Environmental Engineering Reference
In-Depth Information
vegetation factors acting together can reduce the number of animals which can be
supported.
Why do the pastoralists get into this sort of situation, which is clearly harmful? As
with many problems, competition for resources is important. In many countries the
cultivated area has increased to cope with increasing population and higher economic
expectations, pushing grazers into more marginal land. Even the American musical
Oklahoma
reminds us that 'the farmer and the cowman can't be friends'. The sinking of
boreholes to provide reliable water supplies has increased grazing in surrounding areas.
This increase of commercialism in pastoral farming has replaced the natural system of
nomadism where the grazers moved their herds in response to vegetation growth,
irrespective of political or economic controls.
Overcultivation
. Intensification of farming to produce more food can result in fewer
and shorter fallow periods, leading to nutrient decline and a decrease in yield. If organic
matter is not returned to the soil in sufficient quantities, soil structure can be affected.
Mechanized farming has advanced into several dryland regions, with deep ploughing
disturbing soil structure and increasing susceptibility to erosion. Classic examples of this
are the Great Plains of the United States and the Virgin Lands campaign of central Asia
in the former Soviet Union. In both cases major wind erosion resulted.
Deforestation
. Clearance of forest or shrubland to increase the proportion of grasses
for grazing leaves the soil exposed to erosion and may reduce the water table. Fuel wood
is in great demand in many energy-poor countries, which places further demand on forest
resources. For example, few trees survive in the wild within about 90 km of the Sudanese
capital, Khartoum. Often dried animal manure is burnt instead, rather than being recycled
back to the land, where it would improve soil structure. In Saharan Algeria the price of
natural gas for cooking is subsidized to encourage its use. Even in remote oases gas is
available, but it is not a solution for most areas.
RECENT IDEAS
Although the above factors are usually cited as causes of desertification, recent research
has questioned the validity of some of the ideas. The state of vegetation in a dryland
environment is a response to grazing pressures and the recent levels of moisture
availability and fire incidence. There have been few long-term studies of vegetation in
affected areas; most of the assessment has been on the basis of short-term visits, perhaps
separated by a number of years. If the vegetation does decline by either process, it is the
animals that are the first to be affected as the food supply deteriorates. In areas of this
type, supplies of fodder cannot easily be bought or brought in to offset the loss of local
supplies. As the animals die, so the pressure on vegetation should decline.
In many areas affected by desertification the political, social and transport
infrastructures are poorly developed. A study of the relative importance of the factors
behind the Sudanese famine of 1984/5 found that the lack of rain in the 1984 wet season
triggered major speculation in food. People bought stocks of cereals because they
believed the price would rise. The price of food then rose beyond the reach of most rural
people in the drought-affected areas. Food was available on a national level, but the
mechanisms for distributing it were inadequate. Whilst deterioration of natural vegetation
