5.4

Question 1

What is a drought

Answer 1

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

Question 2

What is meteorological drought

Answer 2

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

Question 3

What are the major features of meteorological drought

Answer 3

The major features of meteorological drought are

• Low precipitation
• High temperatures
• strong winds
• increased solar radiation
• Reduced snow cover

Question 4

What are the major impacts of meteorological drought

Answer 4

• the major impacts of meteorological drought are
• loss of soil moisture
• supply of irrigation water decreases

Question 5

What is agricultural drought (soil moisture deficit)

Answer 5

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

Question 6

What is hydrological drought (streamflow defecit)

Answer 6

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

Question 7

What is famine drought ( socio- economic drought)

Answer 7

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

Question 8

How can global atmospheric circulation cause drought

Answer 8

• 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)

Question 9

What is the ITCZ

Answer 9

• The ITCZ is a belt of low atmospheric pressure around the equator
• It moves north and south of the equator seasonally

Question 10

How does the ITCZ cause drought

Answer 10

• 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

Question 11

How is this drought exacerbated by subtropical high pressure zones

Answer 11

-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

Question 12

What are mid latitude blocking anticyclones

Answer 12

• 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

Question 13

What sort of weather conditions do these anticyclones bring

Answer 13

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

Question 14

How long do these anticyclones last

Answer 14

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

Question 15

What is the El Niño cycle

Answer 15

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

Question 16

How can the El Niño cycle create drought

Answer 16

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

Question 17

How does La Niña operate

Answer 17

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

Question 18

How does La Niña create drought

Answer 18

• 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

Question 19

How are humans directly causing droughts

Answer 19

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

Question 20

How are humans indirectly affecting droughts

Answer 20

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

Question 21

Describe the context to the area studied as a case of a region affected by drought

Answer 21

• 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

Question 22

What are the physical causes of drought in the Sahel

Answer 22

• 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

Question 23

What are the human causes of drought in the Sahel

Answer 23

• 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

Question 24

What is the background to another case study area affected by drought

Answer 24

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

Question 25

What are the physical causes of the millennium drought

Answer 25

• The El Niño events between 2002-03 and 2006-7 are partly responsible and are an explanation for 2/3 of the rainfall deficit in SE Australia
• The strengthening of the high pressure belt known as the sub tropical ridge accounts for 80% of the decline. It blocks storm tracks ,forcing them to higher latitudes and therefore reducing frontal rainfall

Question 26

What are the human factors contributing to the millennium drought

Answer 26

• The changes to the Hadley cell and STR as a result of global warming stand as human factors
• The STR intensifies as surface temperatures increase
• Anthropogenic warming also reduced the temperature gradient between the pole and the equation which reduced the energy for the polar front jet stream and mid latitude storm systems

Question 27

What was the human affect of the 2006 Big Dry

Answer 27

• The Big dry of 2006 affected more than half the farmlands in the Murray-darling basin and therefore 50% of the nations agricultural outputs
• This had disastrous affects on food supplies and wool,wheat and meat exports
• reservoirs fell to 40% of capacity
• Adelaide drew 40% of its drinking water from the river Murray so it was therefore vulnerable

Question 28

What is resilience in the context of droughts

Answer 28

Resilience is the capacity of an ecosystem to function in droughts and persist in its current state

Question 29

What is the value of wetland ecosystems

Answer 29

• Wetland ecosystems provide a habitat to many types of plant and animal species. Many of which are used by man for food
• Wetlands act as buffers to reduce tidal erosions but also help to absorb storm surges, reducing flooding by hurricanes
• Wetlands absorb excess water from rainfall and release it slowly back into waterways , controlling flooding and droughts. The plant species in the wetlands slow the flow of water helping to control flooding downstream. These plant species also help to control erosion by anchoring the soil with their roots
• Wetlands act as water purification systems as they absorb excess nitrogen and other pollutants and prevent them from entering water ways
• Humans use wetlands for recreational facilities e.g fishing
• The net primary productivity( which measures the amount of carbon being taken in and chemical energy being produced by plants) was at 1000 g/cm2/y

Question 30

How resilient are wetland ecosystems

Answer 30

In general wetlands are not revealing to droughts due to owing their reliance to their submerge of water for long periods of time

However resilience varies between type of wetland. In general river fed wetlands experience a wider range of water levels so naturally support communities may be adapted to fluctuations. Groundwater fed wetlands experience a smaller range of water levels so are more vulnerable to the affects of drought

Question 31

Why are wetlands being destroyed

Answer 31

• 2.5 million square kilometres of wetland has been destroyed in Europe and the USA for agriculture and urban development
• Transfer schemes such as the Jonglie Canal project have destroyed wetlands. It diverted the white Nile discharge away from the Sudd Swamp to the dry lands of South Sudan
• The marshland of southern Iraq has been completely destroyed by dams on the Tigris and Euphrates and also by Saddam Husseins drainage schemes
• Local,national and international schemes have been set up to protect wetlands. The 1991 Ramsar convention of wetlands listed 1800 wetlands of international importance

Question 32

What is the affect of droughts on wetlands

Answer 32

• With limited precipitation there will be less interception as vegetation will deteriorate, and less infiltration and percolation to groundwater stores, causing water table levels to fall
• Transpiration rates will decrease but evaporation will continue ,making the wetland less functional
• The open water areas will shrink or dry up resulting in a loss of habitat
• As soil moisture is reduced the drying can lead to soil erosion and a reduced ability to store water in times of flood, or to release water in times of drought
• Organic soils May oxidise, releasing carbon into the atmosphere and concentrations of pollutants may increased
• drought can alter communities by eliminating some species and creating gaps in the food web e.g species such as spine and thrush will wane as the dry soil surface will be less penetrable , limiting their ability to feed on soil invertebrates

Question 33

What are the benefits of forest ecosystems

Answer 33

Forest ecosystems provide goods such as timber,food,fuel and bio products

• they provide ecological functions such as carbon storage,nutrient cycling, water and air purification and maintenance of wildlife habitat
• They also provide social and cultural benefits such as spiritual and aesthetic pros
• frosts store an average of 0.73 tonnes of carbon per hectare per year

Question 34

How resilient are forests to drought

Answer 34

• living trees take an average of two to four years to recover and resume normal growth rates following drought
• Resilience was even lower for so,e species such g as pine which tend to keep using water at a high rate

Question 35

What are the affects of droughts on trees

Answer 35

• The long term harm to drought stressed trees included foliage loss, impairing growth, increased accumulation of pests and disease and lasting damage to vascular tissues, impairing water transport
• Piñon pines are particularly affected with 90% of them in some areas being destroyed
• Droughts also lead to forest fires due to the drying out of vegetation making it flammable and the lowering of the water table reducing natural protection from burning