P4

Question 1

Drought

Answer 1

very complex geographical phenomenon.
Although the fundamental definition is a ‘shortfall’ or deficiency of water over an extended time period, usually at least a season, there are a wide variety of drought types, as shown in Figure 2.1.
Three major types of drought

Question 2

Figure 2.1 A model of drought development and its impacts

Answer 2

Question 3

1 Meteorological drought

Answer 3

is defined by shortfalls in precipitation as a result of short-term variability, or longer-term trends, which increase the duration of the dry period. Precipitation deficiency is usually combined with high temperatures, high winds, strong sunshine and low relative humidity, all of which increase evaporation. The causes of rainfall deficiency can be natural variations in atmospheric conditions or desiccation caused by deforestation, or longer term such as occurrence of El Niño events and climate change.
Figure 2.1 (page 23) shows how these conditions impact on the hydrological cycle, with decreases in infiltration, percolation and groundwater recharge, and increases in evaporation and transpiration.

Question 4

agricultural drought

Answer 4

Some farming practices such as overgrazing can accelerate the onset of agricultural drought. The rainfall deficiency leads to deficiency of soil moisture and soil water availability which has a knockon effect on plant growth and reduces biomass.
Soil moisture budgets (see page 13) can show if the deficit stage is protracted and more severe than normal. The outcomes, with falling groundwater levels, are poor yields from rain-fed crops, failure of irrigation systems, decline in pasture quality and livestock well-being, and a knock-on effect on the economy of rural areas with many subsistence farmers requiring government aid.

Question 5

Hydrological drought

Answer 5

is associated with reduced stream flow and groundwater levels, which decrease because of reduced inputs of precipitation and continued high rates of evaporation. It also results in reduced storage in any lakes or reservoirs, often with marked salinisation and poorer water quality. There are also major threats to wetlands and other wildlife habitats. Hydrological droughts are also linked to decreasing water supplies for urban areas, often in developed countries, which inevitably results in water-use restriction to control abstraction rates, as in 1973 in the UK when a Minister for Drought was actually appointed to manage the crisis!

It can be a particular problem in areas such as rural north-eastern Brazil, where there are no permanent rivers and water supplies depend on seasonal rainfall stored in shallow reservoirs and ponds. As well as leaving many rural dwellers with less access to water, the quality of the water declines, leading to ill health and a reliance on high-cost water distributed by road tankers.

Question 6

Fourth type of drought

Answer 6

Resulting from these droughts, a fourth type of drought can occur, often called famine drought.
With widespread failure of agricultural systems, food shortages develop into famines that have severe social, economic and environmental impacts. Humanitarian crises such as those associated with the Horn of Africa in 2012-14 require international solutions.

As populations grow and become wealthier, their demand for water also increases. At the same time, natural variability in climate can cause a temporary decline in supply, and stores are not replenished. To this can be added more long-term susceptibility to drought brought about by ENSO and climate change associated with global warming.
Higher temperatures lead to increased evaporation.
Areas that are severely affected by drought have doubled to include more than 30 per cent of the world’s land area in the last 30 years - especially in southern Europe, many parts of USA such as California, parts of the Asian landmass and eastern Australia.
Droughts (known as ‘creeping hazards’) typically have a long period of onset, sometimes several years, which makes it difficult to determine whether a drought has begun or whether it is ‘just a dry period’.
Table 2.1 shows the various indices used to define drought. Most use a water balance approach: inputs of precipitation and losses due to evapotranspiration, and run-off (where applicable).
The physical causes of droughts are only partially understood. Climate dynamics is the study of the interlocked systems of the atmosphere, oceans, cryosphere, biomass and land surface, all of which 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 mean that development of the ENSO within the Pacific Ocean has an impact on climates around the world.

Question 7

Measurement of drought

Answer 7

Palmer Drought Severity Index (PDSI)
Crop Moisture Index (CMI)
Palmer Hydrological Drought
Index (PHDI)

Question 8

Palmer Drought Severity Index (PDSI)

Answer 8

This applies to long-term drought and uses current data as well as that of the preceding months, as drought is dependent on previous conditions. It focuses on monitoring the duration and intensity of large-scale, long-term, drought-inducing atmospheric circulation.

Question 9

Crop Moisture Index (CMI)

Answer 9

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.

Question 10

Palmer Hydrological Drought
Index (PHDI)

Answer 10

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.

Question 11

Key concept: The El Niño-Southern Oscillation (ENSO)

Answer 11

Figure 2.2 shows normal conditions in the Pacific basin, and then conditions during an El Niño event when the cool water normally found along the coast of Peru is replaced by warmer water. At the same time the area of warmer water further west, near Australia and Indonesia, is replaced by cooler water. 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. El Niño events usually occur every three to seven years and usually last for 18 months. A La Niña episode may, but does not always, follow an El Niño event. In terms of drought occurrence, El Niño can trigger very dry conditions throughout the world, usually in its second year, especially in South East Asia, India, eastern Australia, south-eastern USA, Central America and north-eastern Brazil, as well as further afield in parts of Africa (Kenya and Ethiopia). In India, El Niño years always lead to relatively weak monsoon rains, exacerbating drought by monsoon failure. La Niña can also lead to severe 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.

Question 12

ENSO

Answer 12

Question 13

Sahel region

Answer 13

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.
Figure 2.3 shows rainfall trends in the Sahel, the name given to the vast semi-arid region on the southern edge of the Sahara, which stretches right across the African continent from Mauritania to Eritrea. It contains several of the poorest developing countries in the world.
As Figure 2.4 shows, the Sahel has high variability of rainfall at all climate scales.
• Seasonally - the African Sahel is drought sensitive as it occupies a transitional climate zone. Under so-called normal conditions, the mean annual rainfall (around 85 per cent) is nearly all concentrated in the summer.
It varies from 100 mm (very arid) on the edge of the Sahara to 800 mm along its southern margins.
• Annually - from year to year there is huge variability, especially on the Saharan fringe. Unusually warm sea surface temperatures (SSTs) in tropical seas favour strong convectional uplift over the ocean that, in turn, weakens the West African monsoon and contributes to drought in the Sahel.
• Decadal anomalies are very clear in Figure 2.3.
Human factors do not cause drought but they act like a positive feedback loop in enhancing its impacts. In the
1999-2000 Ethiopian-Eritrean drought/famine crisis, about 10 million people needed food assistance. The drought impacts were increased by socio-economic conditions due to growing environmental degradation from overgrazing by nomadic tribes, deforestation for fuelwood, as well as high levels of rural poverty. Rural population densities had increased - the population had doubled every 20 to 30 years - so population 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.

the combined processes of drought-induced environmental fragility can combine with poverty-induced human vulnerability to contribute to a very high risk from desertification.

Question 14

Desertification:

Answer 14

Land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities.

Question 15

Figure 2.5 The complex causes of desertification

Answer 15

Question 16

Drought in Australia p1

Answer 16

Drought is a recurrent feature in Australia. The Australian Bureau of Meterology recognises two main types of drought based on rainfall criteria:
• Serious deficiency - rainfall totals within ten per cent of values record for at least three months.
• Severe deficiency - rainfall totals within the lowest five per cent of values on record for at least three months.
There are major physical reasons why Australia is so drought-prone, with 30 per cent of the country usually affected in any one year:
• Low, highly variable rainfall totals occur because the climate is dominated by the sub-tropic high-pressure belt of the southern hemisphere.
• The droughts vary considerably - some are intense and short lived; some last for years; some are very localised; others, such as the ‘Big Dry’ of 2006, cover huge areas of Australia for several years (Figure 2.6, page 28).
• Most Australian droughts are closely linked to El Niño events, for example the East Coast drought of Southern Queensland in 2002-3.
• Since the 1970s there has been a shift in rainfall patterns with the eastern area, where most people live, becoming drier compared to north-western areas.
• The ‘Big Dry’ is thought to have been associated with longer-term climate change, leading to a trend of a warmer, drier climate for south-eastern Australia.

Question 17

Drought in Australia p2

Answer 17

The ‘Big Dry’ was assessed as a 1-in-1000-year event as it spread nationwide. It affected more than half the farmlands, especially in the Murray-Darling Basin (the agricultural heartland), which provides 50 per cent of the nation’s agricultural outputs. This had disastrous impacts on Australia’s food supplies and wool, wheat and meat exports. Farmers also rely on water for their irrigated farming of rice, cotton and fruits (newer crops).
Despite the fact that most of Australia’s cities are served by sophisticated water supply schemes designed to withstand multiple episodes of low run-off, reservoirs fell to around 40 per cent of their capacity.
Adelaide, in South Australia, was especially vulnerable because it drew 40 per cent of its drinking water from the River Murray. In recent years the river has been so over-extracted that no water has flowed at its mouth.
With a growing population used to an affluent water-consuming lifestyle, per capita water consumption is one of the highest in the world. Many surface and groundwater resources have been over-extracted for agricultural, industrial and urban usage.
With future demands likely to exceed supply (threatened by more El Niño events and climate change, 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. With limited supplies of water, inevitably there is competition between farmers and urban dwellers. The farmers claim they need the 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 drought’s impact.

Question 18

Wetland:

Answer 18

An area of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt.

Question 19

Wetlands perform a number of key functions:

Answer 19

• 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 water 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 their value for human society, as providers of resources (fish, fuelwood, etc.), of services in terms of hydrology within the water cycle, and as carbon stores (peat) within the carbon cycle. Figure 2.9 summarises their value.

Question 20

The value of wetlands

Answer 20

Question 21

Drainage and destruction p1

Answer 21

Drought 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.
In addition to the physical causes of wetland loss, perhaps 2.5 million square kilometres, mostly in the developed world, has been destroyed in Europe and the USA largely for agriculture and urban development (for example, in Florida).
There are many other schemes that have led to wetland drainage, including water transfer schemes such as the Jonglei Canal Project, which diverted the White Nile discharge away from the Sudd Swamp to the dry land areas of South Sudan, or the degradation of the Okavango Delta jegn in Botswana for cattle rearing. Exploiting fuel resources, such as peat, is another reason for wetland habitat loss.

Question 22

Drainage and destruction p2

Answer 22

The marshland of southern Iraq has been almost completely destroyed by dams on the Tigris and Euphrates, substantially reducing their flow, and also by Saddam Hussein’s drainage schemes, largely designed to destroy the lifestyle of the 250,000 Marsh Arabs who lived there. For a time the marshes became largely desert with great ecological losses. Since the overthrow of Saddam in 2003, water has been allowed to flow again and some level of vegetation has spread over half of the original area, but so far it is of a lower ecological quality.
As a result of concerns about wetland habitat destruction, schemes have been developed at a local, national and international scale to protect them. The 1991 Ramsar Convention on Wetlands has listed over 1800 wetlands of international importance, covering 1.7 million square kilometres in 160 states, to promote their conservation.
As ecosystems play such a vital role within the hydrological cycle, it is clearly important to keep them in a pristine state where possible, and to ensure their sustainable use.