Topic 5 - EQ2 Flashcards
What is a drought?
This is a a complex geographical phenomenon. Although fundamentally it is defined as a ‘shortfall’ or deficiency of water over an extended period ,usually a season at least.
What are the 4 types of droughts?
-Meteorological drought (least severe)
-Hydrological drought
-Agricultural drought
-Famine drought (most severe)
What is a meteorological drought?
These are defined by shortfalls in precipitation as a result of short-term variability, or longer term trends which increase the duration of a dry period. Precipitation deficiency is usually combined with high temperatures, high winds, strong sunshine and low relative humidity. All of these increase evaporation. The causes of rainfall deficiency can be natural changes in atmospheric conditions, or caused humanly (e.g deforestation). They can also be caused by even longer term events such as El Niño and climate change.
What is an agricultural drought?
Some farming practices such as overgrazing can accelerate when an agricultural drought occurs. The rainfall deficiency leads to deficiency of soil moisture and soil water availability which then has knock on affects on plant growth, and reduces biomass. Soil moisture budgets can show if the deficit stage is protracted and more severe than normal. The outcome of this falling groundwater level are poor yields from rain-fed crops, failure of irrigation systems, decline in pasture quality and livestock well being. This then has a knock on effect on the economy of rural areas as farmers require hover aid.
What is a hydrological drought?
These are associated with reduced stream flow and groundwater levels, which decrease because of reduced inputs of precipitation and continued high rates of evaporation. It results in reduced storage in lakes and reservoirs, often with marked salivation and poorer water quality. They also cause a major threat to wetlands and other wildlife habitats. This type of drought is also linked to decreasing water supplies for urban areas, which inevitably results in water-use constriction in order to control abstraction rates.
This is a particularly big issue in areas like rural NE Brazil, where there’s no permanent rivers and water supplies depend on seasonal rainfall stored in shallow reservoirs and ponds. This leaves people with less access to water, and a lower quality of water, meaning health declines and reliance on water distribution by road tankers rises.
What is a famine drought?
This is a humanitarian crisis in which the widespread of failure of agricultural systems leads to food shortages and famines with severe social, economic and environmental impacts. These result as a build up of the other three types of droughts. When these scenarios occur, e,g the Horn of Africa in 2012-14, they often require international solutions. The rural economy can collapse, malnutrition increases and so does related mortality. There is a loss of natural vegetation, increased risk of wildfires and wind-blown soil erosion.
What has caused droughts to become more frequent in recent times?
As the population grows, they become wealthier and their demand for water also increases. At the same time, natural variability in climate can also cause a temporary decline in supply, and stores are not replenished. These factors such as the ENSO cycle and climate change associated with global warming make places more susceptible to droughts. High temperatures lead to increased evaporation, areas that are severely affect by droughts have doubled to include more than 30% of the worlds land in the last 30 years; this is especially in Southern Europe, many parts of the USA like California and Asian + eastern Australia.
Why is it often hard to predict/determine a drought?
They often have long periods of onset, sometimes even several years, which makes it difficult to determine if a drought has begun or if it’s just a dry period. Droughts are known as ‘creeping hazards’.
Are the causes of droughts fully understood?
No, only partially. Climate science is the interaction of interlocked systems including atmosphere, oceans, cryosphere and biomass + land surface. These all interact to produce the global climate. Individual research on various drought occurrences has suggested that sea surface temperature anomalies are a very important factor. Teleconnections means that development of the ENSO cycle in the Pacific Ocean has an impact on the climate globally.
What are Teleconnections?
In atmospheric science, refers to climate anomalies which relate to each other at large distances.
What are some Measurements of droughts?
Palmer drought Severity Index (PDSI) - applies to long-term drought and uses current data as well as that of the preceding months, as drought is dependent on previous conditions. Focuses on monitoring duration and intensity of large scale, long term, drought inducing long term circulation.
Crop moisture index - CMI. This is a measure of short-term drought on a weekly scale and is useful for farmers to monitor water availability during the growing season.
Palmer drought severity index - The hydrological system responds slowly to drought, both in reacting to drought and recovering from it. So different models need to be developed for rivers, lakes etc.
What is the El Niño event?
This is when the cool water normally found along the coast of Peru is replaced by warmer water. At the same time area of warmer water further west, near Australia and and Indonesia is replaced with cooler water. The El Niño event usually occurs event 3-7 years and lasts for roughly 18 months. El Niño events can trigger very dry conditions throughout the world, usually in its second year and especially in SE Asia, India, East Australia, Central America and SE USA. In India, El Niño years always lead to relatively weak monsoon rains, causing drought due to monsoon failure.
What does the global atmospheric circulation system do?
1) Intense solar radiation at the equator warms the air, this rises and starts convection. The air cools as it rises and water vapour condenses forming clouds.
2) Subtropical high-pressure zones are created where air has risen at the equator and cooled and sunk to form a belt of high-pressure and hot, dry conditions.
3) The air returns to ground level at the equator, causing trade winds.
4) Trade winds meet at the ITCZ, where warmed air rises. The position of the ITCZ moves with the seasons, in the northern hemisphere, the ITCZ is north of the equator, but in December - February, the Southern Hemisphere is tilted towards the sun and therefore the ITCZ is south of the Equator. This causes alternating wet and dry seasons in the tropics.
5) The warm air moving from the subtropics to the mid-latitudes meets cold polar air at the polar front. Here, warm less dense air rises, causing condensation and rainfall.
6) The warmer air rises into the polar front jet stream and is transferred at high altitude towards the poles, where it cools and sinks. This creates a movement of air at ground level back towards the equator.
What is the intertropical convergence zone (ITCZ)?
This is a belt of low pressure located around the equator. It moves north or south of the equator seasonally. In the ITCZ the air rises due to intense heating from the sun’s energy. There is also high evaporation, especially from oceans. It therefore causes an alternating wet season and dry season in some world regions. Sometimes, the sub-tropical high-pressure zones associated with the descending part of convection cell block the high humidity, rain-bearing air masses associated with the ITCZ, so that pattern is modified. Over continental areas such as Africa, there may be lower humidity levels because less water evaporates, and if high pressure blocks the arrival of the wet season, a severe drought can occur in the Sahel.
What is the La Niña event?
A La Niña is an episode which may, but won’t always follow an El Niño event. La Niña is preceded by a build-up of cooler than normal subsurface water in the tropical pacific - an extreme case of the normal situation. La Niña can also lead to sever drought conditions, but these are usually localised on the western coasts of South America. Cooler than normal ocean temperatures can generate anticyclonic weather and, therefore, very dry conditions associated with descending air.
What has the study of the Sahel region shown?
A study of the Sahel region of Africa shows how, although there are physical factors associated with drought development, human activity plays a major role in making droughts even more severe.
There are many causes of high rainfall variablikity at all climate scales:
Seasonally- the African Sahel is drought sensitive as it occupies a transitional climate zone. Under so-called normal conditions, the annual rainfall (around 85%) is nearly all concentrated in the summer. It varies from 100mm on the edge of the Sahara to 800mm along its southern margins.
Annually - from year to year there is huge variability, especially on the Saharan fringe. Unusually warm sea surface temperatures in tropical seas favour strong convectional uplift over the ocean that, in turn, weakens the West African monsoons and contributes to Sahel droughts.
-Human factors (another flash card)
How do human factors affect the Sahel?
The human factors do not cause drought but they act like a positive feedback loop in enhancing its impacts.
Drought impacts were increased by socio-economic conditions due to growing environmental degradation from overgrazing normadic tribes, deforestation for fuel wood, as well as high levels of rural poverty. Rural population densities had increased - with the population doubling every 20-30 years, so this growth had outstripped food production in many areas. Any agriculture was rain fed, making it very vulnerable to droughts, with some over-cultivation. Additionally, Ethiopia and Eritrea were at war, which blocked access to food for many people.
What are the types of droughts in Australia?
In Australia, drought is a recurrent feature. The Australian Bureau of Meterology recognises two main types of drought based on rainfall criteria:
-Serious deficiency - rainfall totals within ten percent of values record for at least three months.
-Severe deficiency- rainfall totals within the lowest five percent of values on record for at least three months.
What are the major physical reasons why Australia is so drought-prone?
-Low, highly variable rainfall totals occur because the climate is dominated by the subtropic high-pressure belt of the Southern Hemisphere.
-The droughts vary considerably- some are intense and short lived; some last for years; some are localised; others such as the ‘Big Dry’ of 2006, cover huge areas of Australia of several years.
What was the ‘big dry’?
It was assessed as a 1-in-1000 year event as it spread nationwide. It affected more than half of farmlands, especially in the Murray-Darling Basin (the agricultural heartland), which provides 50% of the nation’s agricultural outputs. This had disastrous impacts on Australias food supplies and wool, wheat and meat exports. Farmers also rely on water for their irrigated farming of rice, cotton and fruits.
Despite the sophisticated schemes Australias cities have to deal with episodes of low run-off, reservoirs still fell to about 40% of their capacity. Adelaide was especially vulnerable because it drew 40% of its drinking water from the river Murray, however it has recently been over-extracted with little water even reaching the mouth. Therefore, with future demands likely to exceed supply, new schemes for urban areas must be developed to include desalination plants, large-scale recycling of grey-water and sewage, and more strategies for water conservation.
Does Australia have competition over water?
With limited supplies of water, inevitably there is competition between farmers and urban dwellers. The farmers claim they need water for vital irrigation, but they need to look towards smart irrigation. The over-abstraction of water in the past, during normal periods, was the root cause of the severity of the droughts impact.
What are wetlands?
They’re an area of marsh, fen, peatland or water, wether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt.
What are the key functions of wetlands?
-They act as temporary water stores within the hydrological cycle, thus mitigating river floods downstream, protecting land from destructive erosion by acting as washlands, and recharging aquifers.
-Chemically, wetlands act like giant filters by trapping and recycling nutrients, as well as pollutants, which helps to maintain water quality.
-They have very high biological productivity and support a very diverse food web, providing nursery areas for fish and refuges for migrating birds.
-All these functions contribute towards value for human society, as providers of resources (fish, fuelwood etc, of services in terms of hydrology in the water cycle, and carbon stores (peat) for the carbon cycle.
What are the ecosystem services of wetlands?
Supporting - primary production at a very high level. Nutrient cycling, Food chain support, carbon state within life support systems of carbon cycles.
Provisioning - Fuelwood, peat, fisheries, mammals and birds as tourism.
Regulating - flood control, groundwater recharge/discharge, shorelines as change and to protect, water purification.
Cultural- Aesthetic value, recreational use, cultural heritage.
What is the impact of droughts on the wetlands?
Droughts can have a major impact on wetlands -with limited precipitation, there will be less interception as vegetation will deteriorate, and less infiltration and percolation to the groundwater stores, causing water table levels to fall. The processes of evaporation will continue and might increase from the less-protected surface, while transpiration rates will decrease, making wetlands less functional. Desiccation can also accelerate destruction by wild fires.
Many other schemes have also led to wetland drainage, including water transfer schemes such as the Jonglei Canal Project, which diverted the While Nile discharge away from the Sudd Swamp to the dry land areas of South Sudan. The exploitation of resources such as peat or cattle rearing is another reason for wetland habitat loss.
A further ≈ 2,500,000 km squared (the majority of which is in the developed world) has been destroyed for agriculture and urban development.
What causes hydrological surpluses?
A number of physical factors lead to very high flows of water drainage in a basin. If the discharge is of sufficient quantity to cause a body of water to overflow its channel and submerge the surrounding land, flooding is deemed to have occurred.
Which environments are most as risk of flooding?
-Low lying parts of floodplains and river estuaries. These are not only subject to river flooding, but also to groundwater flooding after the ground becomes saturated from prolonged heavy rainfall.
-Where low lying areas are partially urbanised with impermeable surfaces, there is a greater danger of temporary surface water flooding as intense rainfall has insufficient time to infiltrate the soil, so flows overland.
-Small basins, especially in semi-arid and/or arid areas are subject to flash flooding. These are floods with an exceptionally short lag time - often minutes of hours - which are therefore extremely dangerous. They’re usually associated with very intense convectional storms, so again infiltration, especially on semi-impermeable surfaces, and steep, unvegetated slopes, is vert limited. This allows surface overland flow to develop rapidly.
What physical factors cause flooding?
The primary causes of floods are either meteorological (short term weather events) or longer-term climatic causes such as rainfall patterns.
How do low pressure systems cause flooding?
In areas such as the UK, the usual cause of flooding is the prolonged and heavy rain associated with the passage of low-pressure systems or depressions. The traditional time of year for this sequence, known as progressive cycle, is autumn or early winter but, as a result of unusual positions in the jet stream, this sequence can occur at other times of the year (for example the summer floods of 2007 and 2008). The degree of flooding also depends on the precise depression sequence - sometimes a succession of very intense storms, as occurred in the UK from October to December 2015, has a cumulative effect on the drainage system. This resulted from a very sinuous jet stream in a fairly constant track, which means that all high-pressure systems were ‘blocked’. Data for Northern England reported many rivers with flows up to 50x higher than normal, some experiencing their highest ever recorded flows.
How do monsoons cause flooding?
In some areas, particularly southern and Eastern Asia, intense seasonal monsoonal rainfall can result in widespread, damaging flooding. Around 70% of the average annual rainfall occurs during 100 days, usually from July to September. Low lying floodplains of rivers in India, Pakistan etc are most at risk. 80% of Bangladeshi people are at risk to flooding, some areas are more effected by some types of floods than others. This isn’t surprising as half of the country is less than 12.5m above sea level.
