Geoscience Reference
In-Depth Information
It is also worth noting that a number of workers reject the very notion of deser-
tification as defined by international organisations such as UNEP, arguing that local
farmers are far more in tune with environmental fluctuations than outside agencies
are willing to acknowledge and that they show a high degree of resilience and a
considerable capacity to adapt to change (Bassett and Crummey,
2003
).
In addition, there is good evidence in support of 'natural desertification', that is,
land degradation caused by a sequence of geomorphic processes initiated, directly
and indirectly, by long-term changes in climate that have nothing to do with human
activities (Avni,
2005
; Avni et al.,
2006
; Avni et al.,
2010
). We will consider these
studies first before evaluating the impact of human activities on the landscapes of
deserts and their margins. The manifestations of dryland degradation or desertification
are as varied as the individual causes (
Table 24.1
), indicating that it would be naive to
seek a single cause. A potentially useful approach would be to examine a number of
examples drawn from different parts of the desert world in order to learn from history
and see if we can identify any general prerequisites for achieving optimum land use
in arid lands. This chapter seeks to do that.
24.2 Natural desertification
Gully erosion has been widely considered as one of the more obvious signs of accel-
erated soil loss brought about by human mismanagement, and it is not hard to find
apparently convincing evidence of such processes in many parts of the desert world.
However, a degree of caution is advisable before attributing the cause of gully erosion
to human actions. In a number of instances, the gullies were already active along
the valley floors well before humans had occupied those valleys. Yoav Avni and his
co-workers monitored a number of gullied valleys in the semi-arid Chifeng region
of Inner Mongolia (northern China) and in the Negev Highlands of southern Israel
(Avni,
2005
; Avni et al.,
2006
; Avni et al.,
2010
). They found that initial deposition
of loess (wind-blown desert dust, see
Chapter 9
) across the hills and valley sides
during the late Pleistocene (between about 70 and 15 ka) was followed by erosion
and redeposition of the valley-side loess mantles and their subsequent accumulation
in alluvial fans and valley fills. A change in rainfall intensity and a reduction in
dust influx during the end of the Pleistocene triggered a change from reworked loess
accumulation along the valley floors to incipient vertical erosion and gully incision.
Thereafter, the gully network extended up-valley through a combination of headwall
erosion and some lateral erosion from bank collapse, leading to a loss of potentially
valuable soil for cultivation and plant cover for grazing. The key factor here was
the progressive exposure of bedrock surfaces along the valley sides and the ensuing
increase in run-off and its capture by the expanding gully system.
In one instance in the Negev, two valleys of similar size and geologywere compared,
since one had been farmed in medieval times more than a thousand years ago and the
