Geoscience Reference
In-Depth Information
22
The human impact
Nick Middleton
22.1
Introduction
A useful distinction can be made between direct and de-
liberate modifications of landforms and processes, on the
one hand, and indirect inadvertent changes, on the other.
Generally, the former are much more easily recognised
than the latter. The construction of terraces on a hillside,
for example, is an ancient form of agricultural land man-
agement that is clear-cut. The acceleration of erosion in
an area where the natural vegetation cover has been mod-
ified by some human activity, such as grazing or burning,
is a much more difficult situation in which to attribute
causation.
This chapter will assess the human impact on arid-
zone geomorphology, ancient and modern, deliberate and
inadvertent, direct and indirect, with a range of examples
illustrating effects at a variety of spatial scales.
Human use and occupation of desert and semi-desert envi-
ronments has persisted over millennia and has inevitably
left its mark on dryland geomorphology. The scale of
impacts has grown with the rise in numbers of people
living in desert areas and the efficacy of their technolo-
gies, although significant effects can also be noted by
small numbers of people using rudimentary techniques.
The range of impacts is wide and occurs both deliberately
and inadvertently, directly and indirectly.
The number of people living in deserts was put at just
over 500 million by Ezcurra (2006), where 'desert' was
confined to arid and hyper-arid regions. A broader defini-
tion, adding semi-arid and dry subhumid climatic zones
and more frequently called the 'drylands' (Middleton and
Thomas, 1997), increases that population to about 2.1 bil-
lion people (Safriel
et al.
, 2005). These totals have been
reached by rapid rates of growth in many parts of the
world in recent decades. Some desert cities, in particular,
have experienced very fast rates of urbanisation. They in-
clude Cairo, Nouakchott, Riyadh, Sana'a, Dubai, Tehran,
Las Vegas and Phoenix. Such large agglomerations of
people have made an impact on-site but also off-site via
the significant ecological footprints of major urban ar-
eas. The human impact on geomorphology is not simply
a numbers game, however. Many changes to landforms
and geomorphological processes can also be attributed to
human activities carried out at relatively low population
densities, such as agricultural activities. Indeed, some of
the largest human impacts on geomorphology - those at-
tributable to desert nuclear weapons testing - have been
allowed in dryland areas simply because very few people
inhabit them (although such effects have actually received
little attention in the geomorphological literature).
22.2
Human impacts on soils
Soil cover is patchy in most deserts but becomes more
continuous towards desert margins. Given than human
use of desert resources is generally easier in the less arid
parts of drylands, most of the examples of human impact
on arid-zone soil geomorphology have occurred in the
wetter parts of desert regions.
22.2.1
Terracing and rainwater harvesting
A number of agricultural techniques have been developed
over the generations to maximise the utility of what little
surface water is available in drylands. This is achieved by
modifying the land surface in some way to concentrate
water into relatively small areas, which are then culti-
vated or used to graze livestock. Many of these ancient
