What you need to know about droughts (2024)

Drought. Most Australians will be familiar with the concept, some very familiar of late.

In southern Australia, droughts of the late 20th and early 21st centuries have been found to be the worst in the past 400 years, and droughts are expected to become more prevalent in the future.

Types of drought

Not all droughts are created equal. There are four main interconnected types.

1. 1.Meteorological
   When there is less rain than usual. Or in science speak, when there is a significant negative deviation from mean precipitation.
2. 2.

   Agricultural
   An agricultural drought occurs when the soil moisture decreases because of evapotranspiration (drying because of the sun) faster than it is replenished by rain.

   This leads to a soil moisture deficit or a situation where there is not enough water in the soil for plants to grow. This is common in summer in southern Australia, where the dry period coincides with the hottest part of the year.

   The problem is when soil moisture deficit creeps into the growing season or extends for so long that there is no feed for livestock.

3. 3.

   Hydrological
   This is when stream flow decreases to the point that it threatens water resources. In an Australian setting, we are usually talking about dams getting low or running dry.

   This usually starts to be felt after meteorological and agricultural droughts have set in. Brisbane's major dam, Wivenhoe, got down to 15 per cent capacity in August 2007, at the end of the millennium drought.

4. 4.Socio-economic
   When there is not enough water to meet demand. Usually, development decreases the chances of socio-economic droughts because of increased water storage capacity and improved connectivity of water supply systems.

A new term has cropped up recently — the flash-drought. The term appears to have mainly been used in the United States so far, and refers to a drought where rainfall drops dramatically or the temperatures increase rapidly to dry out soils quickly.

The speed with which the impacts are felt is the flash drought's defining feature.

It can be difficult to define exactly when a drought starts — rain could be just around the corner, after all. It can also be difficult to define exactly when a drought breaks.

A passing shower might make things look better temporarily, but it will not put a real dent in the damage done.

For a drought to truly be over, a return to normal rainfall patterns and amounts is needed.

Why do we get droughts?

Australia is the second-driest continent in the world, with mean annual rainfall less than 600mm for more than 80 per cent of Australia.

Australia is so dry because we sit under the subtropical high-pressure belt, which encourages the air to push down, preventing the lift required for rain.

Being under the high-pressure belt is enough to make it dry, but there are other systems called climate drivers that can make things worse from one year to the next.

El Nino has become a bit of a household name after it got a lot of press during the millennium drought.

El Nino is a phase of the El Nino Southern Oscillation (ENSO), the phenomena that drives the dominant flows of air over the Pacific Ocean.

It is commonly associated with drought in the east of Australia because it leads to more air being pushed down, which again prevents rain.

Its sister, La Nina, is generally the opposite, although it is all a bit messy when you get into the details, and every event is different.

The Indian Ocean Dipole (IOD) is the other well-known major player.

Like the ENSO in the Pacific Ocean, the dipole is the driver behind the mass movement of air over the Indian Ocean. In this case a positive IOD is associated with drought in Australia.

One that is less well-known is the Southern Annular Mode (SAM.) It works to the south and is involved with the extent to which rain-bearing cold fronts make it onto the continent.

A positive SAM in winter is associated with winter drought in the south of Australia.

In the north the big rain changer is the monsoon.

Beyond the major climate drivers, there are also local conditions that affect the weather.

As the major climate drivers are currently neutral in the latest outlook, the bureau has attributed the current dry to cooler ocean temperatures to the north bringing less tropical moisture, and weak westerlies to the south.

Droughts around the world

Australia may be dry, but we are not the only place to get droughts.

On a global scale, we are largely protected from socio-economic droughts, but many others are not.

The International Disaster Database estimates that between 1900 and 2013, nearly 12 million people died because of drought.

Asia accounts for nearly 10 million all on its own.

When it comes to drought, it is rarely the physical disaster alone that costs lives.

The prolonged and pervasive nature of droughts mean they can often lead to complex emergencies, a term for when the natural hazard is only one element of a much wider disaster.

For example, the 1991–1992 African drought had a much higher economic impact on Malawi and Zimbabwe than Botswana.

Botswana's low drought-related death toll has been attributed to its small population, government commitment, and effective food security and monitoring system.

Another complex drought emergency is Lake Chad, which is in central northern Africa on the border of Nigeria, Niger, Chad and Cameroon. It once supplied water for 8.3 million people.

The lake reduced in size by 95 per cent from 1963 to 2000. This rapid reduction has been blamed on reduced rainfall as well as irrigation.

The emergency has been made worse because of Boko Haram, which has been plundering vulnerable villages in the region, murdering civilians and causing a wave of refugees.

Australian droughts on the rise

The Bureau of Meteorology and CSIRO's 2016 State of the Climate report said Australia would need to adapt to increases in the frequency or intensity of heat events, fire weather and drought.

There has already been a significant drying observed across southern Australia, particularly during the April to October growing season.

The report says winter and spring rainfall especially are projected to decrease across southern continental Australia, resulting in more time spent in drought.