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Definitions
List of terms for Desert science
A
Animal life
Deserts and drylands teem with biological life, particularly insects, reptiles and small mammals in addition to camels. Animals are often nocturnal and avoid excessive water loss during the day by finding shelter underground. Strategies employed to obtain water include using night-time dew, as well as condensed fog.
Aquifer
A layer of underground rock that contains water. Water from aquifers is used for domestic consumption as well as in agriculture and industry.
Arid
A climate or habitat that receives less than 250 millimetres of annual rainfall and has high rates of evaporation and sparse vegetation. Semi-arid regions receive between 280 and 400 millimetres of annual rainfall and support limited agriculture.
B
Bedouin
Indigenous people of the deserts in the Middle East, most of whom are nomadic pastoralists. The word 'Bedouin' comes from the Arabic Badawi, or 'desert dweller'.
Bush encroachment
A process, often caused by over-grazing, in which a mix of dryland vegetation, particularly grasses, is replaced by a single species of dense bush or shrub.
D
Desertification
Twelve million hectares of land is being taken out of cultivation every year. The UN Convention to Combat Desertification defines desertification as "land degradation in arid, semi-arid and dry sub-humid areas" brought about by climate change and human activities, such as overgrazing and deforestation. This is not the only definition, however, and desertification is not regarded as being irreversible.
In most cases desertification involves a decline in land productivity, and not advancing desert sands. Land degradation often affects the poorest and most vulnerable in society. An estimated 250 million, for example, have had to leave their homes because soils are longer suitable for cultivation.
Deserts
Deserts are regions of the world that receive large amounts of sunshine, but very little rainfall, making the land unsuitable to grow crops. Strong winds and high temperatures remove moisture through evaporation. Average desert rainfall is less than 250 millimetres per year. Parts of the Sahara and Atacama deserts receive much less – around 1 millimetre per year. Often, the heat turns the rainfall into vapour, which returns to the atmosphere before it has a chance of reaching the ground.
The Sahara is the largest desert in the northern hemisphere. Asia’s great deserts include the Thar in India and Pakistan and the Gobi in China. In the US, deserts are found in the states of Arizona, California, Nevada and New Mexico. In Africa south of the Sahara, deserts are in Botswana, Namibia and South Africa.
Drought
When a region receives less than its normal or predicted amount of water. Drought is caused by decreased rainfall, increased evaporation because of higher temperatures, or a combination of these two processes. In some countries a drought is declared after a minimum number of consecutive days without rain.
Drylands
Drylands are on the margins of deserts and contain land that can be suitable for agriculture if irrigated. Drylands in the Middle East and North Africa, for example, are often the original homes for many of the world’s most important food crops such as wheat, barley, millet and sorghum.
Dunes
Dunes are the large accumulations of sand that can form shapes such as hills, or ridges in a desert landscape. Half of all dunes are parallel ridges of sand that can extend for 20 kilometres or more.
E
Ecosystem services
Natural processes carried out by ecosystems that support life on Earth. Also known as 'environmental services', they include water supply, cycling of soil nutrients, pollination, natural pest control and carbon sequestration. Conservationists and some economists argue that these services have a monetary value that should be included in a country's national accounts.
F
Fog-harvesting
The systematic collection of fog droplets into usable water (also known as fog-catching). It is common in arid countries like Chile, India, Peru and Yemen, where rainfall is scarce. Water is harvested by placing large nets, or metal sheets, in the path of fog clouds. Fog droplets condense on these, and the resulting water is passed through a network of pipes and channels.
G
Global Environment Facility
The Global Environment Facility is a funding agency for environmental projects. Based in Washington DC, it is jointly administered by the World Bank, the UN Development Programme and the UN Environment Programme. It supports projects related to land degradation, climate change, biodiversity, international waters, the ozone layer and persistent organic pollutants.
Groundwater
Water that infiltrates soil and rock, accumulating in underground streams and aquifers. See also 'Water'.
I
Irrigation
Transporting water from a source of supply, such as a river or aquifer, to an area where it is needed for the cultivation of crops. Types of irrigation include drip irrigation in which water flows through perforated pipes on the ground close to crop plants; and sprinkler irrigation in which plants are watered using an overhead pressurised spray.
L
Land degradation
A decline in the ability of land to store and recycle energy, nutrients and water. Land degradation decreases soil productivity. It has both human-induced as well as natural causes. Some 40 per cent of the world's cropland is degraded, as is 21 per cent of global pastures and 18 per cent of all forests.
O
Overgrazing
Overgrazing of pastures is regarded as the single largest cause of declining land productivity (or land degradation). Government policies to encourage nomadic communities of herders and pastoralists to settle down is one cause of land being overgrazed.
P
Pastoralism/pastoral farming
A type of production based on herding domesticated animals. The areas of vegetation used by pastoral farmers are often called rangelands.
Plant life
Plants in deserts and drylands are equipped to grow in salty soils and long periods of water scarcity. As a result, many tend to grow, flower and produce seeds very quickly, for example when rains have ended. Roots can be very deep enabling plants to tap into underground moisture. Leaves can be small and tough to reduce water loss through evaporation.
Protected areas
An area of land or sea dedicated to the protection and maintenance of biological diversity. By the end of 2003 there were some 30,000 protected areas covering around 12 per cent of the Earth's surface. Protected areas can be important to local communities and economies, especially indigenous people, who often depend on them for a sustainable supply of resources. Protected areas are not the same as hotspots.
R
Runoff
Water that drains or flows over the ground and into streams and rivers.
S
Sahel
The semi-arid African region that lies south of the Sahara Desert, and stretches from Senegal to Somalia.
Salinisation
The accumulation of large quantities of mineral salts in soils. Salinisation affects land productivity and leads to a decrease in water quality.
Semi-arid lands
Semi-arid lands are places which are dry, but contain enough moisture to be able to sustain limited vegetation.
Soil erosion
Dryland soils are vulnerable to degradation. New soils form very slowly and salts tend to build up because of infrequent rains. Moreover, sparse topsoil can be blown away by winds or by rains when they do come. The world loses an estimated 24 billion tonnes of topsoil every year. This decline, in turn, means that land is unable to hold water, which contributes to runoff and flooding.
U
UN Convention to Combat Desertification (UNCCD)
An international environmental agreement, whose principal objective is to "combat desertification and mitigate the effects of drought, particularly in Africa". The agreement came into force in 1996 and has been ratified by 191 countries.
W
Water
Oceans make up 97.5 per cent of the Earth's water supplies. The remaining 2.5 per cent is freshwater, most of which is locked up in ice caps and glaciers. Twenty per cent of all freshwater is groundwater and only one per cent can be found in rivers and lakes.
Water harvesting
The collection of rainwater that would otherwise evaporate, infiltrate the ground, or flow into drainage systems. Rainwater can be collected from rooftops and stored in small water butts for future household use. Alternatively, large reservoirs of rainwater can be kept in underground cisterns and used for agriculture and industry.
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