Causes of Desertification (AQA A Level Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Natural Causes of Desertification
Desertification is:
The process of desert-like conditions spreading into what were previously semi-arid areas, as the quality of soil and its fertility declines over time
Desertification is a form of land degradation
Land degradation indirectly affects about 350 million people in the developing world
Land degradation occurs through changes in soil character, wind erosion, or water shortage (droughts) which leaves the land unproductive or lost
Soils in arid and semi-arid regions are potentially very fertile, as there is insufficient water to leach minerals from the soil
These soils have high pH values and intense evaporation at the surface results in the process of capillary rise of soil moisture and minerals - this is salinisation
When irrigation water is added to land and allowed to evaporate, this has the effect of increasing salinisation
This process has made a lot of land in the hot deserts infertile
Increased soil salinity restricts most plant growth and is also phytotoxic when there are high concentrations of sodium in the soil
Soil erosion is the washing away or blowing away of topsoil, which greatly reduces the fertility of the remaining soil
This is a natural process but made worse through human activity
There is a link between soil erosion and desertification
A changing climate causes a physical response as a positive feedback loop
The main physical changes are to precipitation and temperatures
Lowered precipitation rates | Increased intensity of the precipitation | Increased temperatures |
|---|---|---|
Precipitation patterns are becoming unpredictable, with drought events increasing and extending for several years Less precipitation decreases the amount available for vegetation and growth is reduced Roots bind the soil and fewer roots expose soil to erosion by the wind Soil becomes less fertile and desertification sets in | Increased intensity of rainfall increases surface runoff as the ground is dry and compacted Soil is unable to capture and store water Soil is exposed to sheet wash and is carried away, leaving bare soil behind Vegetation is unable to grow and therefore, no roots to bind soil Soil becomes less fertile which leads to a loss of productivity and increased desertification | From 1900 to 2020, global surface temperatures have increased by 1.08°C and are set to continue to rise As temperatures increase, the rate of evaporation and transpiration will also increase This leads to the drying up of watercourses and an absence of surface water This lowers the water table, increasing salinisation, and causing the soil to become toxic Vegetation growth decreases, exposing the soil to wind erosion There is an increased presence of dry, loose soil, that is easily lost as dust storms |
Characteristics of desertification include:
Absence of surface water
Dried up watercourses
Lowered water table
The increased salt content of the soil
Degraded or lost vegetation
Exposed areas of soil
Presence of dry, loose, infertile soil
Worked Example
What evidence is there of desertification in the landscape shown in Figure 1?
[4 marks]
Figure 1: Landscape in Senegal, West Africa
Answer:
There is an absence of surface water, which will increase in the salt content of the soil [1]
Ground vegetation is sparse with large patches of dry, loose soil showing [1], which will increase soil erosion as bare soil is exposed to wind [1d]
Vegetation is becoming degraded - brown and dry, indicating the soil is becoming less fertile [1], increasing the presence of dry, loose sand [1d]
Human Activities & Desertification
People enhance the process of desertification through:
Population growth – more people need more food which puts pressure on the land
Overgrazing – overstocking animals exceeds what the land can support and soils suffer
Overcultivation- grow too much without replenishing the soil and it becomes exhausted
Deforestation – trees are cut down for fuel and building. The loss of roots to hold the soil down gives rise to erosion
War – many sub-Saharan countries have suffered for years from civil war, where crops and animals have been destroyed leading to famine
Leisure and tourism - 4x4s have begun to replace camel transport in hot deserts. These vehicles damage the delicate surface and expose the ground to wind erosion. Safari minibusses cause damage to vegetation leading to soil erosion
Migration – as desertification takes hold in one area, local people migrate elsewhere in search of food and water. Unfortunately, wherever they settle, they increase the population pressure on that environment
Global changes in food trade - growth in demand for cash crops (bananas, coffee, cocoa etc.) has led to the best land being used for growing these crops, leaving unsuitable land for general food crops
Irrigation can cause desertification in a number of ways:
River channels can be diverted elsewhere to irrigate crops
Channels feeding the Aral Sea were diverted by the Soviet Union to irrigate cotton fields
Lake level decreased by 90%
The salinity of water increased, poisoning fish and affecting the fishing industry
The wind blows dust polluted with chemicals from the lake into towns, reducing life expectancy
Irrigation depletes surface water and/or fossil aquifer systems, lowering water levels and water availability
The Nubian Sandstone Aquifer System (NAS), North Africa, is the world’s largest known fossil water aquifer system and spans four countries with an underground land area of 2.6 million km², and holds roughly 150,000 km³ of groundwater
All 4 countries face similar problems of aridity, scarce surface water, droughts and fragile ecosystems
This makes the aquifer an important source of water and a priority for sharing equitably between these countries
It is of course a source of tension
Over-irrigating crops can lead to raising groundwater levels, causing salts to be dissolved and form a salty crust on the surface, creating an impermeable, infertile layer that is toxic to plants, reducing ground cover and increasing soil erosion
Excess water is lost due to poor irrigation techniques and essential minerals and soil are washed away
Examiner Tips and Tricks
Remember that it is all these factors (physical and human) that combine and lead to further reductions in the root mat holding the soil together. Once this is gone, the soil is lost to wind and rain, which leads to desertification. This is an example of a positive feedback loop where human and physical processes work in an ever-damaging way
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