What causes drought?
Most simply, drought happens in Australia when there's not enough rain.
Australia is prone to drought partly because of its geography. Our continent spans the latitudes of the subtropical high pressure belt, or subtropical ridge. This is an area of sinking, dry, stable air and usually clear skies.
This means most of the country has low and erratic rainfall. Even in the wetter areas, very dry years can disrupt normal activities and lead to water shortages.
Find out who is responsible for declaring a drought.
Influence of climate drivers
A lot of our rainfall variation, but not all, can be explained by what's happening in the climate system. Australia's climate is affected by many different climate drivers. Some of them influence rainfall.
Two key climate drivers of Australian rainfall are the:
- El Niño–Southern Oscillation, which includes El Niño and La Niña
- Indian Ocean Dipole (IOD).
An El Niño and positive IOD happening at the same time can reinforce each other’s drying effects.
Types of drought
Generally, there are 4 types of drought. This is because it's measured in different ways and across different timescales:
Meteorological drought
Climatologists monitor the extent and severity of drought in terms of rainfall deficiencies (or shortages, compared to average rainfall for the period).
Hydrological drought
Hydrologists examine:
- soil moisture
- river flows
- surface and groundwater levels. Surface water is the water storage levels in dams.
Agricultural drought
Agriculturalists rate the impact on primary industries.
Socioeconomic drought
Sociologists define drought by:
- social expectations and perceptions
- impact on the community.
Breaking a drought
A drought isn't over until people have enough water to meet their needs. This means there's no specific amount of rain that is 'drought breaking'. It depends on the average rainfall and water use in each area.
There may be enough rain to keep a flush of green growth in the landscape, but the underlying soil may be too dry to grow enough pasture. This is sometimes called a 'green drought'. Or there may be sufficient rain at the right time of year to grow a crop, but not enough to replenish water storages.
Sometimes it can take many months of above-average rainfall before enough water is available. If the above-average rainfall happens at the 'wrong' time of year – such as outside growing season for farmers – then it isn't a good indication of whether the drought is broken.