A
drought is an extended period of months or years
when a region notes a deficiency in its water supply. Generally,
this occurs when a region receives consistently below average
precipitation. It can
have a substantial impact on the
ecosystem
and
agriculture of the affected region.
Although droughts can persist for several years, even a short,
intense drought can cause significant damage and harm the local
economy. This global phenomenon has a
widespread impact on agriculture.
The United
Nations estimates that an area of fertile soil the size of
Ukraine is lost every year because of drought, deforestation, and climate instability.
Lengthy periods of drought have long been a key trigger for
mass migration and played a key role
in a number of ongoing migrations and other humanitarian crises in
the
Horn of Africa and the
Sahel.
Implications
Drought is a normal, recurring feature of the climate in most parts
of the world. It is among the earliest documented climatic events,
present in the
Epic of Gilgamesh
and tied to the
biblical story of
Joseph's arrival in and the later
Exodus from
Ancient Egypt. Hunter-gatherer migrations in
9,500BC Chile have been linked to the phenomenon, as has the exodus
of early man
out
of Africa and into the rest of the world around 135,000 years
ago. Modern peoples can effectively mitigate much of the impact of
drought through irrigation and crop rotation. Failure to develop
adequate drought mitigation strategies carries a grave human cost
in the modern era, exacerbated by
ever-increasing population densities.
Recurring droughts leading to
desertification in the
Horn of Africa have created grave
ecological catastrophes, prompting massive
food shortages,
still recurring.
To the
north-west of the Horn, the Darfur conflict in neighboring Sudan, also
affecting Chad, was fueled
by decades of drought; combination of drought, desertification and overpopulation are among the causes of the
Darfur conflict, because the Arab Baggara nomads searching for
water have to take their livestock further south, to land mainly
occupied by non-Arab farming peoples.
According
to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers - Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow - could disappear by 2035 due to global
warming. Approximately 2.4 billion people live in the
drainage basin of the Himalayan
rivers.
India, China, Pakistan, Bangladesh, Nepal and Myanmar could
experience floods followed by droughts in coming decades.
Drought in India affecting the
Ganges is of particular concern, as it provides
drinking water and agricultural
irrigation for more than 500 million people.
The west coast of
North America, which
gets much of its water from glaciers in mountain 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
reported
Woods Hole Research
Center results showing that the forest in its present form
could survive only three years of drought. Scientists at the
Brazilian
National Institute of
Amazonian Research argue in the article that this drought
response, coupled with the effects of
deforestation on regional climate, are pushing
the rainforest towards a "
tipping
point" where it would irreversibly start to die. It concludes
that the
rainforest is on the brink of
being turned into
savanna or
desert, with catastrophic consequences for the
world's climate. According to the
WWF, the combination 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
by Australian and American researchers investigated the
desertification of the interior, and suggested that one explanation
was related to
human settlers who arrived
about 50,000 years ago. Regular burning by these settlers could
have prevented
monsoons from reaching
interior Australia. In June 2008 it became known that an expert
panel had warned of long term, maybe irreversible, severe
ecological damage for the whole
Murray-Darling basin if it does not
receive sufficient water by October. Australia could experience
more severe droughts and they could become more frequent in the
future, a government-commissioned report said on July 6, 2008.
The
Australian of
the year 2007, environmentalist Tim
Flannery, predicted that unless it made drastic changes,
Perth in Western Australia could become the world’s first ghost metropolis, an abandoned city with no more
water to sustain its population.
Causes
Generally, rainfall is related to the amount of water vapor in the
atmosphere, combined with the upward forcing of the air mass
containing that water vapor. If either of these are reduced, the
result is a drought. This can be triggered by an above average
prevalence of high
pressure systems,
winds carrying continental, rather than oceanic
air masses (ie. reduced water content), and ridges of
high pressure areas form with behaviors
which prevent or restrict the developing of thunderstorm activity
or rainfall over one certain region. Oceanic and atmospheric
weather cycles such as the
El Niño-Southern
Oscillation (ENSO) make drought a regular recurring feature of
the Americas along the Pacific coast and Australia.
Guns, Germs, and Steel author
Jared Diamond sees the stark impact of
the multi-year ENSO cycles on Australian weather patterns as a key
reason that
Australian
aborigines remained a
hunter-gatherer society rather than
adopting agriculture.
Human activity can directly trigger exacerbating factors such as
overfarming, excessive irrigation,
Deforestation, and
erosion adversely impact the ability of the land to
capture and hold water. While these tend to be relatively isolated
in their scope, activities resulting in global
climate change are expected to trigger
droughts with a
substantial impact on
agriculture throughout the world, and especially in
developing nations. Overall, global
warming will result in increased world rainfall. Along with drought
in some areas, flooding and erosion will increase in others.
Paradoxically, some proposed
solutions to global warming
that focus on more active techniques,
solar radiation management
through the use of a
space sunshade
for one, may also carry with them increased chances of
drought.
Consequences
Periods of drought can have significant environmental,
agricultural, health, economic and social consequences. The effect
varies according to vulnerability. For example, subsistence farmers
are more likely to migrate during drought because they do not have
alternative food sources. Areas with populations that depend on
subsistence farming as a major
food source are more vulnerable to drought-triggered famine.
Drought is rarely if ever the sole cause of famine; socio-political
factors such as extreme widespread poverty play a major role.
Drought can also reduce water quality, because lower water flows
reduce dilution of pollutants and increase
contamination of remaining water sources. A
few common consequences of drought include:
Types of drought
As a drought persists, the conditions surrounding it gradually
worsen and its impact on the local population gradually increases.
People tend to define droughts in three main ways:
- Meteorological drought is brought
about when there is a prolonged period with less than average
precipitation. Meteorological drought usually precedes the other
kinds of drought.
- Agricultural droughts are droughts
that affect crop production or the ecology of the range. This condition can also arise
independently from any change in precipitation levels when soil conditions and erosion triggered by poorly planned
agricultural endeavors cause a shortfall in water available to the
crops. However, in a traditional drought, it is caused by an
extended period of below average precipitation.
- Hydrological drought is brought about when the water reserves available in sources such as
aquifers, lakes and
reservoirs fall below the statistical average.
Hydrological drought tends to show up more slowly because it
involves stored water that is used but not replenished. Like an
agricultural drought, this can be triggered by more than just a
loss of rainfall. For instance, Kazakhstan was recently awarded a large amount of money by the
World Bank to restore water that had been
diverted to other nations from the Aral Sea under Soviet rule.
Similar
circumstances also place their largest lake, Balkhash, at risk of completely drying out.
Mitigation strategies
- 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 prevent man-made
drought. For instance, analysis of water usage in
Yemen has revealed that their water table (underground water level) is put at
grave risk by over-use to fertilize their Khat
crop. Careful monitoring of moisture levels can also help
predict 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-dependent crops in drier
years.
- 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.
- Outdoor water-use
restriction - Regulating the use of sprinklers, hoses or
buckets on outdoor plants, the washing of motor vehicles or other
outdoor hard surfaces (including roofs and paths), topping up of
swimming pools.
See also
References
External links