5.4 Flashcards
What is a drought
A drought is an extended period (season,year,several years) of deficient rainfall relative to the statistical multi year average for that region. 38% of the world has some level of drought
What is meteorological drought
Meteorological drought is defined by shortfalls in precipitation as a result of short term variability or longer term trends, which ultimately increase the duration of the dry period
-This precipitation deficiency is usually combined with high temperatures, high winds, string sunshine and low relative humidity, all of which increase evaporation
+It is region specific
What are the major features of meteorological drought
The major features of meteorological drought are
- Low precipitation
- High temperatures
- strong winds
- increased solar radiation
- Reduced snow cover
What are the major impacts of meteorological drought
- the major impacts of meteorological drought are
- loss of soil moisture
- supply of irrigation water decreases
What is agricultural drought (soil moisture deficit)
Agricultural drought occurs when there is insufficient soil moisture to meet the needs of a particular crop at a particular time.
- It is caused by a number of factors such as precipitation shortages, differences between actual and potential EVT, soil water deficits and reduced groundwater/ reservoir levels
- It can result in crop failures or undeveloped crops with greatly depleted yields
What is hydrological drought (streamflow defecit)
Hydrological drought occurs when there are deficiencies in surface and subsurface water supplies
- It is caused by reduced rates of precipitation but continued high rates of Evaporation
- It results in reduced storage in lakes and reservoirs, with marked salinisation and poorer water quality
- It is linked to decreasing water supplies for urban areas,often in developed countries, which results in water use restrictions to control abstraction rates
- Has had an impact in north eastern Brazil where there are no permanent rivers
What is famine drought ( socio- economic drought)
Famine drought occurs when the water demand for socio-economic purposes (such as crop irrigation / HEP) exceeds water availability
-It could be as a result of weather related shortfall or overuse of available water supplies.it differs from other types of drought as it depends on variations in supply and demand
How can global atmospheric circulation cause drought
- Intense solar radiation at the equator warms air which rises and starts convection. The air cools as it rises and condenses to form clouds and rain
- The SUBTROPICAL HIGH PRESSURE ZONE is created by the air that has risen at the equator and has cooled so sinks to form a belt of High pressure and hot dry conditions ( creating drought)
What is the ITCZ
- The ITCZ is a belt of low atmospheric pressure around the equator
- It moves north and south of the equator seasonally
How does the ITCZ cause drought
- At the ITCZ air rises as a result of intense heating by the suns energy (due to it being near the equator) there is also high evaporation
- It therefore causes alternating wet seasons ( when it arrives as air rises, cools and condenses) and dry seasons ( when it leaves) in some areas
- This creates relative drought during the dry season
How is this drought exacerbated by subtropical high pressure zones
-This level of drought is exacerbated if subtropical high pressure zones( formed with the descending part of the Hadley cell) block the High humidity , rain bearing masses associated with the ITCZ as the blockage means some areas don’t get a wet season
What are mid latitude blocking anticyclones
- In mid latitudes, frontal precipitation is created in low pressure systems that form along the polar front, where warm tropical air rises over cold polar air
- Depressions and therefore frontal precipitation move from east to west in the mid latitudes as a result of the Coriolis force (caused by rotations of the earth ) and their track is direction by the polar front jet stream
- The loops of Jet stream occasionally stabilise or even break up and this allows high pressure ( mid latitude blocking anticyclones) from the subtropics to move northwards
What sort of weather conditions do these anticyclones bring
These anticyclones bring stable weather conditions with very little precipitation (heat waves) while rain bearing masses are usually forced around them, usually to the north but occasionally to the south, causing drought in mid latitude countries such as the UK
How long do these anticyclones last
The stability of theses anticyclones, with their sinking air and calm conditions means they can persist and block weather systems from the west for up to two weeks
-If this situation is repeated over space of a few months the normal precipitation levels are greatly reduced and this may cause extended periods of drought
What is the El Niño cycle
The El Niño is a naturally occurring large mass of very warm seawater in the equatorial pacific
- It is normally located in the western pacific where it is pushed by ocean currents, trade winds and the Walker circulation cell in the atmosphere
- On average every 7 years these pushing forces weaken and this allows the mass of warm water to move eastwards to the west coast of central and southern America
- Wherever the mass of warm water is located, evaporation rates are higher and precipitation greater
- In the areas that experience the cooler water there is drier weather
How can the El Niño cycle create drought
As well as triggering drought in areas wheee the mass of cold water is located, El Nino can trigger very dry conditions through the world
- It’s effects are especially felt in south east Asia, India , north eastern Brazil, SE USA, eastern Australia and Central America
- In India specifically El Niño years always lead to weak monsoon rains , exacerbating drought by monsoon failure
- affects of La Niña can be exacerbated by anti cyclonic weather caused by cooled ice a temperatures than normal
How does La Niña operate
La Niña occurs when the warm mass of water is pushed even further west that normal, which causes drought in Peru and California for example. It is intensification
How does La Niña create drought
- as mentioned , La Niña usually creates localised drought conditions in the western coast of South America
- however it has also be found to create more widespread drought - the severe East African drought of 2011 was attributed to a strong La Niña for example
How are humans directly causing droughts
Humans are directly causing droughts by abstracting from rivers and groundwater supplies and reducing downstream water supplies by building reservoirs and water transfers.
-Activity with dams has left 23% of the global population with less water
How are humans indirectly affecting droughts
Humans are indirectly affecting droughts by changing land uses and altering hydrological processes.
For example, deforestation and overgrazing ( which come as a result of changing land use to pastoral uses from forestland) , reduce vegetation cover, so reducing evapotranspiration rates and atmospheric moisture and precipitation leading to droughts
-The removal of vegetation also alters soil conditions through compaction, reduced moisture retention and organic matter. This reduces infiltration and increases surface run off which reduces water storage leading to drought
-Other affects of deforestation include lower latent heat as well
Describe the context to the area studied as a case of a region affected by drought
- The Sahel is a vast, semi arid region across Africa covering parts of 11 countries including Eritrea, Sudan and Chad and affecting 15.6 million people
- Since the 1960s there has been a pronounced decline in annual rainfall in the Sahel region with the largest negative anomalies coming in the early 1980s
- The most recent severe drought were in 2012-2013
What are the physical causes of drought in the Sahel
- The Sahel is physically drought sensitive as it occupies a transitional climate zone. In normal conditions the annual rainfall is nearly all concentrated in the summer (85%). The annual rainfall ranges from 100mm-800mm
- The met office Hadley centre forecast that the 2015/16 El Niño events would increase water stress in the western Sahel region as temperatures were 2 degrees higher than average in December and January and there was lower rainfall. These were similar conditions to the ones that caused the 1972-73 Sahel drought and famine
- In 2011 an unusually strong La Niña interrupted seasonal rainfall in East Africa which no doubt added to drought
What are the human causes of drought in the Sahel
- In 2002 a study by the commonwealth scientific and industrial research organisation suggested that air pollution from North America and Europe may have caused the drought of the 20-21st century (sulphur based aerosols). Theses pollutants were thought to have caused atmospheric cooling by changing the global heat balance and atmospheric circulation so tropical rains associated with the ICTZ did not fall, causing drought
- In 2005 a study conducted by the NOAA found that the droughts could have been caused by higher sea surface temperatures caused by global warming. The rain bearing winds moving over the Sahel appear to fail when sea temperatures are above average
- The Sahel region is experiencing some of the highest population growth rates in the world. This is partly due to rising birth rates but also due to increase in migration due to conflicts e,g Ethiopian- Eritrean conflict. There has been growth rates of 2.5-4% in some areas. This has increased the demand for food and fuel wood and natural dry land ecosystems then being over cultivated and overgrazed, causing desertification
- The population growth has also caused an increase in demand for firewood which has led to deforestation and vegetation being removed and desertification
- There has been an increase in animals (especially within nomadic communities)due to the population growth and due to the demand for food partly to do with locust plagues ( so much so that it is above the carrying capacity of the fragile grasslands). In some areas livestock numbers have increased by 40%. This has led to overgrazing and the soil being depleted of nutrients and vegetation not being able to re establish itself and ultimately desertification
What is the background to another case study area affected by drought
The Millennium drought in southern Australia 97-09 was the longest series of uninterrupted years with belong median rainfall in southern Australia since 1900 , with rainfall 12.4% below mean
