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– A Form of Land Degradation (Page 2)
By Professor Nils Nykvist
Salinization worsens the desertification problem
Even in arid areas, flooding
can occur on clay soils when the rainfall in a short time is greater than the rate at which water infiltrates into the soil.
When the flooded areas dry out during the dry season, water moves upward in the soil through capillary action, and its content of soluble salts precipitates out
when the water evaporates from the soil surface. This salinization of soils can in arid areas be a big problem, because the content of salts is often too high for most field crops.
For cultivation of the soil, the drainage must therefore be so large that the salt is washed out from the soil, which increases the salinity of the drainage water. When rivers from irrigated areas flow through several countries, the increased salinity could have political consequences.
A frightening example of environmental degradation is the Aral Sea, where over-intensive irrigation of cotton farms upstream has converted most of the sea into a salt desert.
In many areas where the drainage was inadequate, salt has been concentrated on the soil surface, and the fertility of the soil substantially decreased. In Agenda 21, the Rio Declaration on Environment and Development, it was estimated that about 30% of the irrigated area in the world has been destroyed by salinization .
Eckholm, writing in 1976, estimated that ‘Pakistan was losing a hectare of good agricultural land every twenty minutes, but gaining a new claimant on that land by birth every twenty-four seconds’ [15, p. 120].
Desertification could have been prevented
Such existing land degradation around villages could have been significantly less, if the utilization of the vegetation had from the beginning been distributed over a larger area. Plants would have become larger and produced more organic substance per unit of area for people and animals to exploit.
The loss of plant biomass is repaired surprisingly fast, even in dry areas, when the plants are not grazed, cut down, or burnt. Examples of this can be seen in many places where barbed wire barriers for some reason have been left behind, or where the plant community has not been utilized so heavily (Fig. 20).
Figure 20. A camel grazes bushes of Sesbania bispinosa in a semi-desert area in Pakistan, where no firewood was collected and no goats or sheep were grazing because of a feud between two villages.
Thus, desertification does not mean that the soil is destroyed, but only temporarily decreased in fertility, unless the soil erosion
has gone so far that all soil has been eroded away from the bedrock.
In addition to the gradual deterioration of soil and vegetation that occurs when people over-utilize the land, population growth is high in areas affected by this kind of activity.
Desertification has therefore become a major threat to people in arid areas. According to Agenda 21, the land degradation in arid, semi-arid, and dry sub-humid areas affects "about a sixth of the world's population, 70% of all drylands, ie. 3.6 billion hectares, which corresponds to about one quarter of Earth's total land area ".
A degradation of land similar to the desertification in arid areas is also going on in other, more humid areas where land use has been so hard that the soil is largely bare. The risk is particularly high where the soil is prone to erosion.
In Iceland over-grazing on the volcanic soils has resulted in large areas of birch forest being converted into a desert by wind erosion (Fig. 21). On the loess soils in China land use that is too intensive in some areas has led to very severe soil erosion (Fig. 22).
Figure 21. Over-grazing by sheep has opened up the land for wind erosion in some parts of Iceland.
Figure 22. Satellite image of a loess area on both sides of the Yellow River in the Shaanxi Province of China. From Google Earth.
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for this article.
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