Geoscience Reference
In-Depth Information
heterogeneous in their characteristics, so that infiltration capacities and rates of overland flow generation
might vary greatly from place to place (Loague and Kyriakidis, 1997). In many places, particularly on
vegetated surfaces, rainfalls only very rarely exceed the infiltration capacity of the soil unless the soil
becomes completely saturated. Elsewhere, where infiltration capacities are exceeded, this will start in
areas where soil permeabilities are lowest and, since infiltration capacities tend to decrease with inceased
wetting, will gradually expand to areas with higher permeability. Bare soil areas will be particularly
vulnerable to such
infiltration excess
runoff generation since the energy of the raindrops can rearrange
the soil particles at the surface and form a surface crust, effectively sealing the larger pores (see Romkens
et al.
, 1990; Smith
et al.
, 1999). A vegetation or litter layer, on the other hand, will protect the surface
and also create root channels that may act as pathways for infiltrating water. Bare surfaces of dispersive
soil materials are particularly prone to crusting and such crusts, once formed, will persist between storms
unless broken up by vegetation growth, freeze-thaw action, soil faunal activity, cultivation or erosion.
Studies of crusted soils have shown that, in some cases, infiltration rates after ponding might increase
over time more than would be expected as a result of the depth of ponding alone (see, e.g., Fox
et al.
,
1998). This was thought to be due to the breakdown or erosion of the crust.
In cold environments, the vegetation may also be important in controlling the degree to which a soil
becomes frozen before and during the build up of a snowpack by controlling both the local energy
balance of the soil surface and the drifting of snow cover. This may have important consequences for the
generation of runoff during snowmelt, even though it may, in some cases, take place months later (Stadler
et al.
, 1996). It is worth noting that a frozen topsoil is not necessarily impermeable. There will usually
be some reduction in potential infiltration rates due to freezing, but seasonal freeze-thaw processes can
also lead to the break-up of surface crusts so as to increase infiltration capacities (Schumm, 1956). The
effects of freezing will depend on the moisture content of the soil and the length of the cold period. Even
where widespread freezing takes place, infiltration capacities may be highly variable.
It has long been speculated that during widespread surface ponding there could be a significant effect
on infiltration rates of air entrapment and pressure build up within the soil. This has been shown to be
the case in the laboratory (Wang
et al.
, 1998) and, in a smaller number of studies, in the field (Dixon and
Linden, 1972). It has also been suggested that air pressure effects can cause a response in local water
tables (e.g. Linden and Dixon, 1973) and that the lifting force due to the escape of air at the surface
might be a cause of initiation of motion of surface soil particles. The containment of air will be increased
by the presence of a surface crust of fine material but significant air pressure effects would appear to
require ponding over extensive areas of a relatively smooth surface. In the field, surface irregularities
(such as vegetation mounds) and the presence of macropores might be expected to reduce the build up
of entrapped air by allowing local pathways for the escape of air to the surface.
In the absence of a surface crust, the underlying soil structure, and particularly the macroporosity of
the soil, will be an important control on infiltration rates. Since discharge of a laminer flow in a cylindrical
channel varies with the fourth power of the radius, larger pores and cracks may be important in controlling
infiltration rates (Beven and Germann, 1981). However, soil cracks and some other
macropores
, such as
earthworm channels and ant burrows, may only extend to limited depths so that their effect on infiltration
may be limited by storage capacity and infiltration into the surrounding matrix as well as potential
maximum flow rates. An outlier in the data on flow rates in worm holes of Ehlers (1975), for example,
was caused by a worm still occupying its hole! The effects of such macropores on hillslope response
might still, however, be significant, even in wet humid temperate environments (Marshall
et al.
, 2009).
Some root channels, earthworm and ant burrows can also extend to depths of meters below the surface.
The Jarrah trees of Western Australia are again a particularly remarkable example.
Overland flow may also occur as a
saturation excess
mechanism. Areas of saturated soil tend to occur
first where the antecedent
soil moisture deficit
is smallest. This will be in valley bottom areas, particularly
headwater hollows where there is convergence of flow and a gradual decline in slope towards the stream.
Saturation may also occur on areas of thin soils, where storage capacity is limited, or in low permeability
