Drought is an ‘acute water shortage’ (Australian Bureau of Meteorology) that results from a marked reduction in precipitation (e.g. a lack of rain, hail, snow) over an extended period of time.

Although it is useful to arrive at a definition of drought in purely meteorological terms it is not a simple matter. We might for example consider drought to have occurred in Alice Springs when annual rainfall falls below say 150 mm, but at Mount Hagen in the wet tropics of New Guinea, just a few weeks without rain might be regarded locally as a drought event. The occurrence and characteristics of drought vary from one region to another and have different impacts on the environment and human activities within each region.

In Australia, especially eastern Australia, drought conditions tend to occur more commonly during El Niño episodes when the Southern Oscillation Index (SOI)  is negative for a sustained period, however droughts are also experienced during La Niñas, though with a reduced expectation. Similarly, we cannot exclude the possibility of floods occurring in eastern Australia just on the basis that the SOI has been consistently negative for a period of time. Droughts are predictable only in probabilistic terms.

Figure 1:  Soil that has dried and formed deep cracks during drought
(Source: Australian Bureau of Meteorology)