water cycle- deficits and surpluses

Types of droughts

meteorological- when long-term precipitation is lower than normal

hydrological- when the amount of the surface and subsurface water is deficient

agricultural- when there is not enough soil moisture to allow enough crops to grow

socio-economic- when water demand exceeds water availability, leading to food shortages, famine and starvation

El Niño southern oscillation (ENSO)

This is an irregular, periodic variation in winds and sea temperatures across the Pacific Ocean.

It is driven by trade winds caused by the walker cell ( circulation of air where upper atmospheric air moves eastwards and surface air moves westwards)

normally there is warm water in the west and cool water in the east

EL NIÑO year

these conditions are reversed- west is cool, east is warm

low pressure forms in the East and warm, moist air rises causing heavy rainfall in South America

the air circulates west in the upper atmosphere, creating drier conditions leading to drought in the West

LA NIÑA years

these conditions are intensified- the west is warmer, east is cooler

lower pressure over the west causing higher rainfall in SE Asia

high pressure over there East causing droughts in South America

the Southern Oscillation Index (SOI) measures the change in air pressure between normal years and El Niño years

EL NIÑO 2015/2016

Australia: droughts, heatwaves, wildfires (in Victoria 116 homes destroyed)

India: monsoon season delayed droughts, rainfall 14% below average, reservoirs down by 30%, crop fields dried up

Paraguay: heavy rainfall, flash floods, 100,000 evacuated , agricultural land submerged, aid was slow so families had to use savings instead

Ethiopia: severe droughts, 80% crops failed, 22 million affected, followed by heavy rainfall, increased risk of insect-borne disease

Human influences on Drought

deforestation- less vegetation leads to increased soil erosion and less evapotranspiration

over abstraction: groundwater stores reduced

overgrazing: soil depleted of nutrients, vegetation dies, increased soil erosion

over cultivation: reduced soil fertility, soil exposed, increased soil erosion

climate change: higher global temperatures, increased evaporation, reduced condensation

changes in land use: more agricultural land means more ploughing, increases soil erosion

Damns & reservoirs: reduced channel flow downstream

impacts of droughts

Wetlands:

areas of Mars that cover 10% of the earths land surface. very important ecologically

have been dredged and drained to create space for farmland

pantanal wetland: central south becoming more frequent, 2005 drought, America drought led to high tree caused loss of 35 inches of tree cover mortality and reduced habitats

causes of surpluses

physical factors:

• intense precipitation over a short period of time (flash flooding)

• sudden snow melt

• unusually heavy or prolonged rainfall (monsoonal rainfall)

• steep slopes: less water is absorbed so higher runoff

• impermeable rocks: less infiltration so more runoff

• size and shape of drainage basin: a narrower basin will funnel the water and increase channel flow

causes of surpluses

Human factors: exacerbate flood risk

• mismanagement: eg channelisation of rivers can restrict flows and increase flood risk

• urbanisation: impermeable surfaces, deforestation, drains all increase surface runoff and discharge

• changing land use: agricultural land eg ploughing, draining of wetlands

impact of climate change

• increases precipitation in some regions, leading to higher flood risk

• increased risk of tropical storms as sea levels and temperatures rise

• reduced precipitation in other areas , leading to more droughts

• increases in temperatures can lead to higher evaporation eg from reservoirs, which can have an impact on water supplies

• reductions in temptress can lead to less evaporation

• if temperatures are below freezing, then more water may be store in the cryosphere.