CHAPTER 26 Tropical Deserts and Semi-Arid Environments

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1 CHAPTER 26 Tropical Deserts and Semi-Arid Environments Dryland environments cover a considerable portion of Earth s surface. As water is a vital resource for habitation and economic activity, it is inevitable that, where water supplies fall below the direct precipitation inputs, then efforts have to be made to overcome the deficit. Water demand is increasing in most parts of the world. An increasing population, together with the greater demands of increased hygiene and greater prosperity, push consumption of water per head and in total volume to greater levels. In some parts of the world the problem has been compounded by decreases in precipitation. Interest in the problems of water supply has never been greater. Most students will have views about the importance of a stable water supply, though it may be difficult for them to imagine themselves in a situation where the sole water supply is a single well. Try to recall how often you have used water in the last twenty-four hours. What would happen if your supply was disconnected and you had to use a standpipe? A popular theme of the dryland environment is that of desertification. Ideas of a desert encroaching onto agricultural land have become widespread. In reality the subject appears more complex. It is discussed in this chapter in some detail as it is a topic which suffers from lack of precise information. Deserts The subtropical anticyclone belt is the main area of the drylands. Where the highs are stable in position we find the main desert areas of the world. On their margins areas of seasonal rainfall occur, with annual rainfall totals gradually increasing away from the desert cores. Intense heating during the day creates shallow low-pressure systems over the deserts. At higher levels, dry, subsiding air and a stable atmosphere prevent precipitation forming. Cold oceanic currents can increase the aridity along some coastal areas. Dryland environments may also be found in continental interiors, far removed from sources of moist, rising air. Most deserts and semi-deserts support widespread but relatively sparse vegetation. Many species have evolved methods of coping with lack of water and extreme heat. Desert soils are typically little weathered and lacking in humus. Saline accumulation may be a problem in some areas. Savanna Savanna areas typifically experience seasonal rains during the hot season. Amounts vary from low values near the desert margins to appreciable totals as the savanna merges into closed forest.

2 Savanna vegetation is characterized by open woodland with widely spaced and rather scrubby trees. The formation of savanna vegetation has been the subject of considerable debate. From the amount of annual rainfall it might be expected to have more luxuriant forest growth. It is believed that fire is significant in determining the type of - vegetation. Many savanna trees are xerophytes. Their morphological and physiological resistance to water loss allows them to survive dry periods. The distribution of savanna soils is related to climatic, geological and geomorphological conditions. Soil types include Ferralsols, Acrisols, Vertisols and Luvisols. (See Appendix or Chapter 18). Biogeochemical cycles Primary productivity is lower in savanna areas than in the tropical rain forest. Nutrient cycling is rapid, owing to the speedy breakdown of organic material by soil organisms. Soil erosion is a major agricultural problem in savanna areas and causes nutrient loss. Environmental problems of dry lands Water resources Water resources are of vital importance in dryland areas. Water may be obtained from rivers, by damming them, by transporting it in natural or artificial aqueducts or from the ground water. Rivers provide one of the cheapest and most accessible sources of water where they exist. They have been used for over eight millenia in the dry lands of the Middle East. A variety of methods have been devised to draw water from them. Dams on rivers provide a more stable source of water. Surpluses during the wet season can be stored and used during periods of lower flow. In addition they provide power generation and even tourist potential. Dams can cause a number of environmental problems such as siltation, ecological changes, pests and diseases, as well as social and economic problems for people dispossessed of their land. Aqueducts can be used to transfer water where natural river channels are not available. Irrigation in drylands is vital for many aspects of agriculture. Water may be extracted from rivers, reservoirs, aqueducts or ground water. Water quality in drylands may be poor. The salt content tends to be high and salt can accumulate in the soil in irrigated areas.

3 Soil erosion Soil erosion can take place by wind-blow or by water erosion in sheets or in gulleys. Agronomic measures, soil management techniques and mechanical methods can be used to alleviate soil erosion but they cost money which may not be available. Desertification Desertification is defined by the United Nations as land degradation in arid, semiarid and dry subhumid areas resulting mainly from adverse human impact on the environment. The causes of desertification are debated. Important factors are overgrazing, overcultivation, deforestation and the social, political and economic situation in the area affected. CASE STUDY Water supply problems in Saudi Arabia Saudi Arabia is one of the driest countries of the world. Lying astride the Tropic of Cancer its climate is dominated by sub-tropical anticyclones throughout the year. The northern part of the country receives some precipitation from temperate latitude low pressure systems that pass along the Mediterranean and the south gets some rain in spring and summer on the line of the Sudanese low pressure system. This rainfall is enhanced along the mountains adjacent to the Red Sea and into Yemen where up to 500 mm/year can be received. In general amounts are low with between mm/year in northern areas but dropping below 100mm/year for most of the central and southern areas. Riyadh, the capital, has a mean annual total of 104 mm falling mainly between January and April. As a result of oil wealth, Saudi Arabia has developed rapidly during the second part of the twentieth century. Population has increased accordingly with expectations that it will exceed 29 million by 2010 compared with 7 million in People need water to live, so, if so little falls as rain, how do they cope? Table 1 Growth of Water Use in Saudi Arabia, (millions of cubic metres) Total , , , , , , , , , , ,469

4 2000 2, , , , , ,300 % % Domestic & Industrial Agricultural Sources: International Development Research Center, Canada. Accessed from 2 January 2008 There are four main sources of water in Saudi Arabia. These are shallow ground water reserves from alluvial aquifers, surface water, desalinated water and non-renewable groundwater from deep and fossil aquifers. Only the first two are classed as renewable and are concentrated in the wetter south-west of the country where population density is low. The fossil aquifers have been fed by rainwater that fell a considerable time ago and has been preserved in the sandstone aquifers. Extraction of this water leads to a fall in the watertable as it is not being renewed to any extent. For example, the Al-Ahsa aquifer in the east of the country has experienced a drop of 150 metres since As the usable volume of water in the aquifers is not known, it is impossible to be certain how long these reserves will last. Desalination is a process of extracting salt from seawater to make it useable. Different processes can be used but they all need large amounts of energy. This is less of a problem in an energy-rich country like Saudi Arabia, especially as there are few alternative cheap sources of freshwater. A number of plants have been built and now supply about 70% of the drinking water requirements. Apart from the obvious requirement for drinking water and other domestic services, water is needed for electricity production, industrial processes and for growing crops. All Saudi crop agriculture has to be based on irrigation. Different irrigation techniques are used; surface irrigation is the most frequent for old established systems. Sprinkler irrigation is also used. Central pivot sprinkling is used for virtually all cereal cultivation. Groundwater is pumped from a well and linked to one or two central pivots. More intensive agriculture such as vegetables and fruit are normally irrigated by drip and bubbler methods respectively. The decision to grow cereals under irrigation was taken as a political decision to ensure sufficiency of home-produced food. Indeed the programme was so successful that the wheat surplus was exported. In 1992, wheat production was almost 4.1 million tons whilst national demand was only 1.2 million tons. This yield was entirely dependent upon the availability of cheap fossil groundwater. Subsequently the decline in level of the water table has changed the emphasis on water use so that less is used on crops like cereals which are relatively cheap in the international market and more on crops such as vegetables. As a result, subsidies have declined and production of wheat in 2000 had fallen to 1.79 millions tons. Saudi Arabia may be an exceptional case as it has both an abundance of energy in the form of oil and also an extensive fossil groundwater resource. Some countries have one or the other but not both. Libya has followed the path taken by Saudi Arabia to some extent with central pivot sprinkler systems being used for irrigation in the desert. Australia has its Great Artesian Basin where fossil groundwater, up to 2 million years

5 old, used to emerge under pressure and provided a source of water in the drier parts of central Australia (see Chapter 26). It covers an area of 1.7 million km 2 and in 2000 had an estimated total flow of 1500 megalitres per day. Water from shallow parts of the basin emerged at a temperature around 30ºC and nearly at boiling point for deeper water where geothermal heating is significant. Because the margins of Australia are much wetter, crops requiring much rain could be grown in those areas and the artesian water was used for watering stock. Nevertheless the level and pressure of water in the Basin have both declined and care has to be taken with its use. Essay and discussion questions 1 Is rainfall more variable in a dry area than in wetter parts of the globe? 2 Examine the ways in which desert vegetation adapts to heat and dryness. 3 What are the main constraints on development in dryland areas? 4 Water is a key national resource that in dry areas is often in short supply. Using an example where several countries compete for water resources, such as the Jordan, the Euphrates, the Indus or the Colorado, examine the problems that have been caused by their exploitation. 5 How would you organize the resettlement of people who will lose their land through inundation by the water behind a new dam? Does dam construction on rivers cause more problems than it solves? 6 How important are political factors in desertification? Further reading Beaumont, P. (1993) Drylands, Environmental Management and Development, London: Routledge. A full discussion of environmental management in dryland areas. Emphasis on regional examples. D'Odorico, P. and Porporato, A. (eds.) (2006) Dryland Ecohydrology. By combining the analysis of biotic and abiotic components of terrestrial ecosystems, this edited book provides a synthesis of material on arid and semiarid landscapes. An advanced text for more information about the links between water and vegetation. Thomas, D.S.G. and Middleton, N.J. (1994) Desertification: Exploding the myth, Chichester: Wiley. A survey of areas believed to be affected by desertification. Puts forward various interesting ideas about the factors, especially political ones, involved in desertification and their validity. Web resources An extensive website with useful links to research and information about drylands.

6 The website of Savanna Explorer, a northern Australian information resource. Examines in some detail a variety of savanna areas in northern Australia from the viewpoint of climate, vegetation, soils and the local economy. Some good photographs. A website that contains frequently asked questions about the subject of desertification. Useful as a teaching aid. A brief but useful summary about the nature of and problems associated with desertification around the world.