Drought, the "creeping disaster"

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As a general phenomenon to cover most situations, drought is difficult to define, though basically a drought is due to a severe, abnormal deficit in the rainfall of a region. Four major types of drought are recognised: meteorological drought, agricultural drought, hydrological drought and socio-economic drought. This report is mainly concerned with hydrological drought, which is manifested in both the availability of surface water and the levels of groundwater. There are numerous indices for drought detection and severity, and those applicable to hydrological drought are briefly examined.

Australia is a drought-prone continent with most of its drought generated by El Niño events. Since reliable records began in 1860 up till now, Australia has had major droughts for 78 of the 146 years. In spite of this, Australia as a nation has not been a leader in drought research, except for research on the climatic and meteorological components of drought.

Knowledge on the extent and severity of past droughts has come from dendrochronology and the examination of lake sediments, notably in North America. Of particular interest is the discovery of mega-droughts (droughts that last longer than one to two decades) in the
Mediaeval Climatic Anomaly (~1150-1400 AD) and the ‘Little Ice Age’ (~1500-1700 AD).

Current knowledge on droughts has been generated with three types of studies. By far the most common type of study—the serendipitous one—occurs when drought intervenes into a study not specifically directed at drought. In this category many of the studies may lack predrought data and recovery from drought may not be followed. In the second and uncommon approach, the studies usually follow the onset and the recovery from drought even though the overall aims of the studies have been long-term ones not focused on drought. Finally and rarely, there are a few experimental field studies of the effects of drought.