Understanding drought

Drought is a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use.

Drought is not simply low rainfall; if it was, much of inland Australia would be in almost perpetual drought.

To understand when droughts are occurring it's necessary to know how much water is around and how that compares to normal conditions.

Because people use water in so many different ways, there is no universal definition of drought. It's measured in different ways and at different timescales:

  • Meteorologists monitor the extent and severity of drought in terms of rainfall deficiencies (or shortages, compared to average rainfall for the period).
  • Agriculturalists rate the impact on primary industries.
  • Hydrologists examine surface and groundwater levels.
  • Sociologists define it by social expectations and perceptions and the impact on the community.

It is generally difficult to compare one drought to another, since each drought differs in the seasonality, location, geographic extent and duration of the associated rainfall deficiencies. Additionally, each drought is accompanied by varying temperatures, soil moisture and water availability. Monitoring the state of these elements and their impacts, and understanding how they compare to normal and historical conditions, helps us know more about the bigger drought picture.

Rainfall averages, variability and trends

An area experiences a rainfall deficit when the total rain received is less than the average rainfall for that period.

Definitions
Serious deficiency
– rainfalls in the lowest 5–10% of historical totals.
Severe deficiency
– rainfalls in the lowest 5% of historical totals.
Lowest on record
– lowest since at least 1900 when the data analysed begin.
Drought Legend
Very much below average
– rainfalls in the lowest 10% of historical totals.
Below average
– rainfalls in the lowest 10–30% of historical totals.
Average
– rainfalls in the middle 40% of historical totals.
Above average
– rainfalls in the highest 10–30% of historical totals.
Very much above average
– rainfalls in the highest 10% of historical totals.