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Droughts occur when there is abnormally low rainfall for an extended period of time. This means that a desert would not be considered in drought unless it had less rainfall than normal, for a long period of time. Droughts can last from weeks to months and even years.

Why are some areas more vulnerable to droughts?

Droughts can occur all over the world, however there is a link between drought and some climate patterns:

• A lack of water vapour in the atmosphere means there is less precipitation and more chance of drought. High-pressure systems reduce evaporation and moisture in the atmosphere.
• El Niño - as the surface temperature of the Pacific Ocean around the central South American coast increases, storm patterns are disrupted. This phenomenon is thought to create droughts in Indonesia and Australia.
• La Niña - as the surface temperature of the Pacific Ocean around the central South American coast decreases, storms are again disrupted and North and South America are prone to droughts.

Causes of droughts

There are three main types of drought:

• Meteorological drought - when the amount of precipitation received in a specific area is less than the average.
• Hydrological drought - when reduced precipitation impacts on water supply, eg there is decreased streamflow, soil moisture, reservoir and lake levels, and groundwater.
• Agricultural drought - when the above two types of drought impact on agricultural activities, eg reduced soil moisture or reservoir levels required for irrigation.

Droughts can also be caused by human activities, for example:

• Agriculture - using large amounts of water to irrigate crops removes water from lakes, rivers and groundwater. Some crops require more water than others, eg cotton.
• Dam building - large dams can be built across a river to produce electricity and store water in a reservoir. This can reduce river water flowing downstream and cause drought below the dam.
• Deforestation - removing trees can reduce the amount of water stored in the soil as rain tends to fall and wash off the land as surface run-off. This leaves the ground vulnerable to erosion and desertification which can lead to drought.
• Climate change - it is predicted that climate change will cause some places around the world to get hotter, which will increase evaporation and some areas to receive less rainfall - both increasing the risk of drought.