The definition and characteristics of drought

A drought is a shortage of water over an extended period of time. Droughts can have significant impacts on people's lives, livelihoods, and the natural environment.

Key characteristics of drought

• Slow-onset hazard - Droughts tend to develop gradually, with many years of below-average water levels before an impact is felt. For example, in the UK, a summer hosepipe ban can be enforced following a year's worth of low rainfall.
• Difficult to predict - It can be challenging to forecast when droughts will start and end.
• Variable duration - Drought length varies considerably. The worst drought in Britain since records began lasted for sixteen months, whereas droughts in some African countries can last for more than a decade.

How water stores are affected during drought

Water stores – such as lakes and rivers – become depleted during a drought because they are not replenished by rainfall while people continue to use them. Droughts are often accompanied by high temperatures, which increase evaporation and deplete water supplies further.

Farmland is particularly vulnerable to drought. If soil loses its moisture, farmers must irrigate their land. This can increase the price of food in the long term or cause food shortages.

Meteorological and hydrological causes of drought

Droughts can be understood through two related types, where one often leads to the other.

Meteorological drought

A meteorological drought occurs when there is less precipitation entering a local system than normal.

Hydrological drought

A hydrological drought occurs when there is less water in groundwater stores, streams, and rivers. A meteorological drought can cause a hydrological drought.

Short-term causes of drought

• Precipitation deficit - Short-term, localised droughts are usually caused by lower than normal precipitation levels, often resulting from changes in the frequency of frontal precipitation.
• Anticyclones - Localised droughts can also occur when areas of cool, descending air create a high-pressure weather system known as an anticyclone. This can block the movement of warm, ascending air that would otherwise form rain clouds, leading to reduced rainfall and drought conditions.

Longer-term changes in drought frequency

• Ocean temperature changes - Climate change causes longer-term warming and cooling of oceans, leading to changes in the frequency of droughts. This means the recovery time between droughts may be much shorter.
• Disruption to seasonal rains - Climate change also affects seasonal rains in the tropics, making the rainy season shorter and more unpredictable. This results in the long-term depletion of groundwater and reservoir stores.

The role of the El Niño-Southern Oscillation (ENSO) cycle

The El Niño-Southern Oscillation (ENSO) cycle is a natural change in the patterns of ocean temperatures in the Pacific Ocean.

Normal conditions

Under normal conditions, there is an area of low pressure over the western Pacific and high pressure over the east. This causes the trade winds to blow from east to west.

El Niño events

Key features of El Niño:

• Pressure increases in the western Pacific and decreases in the east
• Trade winds weaken in strength or reverse direction, blowing from west to east
• The sinking of air in the high-pressure areas over the western Pacific leads to drought conditions in the west (e.g. Australia), as less precipitation falls
• This increases the risk of wildfires
• El Niño events occur every 2 to 7 years and each event typically lasts for 9 to 12 months

La Niña events

A La Niña event is when the normal conditions become more extreme.

Key features of La Niña:

• Trade winds increase in strength, causing more cold water to rise in the eastern Pacific
• This results in drier conditions in the east and wetter conditions in the west
• La Niña events occur every 2 to 7 years

How human activity increases drought risk

Human activity can make low rainfall more of an issue and intensify a hydrological drought.

Over-abstraction from aquifers

An aquifer is a saturated layer of permeable rock below the water table. If the demand for water is greater than the supply from precipitation, surface water resources such as lakes and reservoirs dry up. Humans often then abstract water from aquifers. Abstracting water from an aquifer at a faster rate than it recharges is called over-abstraction, and this depletes underground water stores.

Diversion of water from natural systems

Humans also divert water out of natural systems into storage within homes and businesses. This water is not subjected to natural processes such as evaporation, reducing the supply of water to other stores in the hydrological cycle.

The Millennium Drought - Australia

Southeastern Australia experienced a severe, long-term meteorological drought from roughly 2001 to 2009. The worst-hit area was the Murray-Darling Basin, an important agricultural region responsible for 40% of Australia's agricultural produce.

Causes of the Millennium Drought

Several factors contributed to the severity and duration of this drought.

Natural causes:

• Australia has naturally low rainfall due to the 30°S high-pressure belt passing through it, which causes cooler air to sink and low levels of precipitation
• In many of the drought years, El Niño events caused weather fronts that normally bring rain to southeastern Australia to move further south, increasing drought conditions in the region
• Temperatures were higher than normal, resulting in more evaporation than usual — the increased temperatures were possibly due to climate change

Human causes:

• Years of over-abstraction for farming, industry, and domestic use depleted groundwater levels and made the area more susceptible to drought

Impacts of the Millennium Drought

Water levels in lakes and rivers — particularly the Murray and Darling — fell, so water supplies ran low. This had a large impact on farming and livelihoods.

Key impacts on agriculture:

• Crop yields - Crop yields fell and food prices increased, particularly for crops that relied on irrigation such as rice. Rice yields fell to just 2% of pre-drought levels.
• Livestock losses - The number of sheep in Australia fell by around 7 million between 2002 and 2003 due to dehydration and starvation through lack of pasture.
• Employment - Farmers' incomes fell, and over 100,000 people employed in agriculture lost their jobs.

The effects of drought on ecosystems

The capacity for a natural area to survive and recover from a disturbance such as a drought is called its ecological resilience.

Impacts on wetlands

Wetland areas are ecosystems that are very sensitive to droughts and have limited resilience. They serve as both a water store for humans and a habitat for birds, fish, and other aquatic organisms.

How drought affects wetlands:

• Water purification loss - Wetlands act as water purifiers, as reeds and rushes trap pollutants. In drought conditions, wetland areas can dry up, causing vegetation to die back.
• Habitat loss - Fewer nesting spaces are available for wading birds as vegetation declines
• Algal concentration - Algae becomes more concentrated as water levels drop, reducing oxygen levels for aquatic life
• Migration - Animals that can migrate, such as birds, will leave the area in search of deeper water

Impacts on forests

Forests are also placed under stress in drought conditions.

How drought affects forests:

• Reduced growth - Even in a mild drought, tree growth slows and young trees are vulnerable to die back
• Land exposure - As trees die, land is exposed to warmer temperatures and wind erosion
• Slow recovery - Trees that survive can take a long time to resume their normal growth rate
• Stress on organisms - Organisms that rely on eating leaves for water will also be under stress