droughts

droughts During the 25-year period between 1968 and 1992 there were 446 droughts worldwide, resulting in 1.8 million deaths; a further 1474 million persons were affected or made homeless by drought and famine (global data from the World Disasters Report prepared by the International Federation of the Red Cross and Red Crescent Societies). With the exception of civil strife, which accounted for around 2.5 million deaths, 90 000 injuries, and 139.7 million affected and homeless people, droughts caused more social disruption over this 25-year period than any other natural or non-natural disaster. The geographical distribution of droughts is not uniform: 62.8 per cent of droughts over the same 25-year period occurred in Africa, 18.6 per cent in Asia, 11.8 per cent in America, and 3.4 per cent in each of Europe and Oceania.

Numerous attempts have been made to define droughts on the basis of a range of physical and socio-economic criteria. For example, permanent droughts are characteristic of the arid regions of the world, which make up about a third of the world's land surface. In regions subject to permanent drought, society has adjusted to a lack of rainfall and utilizes a range of alternative strategies in order to provide a reliable water supply. These include the exploitation of deep groundwater (as in Australia, Libya, and Tunisia), the use of desalination (Kuwait, Saudi Arabia, and Iran), abstraction from rivers fed by lakes, rainfall, and snowmelt in areas with more reliable precipitation (the Nile, Tigris, Euphrates, Murrumbidgee), and the development of large-scale water storage reservoirs (Ataturk Dam, Turkey; Aswan High Dam, Egypt; Bhakra Dam, North India). It is interesting to note that the completion of the Ataturk Dam in southern Turkey in 1993 has substantially reduced the quantity of water reaching Iraq through the Tigris and Euphrates river systems; this fact well illustrates the international political dimension to drought amelioration.

Seasonal droughts occur in many areas on the margins of arid regions where there is a strong seasonality in the precipitation regime and in equatorial regions dominated by monsoon climates. Ironically, many of the monsoon regions that experience seasonal drought also experience seasonal flooding.

Contingent drought occurs when lower than average rainfall conditions persist for months or even years in environments where the expectation of rainfall is high and in which an alternative water supply infrastructure has not been developed. Contrary to popular belief, contingent droughts are not limited in extent to the developing world. Examples include the dust-bowl years of the 1930s in the USA, the European drought of 1976, the Sahelian droughts of North Africa in the mid and late 1980s, and the south-east Australian drought of the 1990s.

An agricultural drought, sometimes referred to as an invisible drought, is one in which rainfall appears to be adequate to sustain agricultural production but evaporation rates are higher than normal, causing plant stress and a reduction in crop yield. It is rarely detected as a result of an examination of rainfall statistics alone, because account needs to be taken of the water balance, which determines the relationship between rainfall, run-off, and evaporation (see hydrological cycle).

Physiological drought is a form of drought in which plants suffer from an excess concentration of salt in the soil. It is common in irrigated dry-land areas. Although the soil contains sufficient quantities of water, the high salinity prevents the plants from taking up enough water for proper growth of the crop.

Different criteria are applicable to different parts of the world. This is partly because the expectation of rainfall and the natural variability in rainfall vary in different geographical regions. The spatial pattern in rainfall is mirrored by statistical variability in river runoff, which increases significantly between latitudes of 25° and 45° north and south of the Equator (Fig. 2). In general terms, the greater the rainfall that is expected in a region, the shorter the time interval without rain which is used to define a drought. In the UK, for example, an absolute drought is a period of at least 15 consecutive days during which no more than 0.2 mm of rainfall is recorded in any one 24-hour period. In arid and semi-arid environments, by contrast, a period of several months may elapse before a drought is officially declared.

Droughts are not abnormal phenomena; they are a normal component of contemporary climates in many regions of the world. They often cause severe disruption to agriculture and to human society generally, but they are a perfectly normal aspect of many climatic regions, especially in dry-lands where rainfall is unreliable and sporadic. In all the definitions given above, drought has the same effects: plants die, vegetation is reduced, and soil is either blown away or washed away by subsequent high-intensity rainfall. In most dry areas, natural vegetation is well adapted to drought conditions, and seeds may lay dormant for several years until enough rain falls for germination.

Numerous attempts have been made to explain why droughts occur. In part, a meteorological explanation is appropriate. In middle latitudes, belts of west-travelling cyclones or depressions bring rain to areas of hundreds of square kilometres. In arid areas, the rainfall is caused by convectional instability through heating of the ground surface. Here, areas as small as a few square kilometres may receive rainfall while the surrounding areas remain dry. Since the worlds dry-lands are found in regions where air masses are stable, few rain-bearing systems develop. Research has shown that there is a relationship between rainfall in the Sahel region of Africa and the surface temperature of the tropical parts of the Atlantic Ocean. When surface temperatures are low, droughts are more frequent—perhaps because less evaporation occurs from a cooler body of water. The reason for a cooler sea surface has not, however, been established. The link between climate and the oceans has been shown to be important in other areas. For example, in the Eastern Pacific Ocean a large area of equatorial water is occasionally much warmer than usual in December. This so-called El Niño event appears to be associated with droughts as far apart as Australia and the Sahel.

It is also important that droughts are seen in the context of climate change. They occur repeatedly in historical rainfall records spanning at least a hundred years. Some research workers have been tempted to argue that the persistent droughts in the Sahel in the 1970s and 1980s are indicative of longer-term shifts in the world's climate. Others have argued that these periods of droughts are not abnormal.

In comparison with other types of natural hazard, such as floods, hurricanes, earthquakes, and tsunami waves, droughts are often characterized by their widespread geographical impact, their longevity, their lack of physical impact other than on vegetation, and their dramatic impact on human population displacement. Droughts are often, and incorrectly, directly associated with the problem of desertification. Desertification is the reduction or destruction of the biological potential of the land that can ultimately lead to desert-like conditions. While climate change and drought may exacerbate the problem, many research workers believe that desertification is a human-induced problem caused by overgrazing, the cutting of vegetation for fuel, the burning of animal manure, and the increased pressure of rising human populations on smaller areas of land leading to non-sustainable development.

The most usual long-term defence against drought has been the construction of dams and reservoirs for the artificial storage and transfers of water supplies. This reliance on high-cost technological solutions is demonstrated by the remarkable increase in river regulation, particularly between 1945 and 1970. By the late 1970s, over 40 per cent of the stable river flow in Europe, North America, and Africa was regulated by reservoirs. Although reservoirs often provide a reliable source of water under extreme drought conditions, a number of problems have arisen. First, many of the world's major rivers cross international boundaries. Disputes have consequently arisen over headwater impoundment and abstraction which have been resolved only by international treaty. Secondly, the life expectancies of many reservoirs in the dry-land areas of the world are proving shorter than was originally predicted. For example, the Mohammed V reservoir of north-east Morocco has been silting up at rates approaching 1 m per year since its construction in 1984. A survey of the 21 reservoirs currently operating in Morocco has shown that all have now lost over 20 per cent of their original storage capacity, and some have lost over 70 per cent. Thirdly, in many rural areas, such as the Sahel, which have been suffering severe droughts over the past twenty years, large-scale technological solutions are not available, and 95 per cent of agriculture is rain-fed and not irrigated. This leaves fewer options for mitigating drought than in irrigated areas, where it is possible to ration water and set priorities on its use. Some scientists have argued that one of the greatest social effects of droughts has been caused by the change from traditional to cash-crop agriculture in the Sahel, which has been prompted by a need to earn foreign exchange. This has seen an increase of the area of land under continual cropping, which in many instances has led to declining fertility and the displacement of traditional pastoralists. The pastoralists had developed diverse stocking management practices in keeping camels, cattle, sheep, and goats, all of which have different grazing habits and water requirements. This allowed a range of responses to drought conditions, including free movement to new pastures or the option of selling animals at market.

The greatest single disaster caused by drought is undoubtedly famine. Although severe droughts occur in many developed countries, far fewer deaths are recorded than in less developed countries. Famine appears to be almost endemic in some of the poorer countries of sub-Saharan Africa. Such disasters are multi-causal and often include an element of civil war and unrest. The international response is usually one of aid, the distribution of which is often made difficult by poor communications and civil unrest. Since sub-Saharan Africa has 29 of the 36 World's poorest countries, social scientists have argued that the people of the region are locked into an economic system that obliges them to produce food they do not consume and to purchase goods they do not produce. Some have argued that colonialism and the dominance of ‘Western’ trading systems have reduced the ability of sub-Saharan Africa to cope with variations in their physical and social environments. The worst-hit groups are undoubtedly the poorest members of society—the landless and jobless, the women and children, who cannot ensure the security of their own food supplies.

Ian D. L. Foster

Bibliography

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