Geoscience Reference
In-Depth Information
clearly have the potential to alter the volume and tim-
ing of water resources derived from the snowfields, and,
moreover, such effects may be prone to further strength-
ening if global and regional climate changes cause drying
and increased emission of desert dusts. Additional wider
significance for desert soils comes from the suggestion
that they may constitute a significant sink for methane, a
strong greenhouse gas (Striegl
et al.
, 1992; McLain and
Martens, 2005). In a similar way, the carbonates that ac-
cumulate at depth in many desert soils have a significant
place in global carbon cycling and storage (Hirmas, Am-
rhein and Graham, 2010). Finally, it is not possible to
consider the wider climatic significance of desert soils
without mentioning the effects of the iron fertilisation of
marine phytoplankton that results when the dust settles to
the sea surface (e.g. Jickells
et al.
, 2005; Aumont, Bopp
and Schulz, 2008). Phytoplankton growth then removes
carbon dioxide, another important greenhouse gas, from
the atmosphere. Moreover, the radiative forcing caused
by atmospheric loadings of dust (Yoshioka
et al.
, 2007)
promotes a feedback link between periods of dry, dusty
climates and rainfall suppression. The long-distance trans-
port of desert dust may even be implicated in the transport
of microorganisms, for example, from northern Africa
across the Atlantic Ocean to the Americas (Griffin, 2007).
These few examples show that while they may have lim-
ited fertility and little pedologic development compared
with agricultural soils, desert soils are by no means unim-
portant. It is appropriate therefore to consider that desert
soils have both autochthonous significance (influencing
water partitioning, overland flow and erosion within the
desert itself) and allochthonous significance (the offsite
and global effects of dust on soils, albedos, the oceans
and global climate). Desert soils then are not only af-
fected by climatic changes but are also drivers of global
environmental change. However, in the remainder of this
chapter, our main focus will be on contemporary soils
and the features developed in and on them that are impor-
tant to dryland hydrology, geomorphology and ecology.
Though deserts may superficially appear to be vast and
immune from degradation, the truth is quite the opposite.
In particular, desert soils exhibit features that are in fact
quite fragile and that can easily be damaged by people,
stock or vehicles (e.g. Adams
et al.
, 1982). Informed land
stewardship therefore also benefits from a knowledge of
these soils.
There are marked differences in the hydrologic role
of soils in drylands and in the humid zone. In many hu-
mid areas, soil infiltrability is high when judged against
the common rain rates, and the occurrence of Hortonian
overland flow is uncommon except in rare events with
pacted, and it may be absent altogether. Near-surface soil
horizons are characteristically porous and permeable, ow-
ing to the abundant organic matter and the overturning of
the soil produced by the population of soil fauna that
is supported by the organic matter. Soil depth is a sec-
ond characteristic of enormous significance, since it de-
termines the capacity for water storage within the soil
column. Even in the humid zone, rainfall is intermittent,
and water storage in the soil is vital to the maintenance of
plant cover and of baseflow in perennial streams. Varia-
tions in soil depth can therefore result in wide fluctuations
in soil moisture availability within a single climatic en-
vironment and can magnify the soil moisture variability
caused by climatic gradients in rainfall (e.g. Hamerlynck,
McAuliffe and Smith, 2000). Even if soils are highly per-
meable, if they are shallow and have little capacity to
store water, plant moisture stress arises relatively soon af-
ter rain, and prolonged drought can result in the mortality
of plants. Under the same conditions of rainfall and soil
permeability, plants in deeper soils survive. The available
pore spaces in a shallow soil can fill relatively quickly
during rain and consequently shallow soils may partition
more rain into saturation overland flow, therefore provid-
ing a functionally significant soil moisture store that is
smaller than might be anticipated from climatological in-
formation (such as the mean annual rainfall). In extreme
cases, this can amount to a form of 'pedologic aridity' that
we will shortly see is very common in drylands.
In drylands, soil infiltrability is often only moderate
judged against rain rates and locally may be very low. We
will examine some of the causes of this later, but low lev-
els of organic matter, less abundant biopores produced by
burrowing organisms and various kinds of surface seals
and crusts all contribute. Hortonian overland flow is con-
sequently more common in some drylands. However, as
we shall see, it is primarily the surface properties of dry-
land soils that are the key determinant of their hydrologic
behaviour and not the bulk properties of the deeper soil
column. A large proportion of the surface of dryland soils,
often 70-80%, may be devoid of vegetation cover, owing
primarily to the scarcity of water and other resources, and
is exposed to wind, rain, frost and strong solar radiation.
Allelopathy, the inhibition of the growth of neighbour-
ing plants by the production of inhibitory chemicals, is
also involved in the wide spacing of vascular plants (e.g.
Halligan, 1973).
The lack of cover creates three circumstances of enor-
mous importance to the hydrology of dryland soils:
1. It leaves the soil surface exposed to the energetic im-
pacts of raindrops, as well as to the erosional and depo-
