With rapid urbanization, increase in human activities is always regarded as the cause of desertification due to over-grazing and over-cultivation. However, human impact is not the factor alone. Though men somehow alter the microclimate, the unstable global climate in the natural environment also leads to desertification. To check the spreading of desert area, some soil conservation measures and afforestation projects are carried out to stop it getting more serious.
Desertification refers to the desert expanding, slowly taking in land previously not a desert. The once green landscapes dry out and there is a change in the ecosystem. The land is deteriorating and land productivity decreases, usually at the marginal semi-arid area of the desert.
Take Sahel region in Sahara as an example. Desertification is a part of climatic change, which is a natural process that has been taking place continuously throughout the history of the Earth. Over hundreds of millions of years, there have been huge variations in the climates of all parts of the world. It is especially obvious in desert area, where the annual rainfall gradient is very steep, so the rainfall is highly unreliable. Desert climate is thus not stationary, but fluctuating. Between 1968 and 1974, prolonged drought occurred in the countries in Sahel. The rainfall failed again in 1980 and 1984.
Though change of climate is an important factor leading to desertification, with rapid population growth in recent decades, human over-exploitation can directly induces desertification or indirectly change the micro-climate. In recent decades, population in Sahel doubles in 20 years, causing misuse of land. Nomadic herding is intensified. More livestock are raised. This causes over-grazing, which means too many animals on too little land, and this destroys natural vegetation and encourages wind and soil erosion. Also, overgrazing by cattle and goats in sandy areas, such as around a well, can cause dunes to become active. It may lower the ground surface such that tree roots are exposed.
Over-cultivation is a reason leading to desertification. More mouths to feed means growing more crops, so cultivation extends into low-rainfall marginal zones. Traditionally, long fallow periods allow dry crop lands to regain their fertility. Now, continuous cultivation exhausts the soil and accelerates erosion. It was found that each year, Ethiopia’s cultivated areas lose more than 1000 million tones of topsoil. Worn-out soil cannot support intensive cultivation and crop yields fall. More and more marginal land is being used by inexperienced farmers, so that its yield rapidly declines.
Desertification is associated with poor irrigation practice. People tap groundwater for irrigation. Yet over-use of it may lead to groundwater depletion. The water table has been lowered and wells are dried up. Besides, secondary salinization is induced. The exposure of the former lake bed and the decline in ground water levels both have a knock-on effect on soil quality and contribute to soil salinity. The lowering of the water table leads to higher evaporation rates causing salts to be transferred upwards through the soil, leading to the formation of solonchaks.
Nevertheless, human over-exploitation of land can cause a change in climate. Owing to misuse of land, there will be less vegetation cover, the infiltration rate decreases and surface runoff decreases. At the same time, soil erosion becomes serious. The soil may be too poor for plant growth. The amount of vegetation decreases. Reduction in vegetation will lead to decrease in evapotranspiration rate. Less rainfall throughout the year will be resulted. Wind speed increases. With less vegetation absorbing carbon dioxide for photosynthesis, the carbon dioxide content in air is getting higher, trapping more heat and thus accelerates global warming. Semi-arid areas are likely to turn into desert region. Desertification will be widespread.
To arrest the process of desertification, some measures are carried out. Regarding the problem of overgrazing, settlement in arid areas should be restricted with control over nomadism. Number of stock raised should be reduced to match the natural carrying capacity of the land. Commercial grazing should be avoided, as it involves extraction of groundwater, extensive trampling and destruction of soil and vegetation cover. Pasture conservation should be introduced through re-seeding and controlled grazing.
To tackle the problems of over-cultivation and poor irrigation practice, some new and careful irrigation methods, such as new wells and water holes, and irrigation schemes can be constructed to prevent destructive salinization and alkalinization of the dryland soils. Fallowing should be practiced to allow adequate soil moisture to build up and soil structure and fertility to recover.
In addition, soil conservation projects can be introduced, such as terracing, contour ploughing, and windbreaks should be implemented to check surface runoff and wind. Sand dunes should be stabilized to prevent further wind erosion. Also, soil can be covered by organic residues to reduce erosion and to conserve moisture.
Not only are these, afforestation programmes can be carried out to prevent intrusion of degradation into susceptible areas. Shelter belts can be built to reduce wind erosion, modify microclimate and stabilize soil structure.
Nonetheless, the above conservation measures always meet the financial problems of the local government. Sahel region is still developing, there is insufficient capital and fund to develop a comprehensive remedial system. Still, deforestation is unlikely to stop and catch up the rate of rapid population growth. To be sustainable, some long-term birth control measures to control population growth are needed. Most important of all, the cooperation of other countries is necessary to carry out careful measures in the future.