Deficits in the Hydrological Cycle (Edexcel A Level Geography)

Causes of Droughts

• Droughts are an extended period of time when there is below average rainfall

• There are three types of droughts:

  • Meteorological 

  • Hydrological

  • Agricultural 

• Meteorological drought occurs when there is rainfall deficit and the length of the dry period, it is caused by:

  • Changes in atmospheric circulation leading to:

    • A lack of precipitation as a result of short-term changes or longer-term trends

    • The lack of rainfall is often combined with high temperatures which increases evaporation

• Agricultural drought is when there is a water deficiency in the soil, which leads to:

  • Crop failure

  • Reduced biomass

• Hydrological drought occurs when there is a lack of water stored on the surface and underground in lakes, rivers, reservoirs and aquifers

El Niño Southern Oscillation (ENSO)

• The ENSO cycle is the movement of a warm water mass in the equatorial Pacific Ocean

• It occurs due to the changes in the trade winds, atmospheric circulation and ocean currents

• There are two phases:

  • El Niño

  • La Niña

• El Niño phases occur more frequently than La Niña 

El Niño

• An El Niño occurs when the sea temperatures are 0.5°C above average

• This leads to warmer than average weather in the eastern Pacific

• The peak is usually in December

• It occurs every 2-7 years

• The temperature of the ocean off the coast of Peru rises an average of 6-8°C, causing thermal expansion and sea level rise

• Water off the coast of Australia and Indonesia is cooler and precipitation is reduced leading to droughts in Australia

Causes of El Niño 

• In a non-El Niño year there is descending air over the eastern Pacific and rising warm moist air over the western Pacific 

• In an El Niño year the trade winds are reduced or reverse (going west to east) leading to a reversal of the conditions and rising air over the eastern Pacific and descending air over the western Pacific

El Niño

La Niña 

• La Niña occurs when sea temperatures fall below average

• This brings cooler and drier than average weather in the eastern Pacific

• La Niña occurs every 3-5 years

Causes of La Niña 

• Stronger than usual eastward trade winds and ocean currents

• These bring cold water to the surface through upwelling

• Both El Niño and La Niña impact the global climate due to the changes they cause in the jet stream

• El Niño has the most significant impacts with:

  • Increased rainfall and flooding in South America, Africa and the south of the US

  • Drought in Australia and Southeast Asia

  • Has been linked to a higher risk of colder winters in the UK

• La Niña leads to:

  • Increased rainfall in Australia and Southeast Asia 

  • Drought conditions in the south of the US

  • Increases the risk of tropical storms in the Atlantic

Human Activity & Drought

• Human activity alone does not cause drought 

• The activities enhance the climate conditions increasing the risk of drought

• Human activity increases the risk of drought in a number of ways, the main way is over-abstraction of:

  • Surface water resources

  • Ground water aquifers

• Deforestation also increases the risk of drought because it reduces the ability of soil to retain water

• Over-abstraction occurs due to:

  • Population growth - this increases the demand for both food and water

  • Overcultivation

Australia droughts

• In Australia droughts are a regular occurrence 

• In 2006 the country experienced a 1 in 1000 year drought event - also known as the Millennium drought

Causes

• It started with low rainfall through 1996/7 followed by dry years in 2001, 2002 and 2003 

• Between 2001 and 2003 El Niño exacerbated the drought conditions

Human activity

• Human activity in Australia increased the risk of drought through:

  • Climate change as a result of increased greenhouse gas emissions

  • Widespread deforestation 

  • Overgrazing of sheep and cattle has led to vegetation loss

  • Over-abstraction by farmers - until recently farmers could take as much water as they wanted

  • High water consumption - 340 litres per person per day

Effects 

• The Murray-darling river system was 54% below the record minimum 

• Crop yield was reduced to 10 million tonnes from the 25 million tonnes average due to less irrigation

• Reservoirs were at 40% of their capacity

• Drinking water shortages were experienced in cities such as Adelaide

• It was ended by the occurrence of a La Niña phase in 2010 which led to floods 

Impacts of Droughts on Ecosystems

• Drought has significant impacts on ecosystems

• The resilience of ecosystems varies

• Drought reduces the amount of water available for plants and animals

Forest ecosystems

• Forests cover approximately 30% of the Earth's surface

• They provide vital goods and services including:

  • The release of oxygen

  • High levels of biodiversity 

  • Storage of carbon

• Trees can draw up to 68-680 litres of water a day

• On a hot day trees transpire between 500 - 1000 litres

• Drought conditions leads to forest stress 

  • Younger trees die and this reduces the tree cover

  • Transpiration is reduced and this decreases precipitation and humdity

  • This leads more trees to die 

• The forest becomes less resilient

  • Trees become more susceptible to pests and diseases

  • Wildfire is more common as dry vegetation and litter covers the forest floor

Wetlands

• Covering about 6% of the Earth's surface wetlands:

  • They provide protection from flooding

  • Reduce air temperatures 

  • They improve water quality and have high biodiversity (40% of the world's species)

• Although wetlands rely on occasional droughts to expose sediments and release nutrients too much drought is damaging

• Human activity is intensifying and increasing the frequency of drought

• Wetlands do not have sufficient time to recover

• The lack of precipitation causes vegetation to die, which reduces:

  • Infiltration

  • Percolation

  • Interception

  • Transpiration which in turn reduces precipitation

• In Australia the droughts led to acidification of the wetlands soils and water

• The drying out of wetlands soils may contribute to climate change due to increased rate of decomposition which release carbon dioxide