5.4 deficits & drought Flashcards
what is drought?
a temporary deficit of water over an extended period of time in a given area
(the 2)
types of drought
- meteorological drought= lack of PRECIPITATION and the duration of the dry period. this is very variable so is only measured on a regional scale.
- hydrological drought= lack of STORES, indicators are river and reservoir levels, not as visible as meteorological drought: lesser amounts of water passing through a river catchment area will create a lag affect!
(the 4)
causes of rainfall deficiency (thus meteorological drought)
- natural variation in the atmosphere conditions
- El Nino events
- changing land use
- climate change creating regional/ local effects
4a) causes of hydrological drought
on a river basin scale: impact of changing land use- de=forestation contributes to a drier climate as evapotranspiration decreases. EG Amazon basin particularly the E. coast where 95% of Atlantic rainforest has been cleared. On a massive scale, deforestation feeds into meteorological drought which then affects regions outside the area of changed land use
ENSO
EL Nino Southern Oscillation: occurs as a result of changes in the surface temp. of the pacific ocean between Australasia and S. America. disrupts east-west trade winds causing them to slow/ reverse. this has a knock on effect on the oceans currents.
cold water normally along west coast of peru is replaced by warmer water. similtaneously, warm water normally along australia is replaced by colder water… drought occur in Australia and floods in S. America.
El Nino events occur every 3-7 yrs and last from 12-18 moths. triggers very dry conditions throughout the world esp in 2nd yr eg failing monsoon rain in India and SE Asia
La Nina can sometimes follow El Nino: build up of cooler than usual water in tropical part if Pacific, can lead to severe drought conditions in western coast of S. America.
(the 3)
physical causes of drought
- ENSO
- monsoons: when they are blocked by high pressure or unusually cold oceans: India heavily relies on monsoons: 70% of its rain falls in 4 months of the year!! (1/5 of this fails and causes drought)
- anti-cyclone: a high pressure event that causes dry conditons (only MULTIPLE can cause drought!)
(the 5)
human contributions to the risk of drought
- over- cultivating (when crops are grown on farmland too many times it reduces the soils carrying capacity of water)
- de- forestation (trees are a feeder of evotranspiration- less of them= less moisture in the air, Amazon drought 2015)
- damning (Colorado river is blocked by several dams causing bottom to run dry)
- over- grazing (reduces capacity of soil and its quality)
- over- abstraction (of reservoirs, aquifers and rivers for domestic use in cities, industries and irrigation) POPN GROWTH WORSENS THIS: more pressure on the land to grow more food
drought C/S: Sahel region, Africa
what caused it?
rapid popn growth led to environmental decline: local people overgrazed and cultivated marginal agricultural land,cutting wood for fuel. civil wars have worsened this with crops, livestock and homes being deliberatley destroyed
effects?
N. Sahel rehion is experiencing a 30-40% annual departure from normal rainfall.
drought C/S: Murray- Darling catchment, Australia (causes and effects)
causes?
— increased RISK becuase of sub tropic high pressure climate belt of the southern hemisphere!
— over abstraction of the Murray River: for agriculture and domestic use.
— climate change: worsening the effects of El Nino making it last longer with the consequencial risk of drought increasing, also since 70s there has been a shift in rainfall patterns the east where most people live is becoming drier (more extreme weather)
effects?
— 40% of Adelaldes drinking water was from the River Murray: drinking water shortages
river has been so over abstracted it no longer flows at the mouth
— Big Dry of 2006:
- Since 2001, the government has provided 4.5bn Australian dollars ($4.7bn, £2.9bn) in EC assistance.
- RAINFALL levels during this period were nationally 30-40% LOWER than average
impacts of drought on wetlands
wetlands: currently cover 10% of earths land surface and perform important functions, temp water stores, recharging of aquifers, trapping pollutants and providing habitats for animals. with DROUGHT: organic soils may oxidise releasing carbon into the atmosphere and they may also leak further into the drainage basin contaminating water supplies, soil moisture reduced due to lack of precipitation which leads to soil erosion and the reduced ability to store water, soil quality is poor meaning ability to infilrate and percolate water is reduced this also will reduce water tables and groundwater stores and increase risk of flooding. EG urban development, industry and agricultural pressures destroted more than half of the Everglade wetlands in Florida
impacts of drought on forests
forests: currently cover over 30% of worlds land surface and are responsible for much interception which means reduced surface run off. Transpiration that results from trees is responsible for much of the worlds air moisture which condenses and then falls as rain !!!!!20 billion tonnes of water vapour are carried daily from the forest and is dumped as rain on central and southern brazil!!!!!. with DROUGHT: trees and plants suffer, beginning to die as a result of lack of water nutrients (as bacteria and funghi die) primary consumers lose their habitat and have a depleted foof source: as dying trees cannot produce nuts and seeds) long term drought= shorter trees and thinner canopies consequentially, secondary consumers have reduced food too… death of trees could result in increased carbon emissions (45% of worlds carbon is stored in forests).
ENSO (short definition for exams)
change in atmospheric circulation caused by migrating warm waters in the Pacific which leads to floods and droughts in different areas of the world
