Lecture 5: Drought Flashcards
What is drought?
A drought occurs when there is not enough rainfall to support human or agricultural needs
There are various causes, both human and natural and they interact
It is a weather-related hazard that can develop slowly
Drought is difficult to prevent & can cause crop failure, famine, death, fires & land degradation
Preparedness can help us to cope with the consequences
Meteorological Drought
reduced rainfall compared to the norm for the same place and time of year (e.g. rainfall in Spain in 1990s).
Hydrological Drought
the effect of low rainfall on water levels in rivers, reservoirs, lakes and aquifers.
Hydrological droughts usually appear some time after meteorological droughts (e.g. reservoir levels in Spain).
Agricultural Drought
when there is not enough water available for a particular crop to grow at a particular time. This drought doesn’t depend only in the amount of rainfall, but also on the correct use of that water(e.g. crop growth in Spain).
Socio-economic Drought
Socio-economic drought occurs when the demand for an economic good exceeds supply as a result of a weather-related shortfall in water supply.
Causes of drought- natural/physical
Changes in circulation patterns brought about by normal seasonality, inter-annual or decadal variability, or changes to sunspots, sea surface temperatures, periodic events e.g. ENSO lead to:
Less rain: dominance of high pressure systems (no convection) or wind carrying dry continental air masses (reduced moisture in atmos) brings less rain for a period
More heat= more evaporation
Significant change in land surface (e.g. land cover) lead to changes in albedo, ET and cloud generation
Climate change: A long-term change in climate brings less rain and/or higher temperatures either seasonally or annually
Causes of drought- human causes
Over-population: many people living in an area and using more water than can be provided reliably
Over-cultivation: more agriculture than can be sustained during the dry years
Over-extraction: use of stores at a faster rate than they can be replenished e.g. for irrigation
Inefficient use – pollution or wastage of water
Degradation of soil and landscape leading to reduced capacity of soil/ecosystems to provide water to agriculture or urban populations
circulation- drought 1988
In 1988, a strong high-pressure system stalled over the central United States. Winds circled the high pressure ridge, pushing air south over the Midwest. This pattern essentially cut off moist air that would otherwise have flowed north from the Gulf of Mexico.
circulation- floods 1993
In 1993, pulses of moist air moved into the Midwest throughout the summer. Air moved around a high-pressure system in the Southeastern United States (where drought dominated), drawing moisture from the Gulf into the Midwest. The moist air brought flood-inducing rain.
Albedo
Land cover-> climate feedbacks
Land surface affects albedo and thus net radiation at the surface
Land surface affects partitioning between latent and sensible heat
⇓Veg = ⇑Albedo = ⇓LE and ⇑H = ⇓clouds = ⇓rainfall = ⇓Veg
Consequences of drought
Ecological/environmental – impacts on terrestrial and aquatic habitats
Agricultural –crop declines, failures
Water supply e.g. reduced water storage for dry season (lake Mead SW USA)
Humanitarian e.g loss of livelihood, migration, malnutrition
Off-site and further impacts (e.g. dust, fire, navigation)
Dust
Aviation
Infrastructure and transport
Respiratory health
Manufacturing
Ethiopia drought 2006
Sahel region, drought conditions since 1985
(Drought of 1984-1985 = 1 million deaths)
Causes = natural climatic causes (low precipitation), human causes (desertification, overpopulation, over-cultivation)
GDP per capita $110, life expectancy 43yrs
2006 event = affected 2.7million people
UNICEF donated $50million to water aid projects
Consequences = famine, chronic poverty, migration, disease, 46% of cattle died, debt
Australia drought, 2006
Extremely low rainfall season (lowest rain since 1900) across southern Australia
River Murray dried up in places (Murray-Darling Austalia’s most impt. Agric. Area, with vast irrigation)
Triggered by ‘El Nino’ weather event – which caused excessively low rainfall
Human causes = poor use of water resources
Consequences: 6 million sheep died in 2 years, temporary migrations, improvements to water resource use
Floods in 2010-2011
