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.
Changes in rainfall
A lot of our rainfall variation, but not all, can be explained by what's happening in the global climate system. Many different factors affect Australia's climate patterns.
There are 2 key climate factors that can affect Australian rainfall variability for periods of a month or more:
- El Niño–Southern Oscillation, which includes El Niño and La Niña
- Indian Ocean Dipole (IOD).
Sometimes these work together, boosting their effect. An El Niño and positive IOD happening at the same time can reinforce each other’s drying effects.
Australia is prone to drought. Even in the wetter areas, very dry years can disrupt normal activities and lead to water shortages.
Types of drought
Generally, we refer to 4 types of drought. Their impacts are 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
- water storage levels in dams
- groundwater levels.
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.
Sometimes it can take many months of above-average rainfall before enough water is available. 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.