Droughts have been a part of our environment since the beginning of recorded history, and humanity’s survival may be testimony only to its capacity to endure this climatic phenomenon.
Drought is considered by many to be the most complex but least understood of all natural hazards, affecting more people than any other hazard (Hagman 1984).
A. Drought: Definitions and Types
Drought is the consequence of a natural reduction in the amount of precipitation over extended period of time, usually a season or more in length, often associated with other climatic factors (such as high temperatures, high winds and low relative humidity) that can aggravate the severity of the event, (Sivakumar, 2005). It is a normal event that takes place in almost every climate on Earth, even the rainy ones. Drought manifestation varies from region to region and therefore a global definition is a difficult task e.g. Drought is a recurring extreme climate event over land characterized by below-normal precipitation over a period of months to years. Drought is a temporary dry period, in contrast to the permanent aridity in arid areas. Drought occurs over most parts of the world, even in wet and humid regions. This is because drought is defined as a dry spell relative to its local normal condition. On the other hand, arid areas are prone to drought because their rainfall amount critically depends on a few rainfall events, (Sun et al 2006). One might define drought in Libya as occurring when annual rainfall is less than 180 mm, if less than 2.5 mm of rainfall in 48 hours in USA, about 15 consecutive days with daily precipitation totals of less than 25 mm in GB, actual seasonal rainfall deficient by more than twice the mean deviation in India, but in Indonesia, Bali drought might be considered to occur after a period of only 6 days without rain, (Ragab, 2005).
Generally there are three types of conditions that are referred to as drought, Meteorological drought is brought about when there is a prolonged period with below average precipitation; Agricultural drought is brought about when there is insufficient moisture for average crop or range production, this condition can arise, even in times of average precipitation, due to soil conditions or agricultural techniques; and Hydrologic drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs falls below the statistical average, this condition can arise even in times of average (or above average) precipitation, when increased usage of water diminishes the reserves,American Meteorological Society (AMS 1997). (Wilhite, 2000 and Sivakumar, 2005). A lack of precipitation often triggers agricultural and hydrological droughts, but other factors, including more intense but less frequent precipitation, poor water management, and erosion, can also cause or enhance these droughts. For example, overgrazing led to elevated erosion and dust storms that amplified the Dust Bowl drought of the 1930s over the Great Plains in North America, (Cook et al 2009).
B. Drought: Negative Affects
Drought is the most complex of all natural hazards as it affects more people than any other hazard, According to EM/DAT data quoted in the World Disaster Report (2007), about 2.63 million people were affected by Hydro-metrological disasters globally during the period (1997-2006), about 41.82% are affected by drought, 38.87% of them were affected during the year 2002 During 1997/2006, hydro-metrological disasters caused an estimated damage of US$ 66.8 billion per year on average out of this 4.62% caused by drought. Average number of people reported killed by drought in million per year are, Asia (81.11), Africa (26.69), Americas (2.57), Europe (0.14). The effects of droughts are seriously worsened by human factors such as population growth that forces people into drier and drier regions and inappropriate cropping and herding practices. The impacts of drought are likely to become ever more severe as a result of development processes and population increases (Squires 2001).Droughts often stimulate sequences of actions and reactions leading to long-term land degradation, figure (1), after ( Erian 2010) .
The most prolonged and widespread droughts occurred in 1973 and 1984, when almost all the African countries were affected, and in 1992 all southern African countries experienced extreme food shortages. Rainfall has declined by up to 30% in the last 40 years and the Sahara is currently advancing at over a mile per year. The potential for conflict over disappearing pasture and evaporating water holes is huge. The southern Nuba tribe have warned they could restart the half-century war between North and South Sudan because Arab nomads (pushed into their territory by drought) are cutting down trees to feed their camels. Agricultural economist Marshall Burke of the University of California, Berkeley and his colleagues have analyzed the history of conflict in sub-Saharan Africa between 1980 and 2002 in a new paper in Proceedings of the National Academy of Sciences. “We find that civil wars were much more likely to happen in warmer-than-average years, with one degree Celsius warmer temperatures in a given year associated with a 50 percent higher likelihood of conflict in that year,” Burke says. The implication: because average temperatures may warm by at least one degree C by 2030, “climate change could increase the incidences of African civil war by 55 percent by 2030, and this could result in about 390,000 additional battle deaths if future wars are as deadly as recent wars.”