
metre during the storm there are about 5.65
7.5
Inorganic seals at the soil surface
Another very widespread feature of desert soils is the in-
organic seal. Seals are formed at and near the surface of
susceptible soils by several mechanisms related to the be-
haviour of raindrops, surface ponding of water or overland
flow. In detail, there is a great variety in the characteristics
of seals, related to the texture and composition of the soil
materials involved, the rainfall environment, the amount
of plant, litter and stone cover, the landuse and many
other factors. For an example of a detailed classification
system, see Valentin and Bresson (1992). Here, we will
divide seals into two classes, which accommodate most
forms:
1.
Raindrop impact seals
(often called structural seals)
that result from the mechanical work done by raindrops
in rearranging and packing near-surface soil materials.
2.
Depositional seals
that result when transported fine
particles are laid down in layers on the soil surface,
either in ephemeral surface ponding or simply as over-
land flow that is absorbed or comes to rest.
The complexity of seals can be envisaged by consid-
ering intermediate classes resulting from deposition oc-
curring while the last stages of rain were still falling, for
example, or with variations in soil texture, especially the
amounts of silt and clay. Dryland surfaces often contain a
mosaic of inorganic seals over quite small distances, with
raindrop impact seals on the freely draining, higher parts
of the microtopography and depositional seals along the
threads of flow that follow the lower areas (Figure 7.2).
10
6
drop
impacts. Every point on the soil surface would be struck
multiple times. In the case of a freshly tilled or cultivated
soil, these impacts gradually reduce the surface roughness,
flattening and rounding the surface form (e.g. Jester and
Klik, 2005). In drylands, bare soil and sediment surfaces
take on a subdued surface microtopography arising from
raindrop impact, generally into shallow overland flows.
The surface typically is not completely smooth, but rather
has very shallow depressions, which act as splash traps for
coarse sand and granules, and coarse organic fragments.
A distinctive feature that enables these sealed surfaces to
be distinguished from those supporting colonies of crust
microorganisms (discussed later) is that they crack upon
drying, but this leaves the surface flat, without any ten-
dency for curling or disruption of the surface. The effect
of drop impacts on the soil can be especially great in
convective rain events, when rapid updrafts of heated air
×
7.5.1
Raindrop properties and raindrop
impact seals
Given the importance of raindrops in driving the break-
down of soil aggregates and the structural rearrangement
of the breakdown products at the soil surface, we need to
consider briefly the properties of these drops.
Small raindrops are held into nearly spherical shapes
