Chapter 2- Water

Define drought, what are the four main types?

Extended period of time when there is a below average rainfall
Meteorological
Hydrological
Agricultural

Describe meteorological drought

Occurs when rainfall deficit, lack of ppt due to changes in atmospheric circulation, often combined with high temps, increasing evaporation

Describe hydrological drought

Lack of water stored on surface and underground in lakes, rivers, reservoirs and aquifers

Describe agricultural drought

Water deficiency in soil leading to crop failure and reduced biomass

What are some of the physical causes of drought?

Precipitation deficits- continued low ppt compared to expected levels will affect water budget
High pressure areas- anticyclones- can lead to weaks w/o ppt
Long term cycles- ENSO

What is the ENSO?

El Niño Southern Oscillation
Movement of warm water mass in Pacific, occurs due to changes in trade winds, atmospheric circulation and ocean currents
2 phases- El Niño and La Niña

What are the causes of El Niño

Trade winds reduced or reversed, causes reversal of conditions and rising air over eastern Pacific and descending air over western Pacific

What are the impacts of El Niño?

Increased rainfall and flooding in South America, Africa and south US

Rising temp of oceans causes thermal expansion and rising sea levels off coast of Peru

Water off coast of Australia + Indonesia is cooler + ppt is reduced, droughts in Australia and Southeast Asia

Linked to higher risk of colder winters in UK

When does El Niño occur?

When sea temps are 0.5 degrees Celsius above average

How often does El Niño occur?

Every 2 to 7 years

What causes La Niña?

Sea temperatures falling below average

How often does La Niña occur?

Every 3 to 5 years

What are the causes of La Niña

Stronger eastward trade winds + ocean currents
Bring cold water to surface, upwelling

What are the impacts of La Niña

Increased rainfall in Australia + Southeast Asia

Drought conditions in south US

Increases risk of tropical storms in Atlantic

Brings cooler and drier than average weather in eastern Pacific

How can human activity increase drought risk?

Over-abstraction of:

• Surface water resources

• Ground water aquifers

• Occurs due to population growth (increased demand for both food and water) and overcultivation

Deforestation- reduces ability of soil to retain water

Overgrazing of sheep and cattle leading to vegetation loss

Climate change as result of increased risk of drought

Summarise the Australia case study- 2006 Millenium droughts

1 in 1000 year drought event

Started with low rainfall following dry years in 2001, 2002 and 2003

2001- 2003- El Niño exacerbated drought conditions

Causes:

• Overabstraction of dams and reservoirs to support high population in major cities eg. Adelaide

• High water consumption of 340 litres per day

• Overgrazing of sheep and cattle- vegetation loss

Consequences:

• Reservoirs reached 40% capacity

• Murray- Darling river system was 54% below record minimum

What are the impacts of drought on forest ecosystems?

Cover approximately 30% of the Earth’s surface, release oxygen, support biodiversity and store carbon

• Drought conditions lead to forest stress, younger trees shed leaves and die reducing tree cover, transpiration is reduced decreasing precipitation and humidity

• Forest becomes less resilient and more trees die, trees more susceptible to disease

• Wildfires more common due to dry conditions and often warmer climate

What are the impacts of drought on wetlands?

• Cover 6% of Earth’s surface

• Reduce air temperatures, provide protection against flooding, high biodiversity- support 40% of world’s species

• Lack of ppt causes vegetation to die which reduces infiltration, percolation, interception and transpiration

• Australia- led to acidification of wetlands

What are the meteorological causes of flooding?

Intense storms- leading to flash flooding, occurs when ppt exceeds infiltration rate increase surface runoff, river capacity quickly exceeded, common in mountainous and semi-arid areas

Prolonged rainfall- can occur during monsoon conditions, leads to saturation of soil, rainfall can’t infiltrate, increased surface runoff

Snowmelt- rapid melt can lead to flooding, water can infiltrate then surface runoff increases

What are some other physical causes of flooding?

Geology of underlying rock- impermeable clay and granite

Volcanic activity releasing meltwater beneath sheets

Ice dams melt releasing glacial lake water

Impervious surfaces in urban areas

What are some of the human causes of increasing flood risk?

Changing land use
Mismanagements of a river channel

How can changing land use increase flood risk

Deforestation- reduces amount of interception + infiltration

Overgrazing- removes vegetation, increasing bare soil, land compacted, reducing infiltration

Ploughing- compacts soil reducing porosity and storage capacity

Drainage ditches- increases overland flow and changes river’s annual regime

Urbanisation- increases impermeable surfaces, drainage systems built for cities speed up removal of water, more reaches rivers at faster rate
Dams and reservoirs built to supply towns with water, less water reaching further down the course

Sewers feed into channels

How can mismanagement of a river channel increase flood risk?

Channelisation- adds concrete liner to reduce friction, improves flow rate reduces silting

Dams, floodwalls and reservoirs- halt flow of sediment and reservoir gradually fills with silt, downstream erosion is increased

Embankments and levees- can improve capacity of river, however if breached, scale of flooding can be more significant

River straightening- increases flow of water by increasing gradient of river and removing natural meanders, increases flow velocity

Floodplains- natural stores of flood water, self regulating, building on these increases risks of flooding and damage to homes and livelihoods

What are the environmental consequences of flooding?

• Loss of crops

• Loss of habitats

• Pollution of waterways

• Soil and bank erosion

• Eutrophication

• Siltation but also soil replenishment

• Recharge groundwater stores

• Can trigger event for breeding, migration and dispersal

• Recharge of wetlands

• Increase connectivity between aquatic habitats

• Move sediment and nutrients around the landscape

What are the socio-economic impacts of drought?

• Loss of life, property and infrastructure

• Road closure and loss of communications

• Long-term rehoming of people

• Cost of house insurance increases

• Loss of income from employment, agriculture or tourism

• Health risks from water contamination

• Trauma and loss of possessions

• Psychological impacts can be long lasting

What are the impacts of climate change on precipitation

Warmer air- holds more water, increase rainfall frequency and intensity, areas most affected- tropics and high latitudes

Snowfall replaced with rain in northern regions, reducing glacier mass

Decreases in precipitation between 10 and 30 north and south of the Equator

What are the impacts of climate change on stores and flows

Ice- higher temps, negative mass balance, shrinking of glaciers, ice caps etc, reduction in glacial water for human use

Oceans- possible increase in evaporation, rise in seal levels, thermal expansion, rise in frequency and intensity of tropical cyclones

Snow- decreased length of snow cover seasons, negative economic impact, reduction to tourism industry

Surface runoff- increased low and high flow events, reduced infiltration with increased surface runoff.

What are the impacts of melting permafrost?

Permafrost melts, releases methane to atmosphere, increases greenhouse effect, positive feedback loop

Changes to ecosystems and tundra reducing stores of water