Natural Hazards: Causes and Effects - Disaster Management Center ...

effect. Finer materials are lifted as dust into the air and carried away over long distances;
coarser sandy materials drift over the surface until they are trapped by plants in accumulations
of low, rounded hills and small dunes. Removal of fine topsoil materials means the loss of the
most productive and nutritious portions of the soil complex, while sterile sand accumulations
cover plants and good soil. In addition, the blasting impact of moving sand harms young crops.
Fine airborne particles may carry soil-borne diseases, irritate respiratory tracts of humans and
animals, cause wear on machinery parts and reduce visibility.
Salinization and Alkalization of Soils
The principal manifestations of desertification on irrigated lands are soil salinization or
alkalization occurring with inadequate leaching of salts contained in the soil or added in
irrigation water. Salinization and water logging commonly occur together. Where the soil is
waterlogged, the upward movement of saline groundwater leaves salts on the surface where
water evaporates. On soils that are not waterlogged, salinization can still occur when water
containing soluble salts moves from irrigation furrows into the ridges where crops are planted or
to high spots in poorly leveled land. Underirrigation of weakly permeable soils can also lead to
salinization if the irrigation water is salty.
Desertification as Patchy Destruction
Desertification breaks out in areas of naturally vulnerable land subject to pressures of land use.
These degraded patches link up to carry the process over extended areas. It is generally
incorrect to envision the process as an advance of the desert frontier engulfing usable land on
its perimeter: the advancing sand dune is in fact a very special and localized case.
Desertification, as a patchy destruction that may be far removed from any nebulous front line, is
a more subtle and insidious process.
Hazard Analysis and Prediction
Experiences indicate that the long-term progressive deterioration that constitutes desertification
may not be readily identifiable against the background of short-term environmental fluctuations
that spring periodic shifts in rainfall. There is a consequent need for regular monitoring of the
status of dryland ecosystems. Such monitoring can provide early warning of trends, identify
areas in which change is taking place, and provide a basis for the investigation of causes and
processes. It is in terms of such information that measures for prevention or reclamation will
ultimately be designed.
Global surveillance of the status of dryland ecosystems and of the land use can be achieved
most economically through the remote sensing powers of specialized orbiting satellites. The
Landsat system, already in operation, has this capacity. Landsat now provides imagery with a
resolution of at least 50 meters (165 feet), and there are prospects of higher resolutions.
Studies of the relationship between agriculture and climate, such as those carried out by the
World Meteorological Organization in western Asia and Saharan Africa, have done much to
determine the connections between climate and the water needs of cereal crops. This has
enabled them to use climatic records as a basis for determining when suitable growing seasons
will occur. These studies should be extended and improved through additional meteorological
recording and investigations into the water requirements of crops at different stages of growth
and under a range of soil conditions. By providing good estimates of climatic risk, such studies
will support policies of land zoning, and measures should be taken to discourage the extension
of cropping beyond certain environmental limits.