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

enterprises are enormously expensive. Such considerations stress the importance of
maintaining existing irrigation schemes by countering desertification with preventive action
whenever it affects them.
Salinization. The most prevalent form of desertification in irrigated cropping systems occurs
when waterlogging causes salts and alkali to infect soils, particularly where drainage is poor and
leaching inadequate. It is a problem that emphasizes the importance of preliminary surveys and
testing of proposed irrigation projects to assure adequate design. Most salinization problems
arise from irrigation system design deficiencies.
Irrigation System Design. Good design should be based on an understanding of how much
water is available for irrigation and on the knowledge of its silt and salt loads, including seasonal
variations. A close study of the soils must be made in the area embraced by a land-use project.
Such study must consider their texture and salinity, and especially their water properties, as
these will determine drainage requirements and water availability to crops. Water requirements
for proposed cropping systems should be determined. The position and salt content of the
groundwater table should also be determined, as there are seasonal fluctuations in both. This
will require some understanding of the hydraulic properties of the soil’s lower layers, and of how
these layers store and transmit water.
Field Test Results. These investigations should yield a map showing salt hazards and how they
might restrict the proposed cropping system. On the basis of the map and the surveys, design
work can continue with particular emphasis on the distribution of the water and on effective
drainage systems and the subdivisions of the system as determined by estimated water needs.
Finally, design should take account of the services and communications the system will require
and the settlements that serve and are served by it.
Maintaining Balance of Groundwater Supplies. Irrigation projects based on groundwater
supplies encounter special difficulties because groundwater quality is usually lower than that of
surface waters and the threat of salinization is generally higher. Limitations in groundwater
supplies may hinder proper leaching. Groundwater supplies must be kept in balance with the
requirements of land use, and enough water must be provided for both irrigation and leaching.
Generally, discipline applied to water use must be stricter when irrigation is based on
groundwater rather than surface water.
Reclamation of Salinized Lands. When irrigated lands have suffered salinization or other forms
of desertification, they should be surveyed as a first step to reclamation. By determining what
topographic changes have occurred and the degree of salinization of soil and groundwater
levels, an individual can estimate what is needed to leach and drain affected lands and what
else might be required to restore the system—by releveling of ground surfaces, for example, or
renewal of irrigation channels. How drainage will be carried out—whether by tubewells, tile
drainage or open ditches—will depend on groundwater conditions, soil properties and costs of
land and labor. To decide among alternatives, a cost-benefit analysis may be needed.
Secondary Physical Effects
It must also be borne in mind that desertification has an impact beyond the lands immediately
affected. Dust storms can move soil great distances, and increased flooding may occur far
downstream due to overly rapid runoff from lands denuded of trees and plants in upstream
catchments undergoing desertification.