Desertification is the degradation of semi-arid land by human activities and change in climate. It is an environmental degradation process which leads to the decline of productive land. A third of land worldwide is at risk from desertification, and 250 million people are already affected. 46% of Africa is at risk from desertification. Desert is encroaching into semi-arid desert margins.
Physical causes of desertification – climatic change
Lower rainfall – in subtropical areas where most semi-arid areas are, surface water and groundwater will be reduced, because it is used up or evaporated. The volume of water for vegetation is reduced, so the vegetation dies. There are fewer plants and trees, so there are fewer roots to anchor the soil. This leads to soil erosion.
Higher temperatures - global surface temperature has risen by 0.6 degrees over the last century and is predicted to continue to rise. As temperatures increase, the rate of evapotranspiration also increases. This dries out soils, lowering surface water levels. Vegetation dies, so there is less root anchorage and more soil erosion.
Human activity – the main cause of desertification
Overgrazing – if the number of animals increases beyond the carrying capacity, over grazing can become a problem. Overgrazing reduces the amount of vegetation, so there is more soil erosion. Trampling by animals compresses and breaks down the structure of soil making erosion more likely.
Population growth – increases pressure on land and more food is needed to meet the growing demand. This leads to further overgrazing, over cultivation, deforestation and irrigation.
Over cultivation – increases in population through high birth rates and immigration means that there is an increased demand for food. Farmers leave behind traditional land use because more land is required to grow crops. Exploitation of the soil leaves it without enough nutrients to support plants, leaving it unproductive. The soil is then easily eroded.
Deforestation – If population rises, there is a greater demand for wood; it is needed for building materials and fuel. Cutting down areas of woodland also increases the amount of land available for agriculture. This leads to more trees being cut down, so fewer roots mean that soil is more vulnerable to erosion.
Irrigation – Depletes surface water and may involve unsustainable pumping of aquifers. As water levels decrease, water availability for plants decreases leading to less vegetation cover and soil erosion. Some irrigation techniques erode soil directly – surface irrigation washes top soil away. If too much water is used to irrigate the crops, excess can sink into the soil and raise groundwater levels. If the aquifer is saline, high concentrations of salt may be brought to the surface, increasing the salinity of the soil. This is known as Salinisation, and plants may be unable to survive.
As fertile top soil is eroded, land becomes unproductive.
Less plant life can be supported by the soils. This means that fewer animals can be sustained, and so biodiversity decreases.
If agricultural production decreases to the point where farmers cannot feed their families or earn a living, they may have to migrate from the area. The land which people move to will be put under increased pressure. If people are unable to move, a community’s inability to produce the food they require may led to famine.
Desertification in the Sahel
The Sahel is a 3900km long belt that runs East to West across Africa. It separates the hyper-arid Sahara from the wetter Savannah to the South. The Sahel runs through 10 countries, some of which are the poorest in the world, including Niger and Mali.￼
The causes of Desertification in the Sahel
Climate change – between 1968 and 1997, rainfall in the Sahel decreased by between 29 and 49%. A 5-year drought was caused, beginning in 1968, and droughts were common throughout the 1990s. The average temperature of the Sahel has also increased over the past century. This increases evapotranspiration, reducing ground and surface water supplies.
Increased population equals more intensive agriculture - The population increase in the Sahel is 3% per year; between 1968-1998, population increased from 274 million to 628 million. Food production increases by only 2% per year, so additional pressure is being put on land. This has led to over cultivation, over grazing and over-irrigation. All of these factors increase desertification.
Deforestation – More space is needed for agriculture, and so large areas of forest have been cleared to create more space for agriculture, usually by slash and burn. Wood is also needed for energy supply – 82% of all energy used in the Sahel comes from wood. With fewer trees, soil is more vulnerable to erosion.
The Struggle for Survival – effects of desertification in the Sahel
Erosion of topsoil has reduced the area of productive agricultural land in the Sahel. In Mauritania, all that is left in a 200km wide strip running across the country. In Niger, 2500 square kilometres is lost each year to desertification.
As land area decreases, so does the amount of food produced. Niger can now produce less than a 20th of the food they could 40 years ago. This causes loss of livelihoods and famine.
Management strategies in the Sahel
Contour Ploughing – ploughing along contours rather than through them to limit soil erosion from wind.
'Magic' Stones – “Water Harvesting” – rows of stones placed along contours, slowing surface run off and protecting top soil. Water contained long enough for it to soak into soil. Increased yields by 40% in Burkino Faso.
Careful management of irrigation and the implementation of drip irrigation, which reduces waste and prevents excessive Salinisation. In Senegal, farmers are being educated in the use of drip irrigation. Concrete dams have been built next to rock outcrops to catch rainwater and condensed dew. Storage tanks then hold the water, allowing it to be used for irrigation.
Promoting the use of solar ovens, to reduce the need for fire wood and limit deforestation.
Introduction of an efficient version of the traditional Mogogo stove (or solar oven, shown below) in Eritrea, Sudan and Senegal, which needs 50% less wood than the older versions, reducing deforestation.
Growing the Jatropha curcas plant in Mali, around food crops. It can grow in poor quality soil and is not eaten by animals. Its roots help to bind soil together and therefore protects against water and wind erosion. Oil from the plant can be sold providing a valuable source of income. The plant can also be burnt, reducing need for deforestation.
Examples of recent developments relying upon external aid
Tal Rimah Rangeland Rehabilitation Project, funded by USAID and US Forestry Service, in the Badia, Jordan. Stone Walls were built to retain the limited water, called water harvesting. Drought tolerant shrubs like atriplex were planted to provide grazing for animals, encourage biodiversity and hold soil together to prevent erosion.
The Eden Reforestation Project – a charity planted 1 million trees in Ethiopia’s Rift Valley to reduce dust storms after years of desertification.
Projet Agro-Forestiere, funded by Oxfam, in Burkino Faso. Stone lines constructed along the contours of the land, and grasses and tree seedlings are planted alongside the stones during the rains. The contours channel the rain water to the plants, helping them to grow.
Note: the name is French.
Evaluating these strategies
Projects funded by external aid can have negative impacts. During the drought of 1968-73, thousands of wells were built. Farmers used this to increase the size of their herds, but the land could not cope with the overgrazing that followed. Large areas of land became desertified as a result.
Managing Desert Margins: Southwest USA
The great plains of the South West (USA), including Texas, Kansas and New Mexico, have experienced desertification. Drought conditions caused the Dust Bowl effect. This was enhanced by years of bad farming practices which had left the top soil dry and susceptible to Aeolian erosion. Ploughing and growing crops in rows up and down the slopes encouraged channelling and gullying.
Overgrazing reduced vegetation cover and so increased soil erosion. In order to control soil erosion in the semi-arid ecosystem, techniques have been employed;
Drip irrigation minimises the use of water and fertiliser by allowing water to slowly drip onto the roots of plants through a network of valves, pipes, tubing and emitters. Liquid fertiliser is mixed with the irrigation water – this is called fertigation.
Chemigation (a made up word) can also take place (the application of pesticides and other chemicals). Drip irrigation can achieve water conservation; less water is wasted, and more crops are produced for the same amount of water used.
Drip irrigation has advantages:
• Minimised fertiliser and nutrient loss due to localised application
• High water application efficiency
• Minimised soil erosion
• It’s expensive – to install a system is expensive - the ‘high initial cost’
• The system takes a lot of time to install, because factors such as topography, soil, agro-climatic conditions must be studied.
Other methods of preventing desertification
Crop rotation – 4 to 6 year rotations of barley, wheat, potatoes and clover, increasing the water absorbing capacity of the soil￼.
Preventing cattle from grazing – trampling of soil reduced ground cover and reduced the soil’s capacity to hold water. Grazing also delayed tree growth, which means that rain water can more easily wash top soil away, as there is no protection.
Contour Ploughing - ploughing slopes horizontally not vertically to reduce soil erosion by water and wind. Here's one type:
And one with which we are more familiar:
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