Environmental Engineering Reference
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Figure 10-11 Natural capital degradation: desertification of arid and semiarid lands. It is caused by a
combination of prolonged drought and human activities that expose soil to erosion. (Data from UN Environment
Programme and Harold E. Drengue)
Over thousands of years the earth's deserts have
expanded and contracted, mostly because of natural
climate changes. However, human activities can accel-
erate desertification in some parts of the world (Fig-
ure 10-11).
According to a 2004 report by the United Nations,
an area the size of Brazil and 12 times the size of
Texas has become desertified in the past 50 years.
Each year since 1990 about 6 million hectares (15 mil-
lion acres) of land has become degraded and less
fertile from desertification. According to a 2003 UN
conference on desertification, one-third of the world's
land and 70% of all drylands are suffering from the ef-
fects of desertification. UN officials estimate that this
loss of soil productivity threatens the livelihoods of at
least 250 million people in 110 countries (70 in Africa).
China is facing serious desertification, as its portion of
the Gobi Desert expanded by an area half the size
of Pennsylvania between 1994 and 1999.
Figure 10-12 summarizes the major causes and
consequences of desertification. We cannot control
when or where prolonged droughts may occur, but we
can reduce overgrazing, deforestation, and destructive
forms of planting, irrigation, and mining that leave be-
hind barren soil. We can also restore land suffering
from desertification by planting trees and grasses that
anchor soil and hold water, establishing windbreaks,
and growing trees and crops together (agroforestry).
Science: Salinization and Waterlogging
of Soils
Repeated irrigation can reduce crop productivity by
salt buildup in the soil and waterlogging of crop plants.
The one-fifth of the world's cropland that is irrigated
produces almost 40% of the world's food. But irrigation
has a downside. Most irrigation water is a dilute solu-
tion of various salts, picked up as the water flows over
or through soil and rocks. Irrigation water not absorbed
into the soil evaporates, leaving behind a thin crust of
dissolved salts (such as sodium chloride) in the topsoil.
Repeated annual applications of irrigation water
lead to the gradual accumulation of salts in the upper
soil layers, a process called salinization (Figure 10-13).
It stunts crop growth, lowers crop yields, and eventu-
ally kills plants and ruins the land.
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