Environmental Engineering Reference
In-Depth Information
Source
: After Thomas and Middleton (1994).
basin) has a high proportion degraded, whilst the arid continent of Australia with its low
density of population and generally low slopes has a very small proportion degraded.
REMEDIES
There are a number of ways in which cultivated soils can be protected from erosion.
These can be subdivided into three groups: (1) agronomic measures which protect the soil
surface; (2) soil management techniques which improve soil structure; and (3)
mechanical methods which modify surface topography to control wind and water
movements. Where properly conducted, such techniques can prevent soil erosion from
susceptible areas or can be used to help restore damaged areas if the erosion is not too
severe. What must not be forgotten is why the problem developed in the first place.
Normally, even in dry lands, the vegetation cover is of sufficient density to prevent wind
and soil erosion. It can become a problem when human activities put increased stress on
the environment. The driving forces are social, economic and political factors such as
population increase, unequal distribution of resources, land tenure methods, government
attitudes to agriculture and the terms of trade. These factors may limit the options open to
the poorer strata of society, who may have to degrade the soil resources in order to
survive. Tenant farmers may have a short-term view of the land's value, trying to
maximize yields rather than taking a long-term view of soil improvement. Here we have
the ethical and practical question of who should pay: the individual farmer or society as a
whole?
DESERTIFICATION
We have already seen in Chapter 26 that desertification can be a problem in
Mediterranean environments. When we are dealing with even drier environments it is
inevitable that the problem becomes more severe. A UN conference on desertification
was convened in 1977, as, at the time, desertification was seen as a threat affecting dry
lands throughout the world. It was blamed partly on declining precipitation levels, as
dramatically demonstrated in the Sahel (Figure 9.7), and partly on overexploitation of a
limited natural resource by increasing populations. The term is still used to denote the
spread of desert-like conditions into wetter areas, but what does it really mean?
The most recent (1990) UN definition of desertification is 'land degradation in arid,
semiarid and dry subhumid areas resulting mainly from adverse human impact on the
environment'. The idea of desertification as purely a natural phenomenon associated with
declining precipitation has been superseded. However, we must not forget the natural
variability of precipitation in dry lands. It is characterized by high variability in space and
time. Much of the annual precipitation falls during a few events in the rainy season. A
higher frequency of events in one year would lead to a higher annual total and vice versa.
Superimposed on the high year-to-year variability, short-term trends may occur
towards wetter or drier conditions. In practice it is then difficult to distinguish between
adverse effects generated by human action and the dry land response to the natural
climatic variability.
