drought, causes and consequences

7/29/2019 Drought, Causes and Consequences

1/5

Drought

A drought is an extended period of months or years when a region notes a deficiency in its water supply whether surface

underground water. Generally, this occurs when a region receives consistently below average precipitation. It can have a substant

impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, inten

drought can cause significant damage and harm the local economy. Succulent plants are well-adapted to survive long periods

drought. Many plant species, such as cacti, have adaptations such as reduced leaf area and waxy cuticles to enhance their ability

tolerate drought. Some others survive dry periods as buried seeds. Semi-permanent drought produces arid biomes such as dese

and grasslands. Most arid ecosystems have inherently low productivity.

This global phenomenon has a widespread impact on agriculture. Lengthy periods of drought have long been a key trigger for mamigration and played a key role in a number of ongoing migrations and other humanitarian crises in the Horn of Africa and th

Sahel.

Consequences

Periods of droughts can have significant environmental, agricultural, health, economic and social consequences. The effect var

according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not ha

alternative food sources. Areas with populations that depend on as a major food source are more vulnerable to drought-trigger

famine.

Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination

remaining water sources. Common consequences of drought include:

Diminished crop growth or yield productions and carrying capacity for livestock

Dust bowls, themselves a sign of erosion, which further erode the landscape

Dust storms, when drought hits an area suffering from desertification and erosion

Famine due to lack of water for irrigation

Habitat damage, affecting both terrestrial and aquatic wildlife

Malnutrition, dehydration and related diseases

Mass migration, resulting in internal displacement and international refugees

Reduced electricity production due to reduced water flow through hydroelectric dams

Shortages of water for industrial users

Snake migration and increases in snakebites

Social unrest

War over natural resources, including water and food

Wildfires, such as Australian bushfires, are more common during times of drought.

Causes

Generally, rainfall is related to the amount and dew point [determined by air temperature] of water vapour carried by region

atmosphere, combined with the upward forcing of the air mass containing that water vapour. If these combined factors do n

support precipitation volumes sufficient to reach the surface, the result is a drought. This can be triggered by high level of reflect

sunlight, [high albedo], and above average prevalence of high pressure systems, winds carrying continental, rather than oceanic a

masses (i.e. reduced water content), and ridges of high pressure areas from behaviors which prevent or restrict the developing

thunderstorm activity or rainfall over one certain region. Oceanic and atmospheric weather cycles such as the El Nio-Southe

Oscillation (ENSO) make drought a regular recurring feature of the Americas along the Midwest and Australia. Guns, Germs, a

Steelauthor Jared Diamond sees the stark impact of the multi-year ENSO cycles on Australian weather patterns as a key reason th

Australian aborigines remained a hunter-gatherer society rather than adopting agriculture. Another climate oscillation known as t

North Atlantic Oscillation has been tied to droughts in northeast Spain.


2/5

Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation, deforestation, and erosi

adversely impact the ability of the land to capture and hold water. While these tend to be relatively isolated in their scope, activit

resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture throughout the worl

and especially in developing nations. Overall, global warming will result in increased world rainfall. Along with drought in som

areas, flooding and erosion will increase in others. Paradoxically, some proposed solutions to global warming that focus on mo

active techniques, solar radiation management through the use of a space sunshade for one, may also carry with them increas

chances of drought.

Types

As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increase

People tend to define droughts in three main ways:

1. Meteorological drought is brought about when there is a prolonged period with less than average precipitatio

Meteorological drought usually precedes the other kinds of drought.

2. Agricultural droughts are droughts that affect crop production or the ecology of the range. This condition can also ar

independently from any change in precipitation levels when soil conditions and erosion triggered by poorly plann

agricultural endeavors cause a shortfall in water available to the crops. However, in a traditional drought, it is caused by

extended period of below average precipitation.

3. Hydrological drought is brought about when the water reserves available in sources such as aquifers, lakes and reservo

fall below the statistical average. Hydrological drought tends to show up more slowly because it involves stored water th

is used but not replenished. Like an agricultural drought, this can be triggered by more than just a loss of rainfall. F

instance, Kazakhstan was recently awarded a large amount of money by the World Bank to restore water that had bediverted to other nations from the Aral Sea under Soviet rule. Similar circumstances also place their largest lake, Balkha

at risk of completely drying out.

Protection and relief

Strategies for drought protection, mitigation or relief include:

Dams - many dams and their associated reservoirs supply additional water in times of drought.

Cloud seeding - an artificial technique to induce rainfall.

Desalination of sea water for irrigation or consumption.

Drought monitoring - Continuous observation of rainfall levels and comparisons with current usage levels can help preve

man-made drought. For instance, analysis of water usage in Yemen has revealed that their water table (underground wat

level) is put at grave risk by over-use to fertilize their Khat crop. Careful monitoring of moisture levels can also help pred

increased risk for wildfires, using such metrics as the Keetch-Byram Drought Index or Palmer Drought Index.

Land use - Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-depende

crops in drier years.

Outdoor water-use restriction - Regulating the use of sprinklers, hoses or buckets on outdoor plants, filling pools, and oth

water-intensive home maintenance tasks.

Rainwater harvesting - Collection and storage of rainwater from roofs or other suitable catchments.

Recycled water - Former wastewater (sewage) that has been treated and purified for reuse.

Transvasement - Building canals or redirecting rivers as massive attempts at irrigation in drought-prone areas.

Regions

Recurring droughts leading to desertification in the Horn of Africa have created grave ecological catastroph

prompting massive food shortages, still recurring. To the north-west of the Horn, the Darfur conflict in neighborin

Sudan, also affecting Chad, was fueled by decades of drought; combination of drought, desertification aoverpopulation are among the causes of the Darfur conflict, because the Arab Baggara nomads searching for wat

have to take their livestock further south, to land mainly occupied by non-Arab farming peoples.


3/5

Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. India, China, Pakista

Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. Drought

India affecting the Ganges is of particular concern, as it provides drinking water and agricultural irrigation for mothan 500 million people. The west coast of North America, which gets much of its water from glaciers in mounta

ranges such as the Rocky Mountains and Sierra Nevada, also would be affected.

In 2005, parts of the Amazon basin experienced the worst drought in 100 years. A 23 July 2006 article report

Woods Hole Research Center results showing that the forest in its present form could survive only three years

drought. Scientists at the Brazilian National Institute of Amazonian Research argue in the article that this droug

response, coupled with the effects of deforestation on regional climate, are pushing the rainforest towards a "tippinpoint" where it would irreversibly start to die. It concludes that the rainforest is on the brink of being turned in

savanna or desert, with catastrophic consequences for the world's climate. According to the WWF, the combinatio

of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.

By far the largest part of Australia is desert or semi-arid lands commonly known as the outback. A 2005 study b

Australian and American researchers investigated the desertification of the interior, and suggested that oexplanation was related to human settlers who arrived about 50,000 years ago. Regular burning by these settle

could have prevented monsoons from reaching interior Australia. In June 2008 it became known that an exp

panel had warned of long term, maybe irreversible, severe ecological damage for the whole Murray-Darling basinit does not receive sufficient water by October. Australia could experience more severe droughts and they cou

become more frequent in the future, a government-commissioned report said on July 6, 2008. Australi

environmentalist Tim Flannery, predicted that unless it made drastic changes, Perth in Western Australia coubecome the worlds first ghost metropolis, an abandoned city with no more water to sustain its population.

East Africa currently faces its worst drought in decades, with crops and livestock destroyed. The U.N. World Foo

Programme recently said that nearly four million Kenyans urgently needed food.

FLOODS:

A flood is an overflow of water that submerges or "drowns" land. The European Union (EU) Floods Directive defines a flood as

covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to t

inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows

breaks levees, with the result that some of the water escapes its usual boundaries, or may be due to accumulation of rainwater

saturated ground in an areal flood.

While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significa

flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods oft

cause damage to homes and businesses if they are placed in natural flood plains of rivers. While flood damage can be virtual

eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by th

water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That huma

continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost

repeated periodic flooding.

Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of ra

Additionally, floods can be local, impacting a neighbourhood or community, or very large, affecting entire river basins

The word "flood" comes from the Old Englishflod, a word common to Germanic languages (compare GermanFl

Dutch vloedfrom the same root as is seen in flow, float; also compare with Latinfluctus,flumen). Deluge myths amythical stories of a great flood sent by a deity or deities to destroy civilization as an act ofdivine retribution, and afeatured in the mythology of many cultures.

Effects

Primary effects

Physical damage damage to structures, including bridges, buildings, sewerage systems, roadways, and canals.


4/5

Secondary effects

Water supplies Contamination of water. Clean drinking water will become scarce.

Diseases Unhygienic conditions. Spread of water-borne diseases.

Crops and food supplies Shortage of food crops can be caused due to loss of entire harvest. However, lowlands ne

rivers depend upon river silt deposited by floods in order to add nutrients to the local soil.

Trees Non-tolerant species can die from suffocation.

Transport Transport links destroyed, so hard to get emergency aid to those who need it.

Tertiary and long-term effects

Economic economic hardship due to temporary decline in tourism, rebuilding costs, food shortage leading to pri

increase, etc.

Psychological flooding can be highly traumatic for individuals, in particular where deaths, serious injuries and loss

property occurs.

Types and causes

Areal Floods often happen over flat or low-lying areas when the ground is saturated and water either cannot run off, or cannot r

off quickly enough to stop accumulating. This may be later followed by a river flood as water moves away from the are

floodplain into local rivers and streams.

Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot rapidly dissipate (i

gentle orientation or low evaporation).

A series of storms moving over the same area can cause areal flash flooding.

A muddy flood is produced by an accumulation of runoff generated on cropland. Sediments are then detached by runoff a

carried as suspended matter or bed load. Muddy runoff is more likely detected when it reaches inhabited areas. Mudd

floods are therefore a hill slope process, and confusion with mudflows produced by mass movements should be avoided.

Riverine

Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes inclu

heavy rains from monsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pac

Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.

Fast kinds: include river flash floods resulting from convective precipitation (intense thunderstorms) or sudden relea

from an upstream impoundment created behind a dam, landslide, or glacier.

Dam-building beavers can flood low-lying urban and rural areas, often causing significant damage.

Estuarine

Commonly caused by a combination of sea tidal surges caused by storm-force winds and high river stag

due to heavy rain.Coastal

Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from eithe

tropical cyclone or an extratropical cyclone, falls within this category.

Catastrophic

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake

volcanic eruption). See outburst flood.


5/5

Human-induced

Accidental damage by workmen to tunnels or pipes

Methods of control

Dams

Many dams and their associated reservoirs are designed completely or partially to aid in flood protection and control. Many lar

dams have flood-control reservations in which the level of a reservoir must be kept below a certain elevation before the onset of t

rainy/summer melt season so as to allow a certain amount of space in which floodwaters can fill. The term dry dam refers to a da

that serves purely for flood control without any conservation storage (e.g. Mount Morris Dam, Seven Oaks Dam).

Self-closing flood barrier

The self-closing flood barrier (SCFB), is a flood defence system to protect people and property from inland waterway floods caus

by heavy rainfall, gales or rapid melting snow. The SCFB can be built to protect residential properties and whole communities,

well as industrial or other strategic areas. The barrier system is constantly ready to deploy in a flood situation, it can be installed

any length and uses the rising flood water to deploy. Barrier systems have already been built and installed in The Netherland

Thailand, Belgium, Italy, Ireland, UK, Vietnam, Australia and the U.S.A. Millions of documents at the National Archives buildi

in Washington DC are protected by two SCFBs.

River defences

In many countries, rivers are prone to floods and are often carefully managed. Defences such as levees, bunds, reservoirs, and we

are used to prevent rivers from bursting their banks. When these defences fail, emergency measures such as sandbags or portab

inflatable tubes are used.

A weir, also known as a lowhead dam, is most often used to create millponds, but on the Humber River in Toronto, a weir was bu

near Raymore Drive to prevent a recurrence of the flood damage caused by Hurricane Hazel in 1954.

Coastal defences

Coastal flooding has been addressed in Europe and the Americas with coastal defences, such as sea walls , beach nourishment, a

barrier islands. Tide gates are used in conjunction with dykes and culverts. They can be placed at the mouth of streams or sm

rivers, where an estuary begins or where tributary streams, or drainage ditches connect to sloughs. Tide gates close during incomi

tides to prevent tidal waters from moving upland, and open during outgoing tides to allow waters to drain out via the culvert aninto the estuary side of the dike. The opening and closing of the gates is driven by a difference in water level on either side of t

gate.

Below is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.

Death toll Event Location Date

2,500,0003,700,000[21]

1931 China floods China 1931

900,0002,000,000 1887 Yellow River (Huang He) flood China 1887

500,000700,000 1938 Yellow River (Huang He) flood China 1938

231,000

Banqiao Dam failure, result of Typhoon

Nina. Approximately 86,000 people diedfrom flooding and another 145,000 died

during subsequent disease.

China 1975

230,000 Indian Ocean tsunami Indonesia 2004

145,000 1935 Yangtze river flood China 1935

100,000+ St. Felix's Flood, storm surge Netherlands 1530100,000 Hanoi and Red River Delta flood North Vietnam 1971

100,000 1911 Yangtze river flood China 1911

drought, causes and consequences

Documents