4.2 Factors in nature affect the water cycle

4.2

Factors in nature affect the water cycle

In this topic, you will learn that:

• the water cycle can be affected by the movement of water vapour in the air • changes in temperature, the direction of the wind and the number of plants can change the amount of water vapour.

Figure 1 Areas in Australia affected by a drought and b flood

The effect of El Niño on Australia

You may have heard of the El Niño effect. Fisher people in Peru, South America, originally used this term to describe the appearance, around Christmas, of a warm ocean current off the coast that brought heavy rain and fl oods. Nowadays, ‘El Niño’ describes the extensive warming of the central and eastern Pacifi c Ocean. Combined with this ocean warming are changes in the atmosphere that affect weather patterns across countries around the Pacifi c Ocean, including Australia. In Australia, El Niño events usually mean less rain will fall.

El Niño events occur approximately every 4–7 years and usually last for about 12–18 months. They are a natural part of the climate system and have been affecting the Pacifi c countries for thousands of years. Each El Niño event is unique in terms of how much the ocean temperature warms and how rainfall patterns change. El Niño events usually result in less rain across parts of eastern and northern Australia, particularly during winter, spring and early summer. Where and when this happens differs a lot from one event to another, even with similar changes and patterns in the Pacifi c Ocean. In 2015–16, a relatively weak El Niño event resulted in severe drought.

The effect of La Niña on Australia

More recently, in 2020–21, a La Niña event caused widespread fl ooding. The opposite of El Niño, La Niña occurs when the ocean current

Figure 2 During an El Niño event, there is an increased chance of drier conditions in Australia because of the combined effects of ocean warming and changes in the atmosphere that affect weather patterns.

Equator Increased convection

Trade winds

Australia

Less rain

˜El Nino

Warmer than normal Cooler than normal South America

Increased convection

Equator

Australia

Increased rain Strong trade winds

La Nina ˜

Warmer than normal Warmer than normal Cooler than normal Cooler than normal South America

is cooler and the winds are stronger. This results in greater-than-usual amounts of rain falling along the east coast of Australia.

Bushfires

The evaporation of water from plants (transpiration) contributes directly to the amount of water vapour in the air. When plants are burnt in a bushfi re, it can prevent transpiration and decreases the amount of water vapour in the air. Any rain that does fall will wash the soot of dust into water storage areas, contaminating the water supply needed by animals and people. This decreases the amount of drinkable water available.

Volcanic eruptions

Volcanoes erupt regularly around the world every year. Occasionally there is a major eruption, like the 2011 Nabro eruption in Eritrea, Africa, that discharged large amounts of dust and particles into the air. The particles refl ected the sunlight, preventing the Earth (and water) being heated. Less water evaporated, resulting in less water vapour condensing into precipitation (rain). Consequently, some countries experienced a drier climate in the year after the eruption.

Figure 3 A La Niña event results in an increased chance of wetter conditions along the east coast of Australia because of ocean cooling and stronger winds.

volcano a vent or hole in the Earth through which molten (melted) rock, ash and other materials escape to the surface

Figure 4 Volcanoes release large amounts of ash into the air when they erupt.

4.2 Check your learning

Remember and understand

1 Describe how El Niño affects Australia.

2 Describe how La Niña affects Australia.

Apply and analyse

3 Explain why Australians do not need to be concerned about erupting volcanoes in Australia.

Evaluate and create

4 Research where and when the last big volcanic eruption occurred in the world. 5 Draw a fl ow chart of how volcanic dust particles blocking sunlight affect the water cycle. Include all parts of the water cycle in your fl ow chart. 6 In 2019 bushfi res burnt large parts of the Amazon rainforest. Explain how this could affect rainfall across other areas of South America.