Climatic drought

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CLIMATIC DROUGHT
by By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore, Mysore

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Objectives:
Research in the early 1980s uncovered more
than 150 published definitions of drought. The
definitions reflect differences in regions, needs,
and disciplinary approaches.
A drought is a period of below-average
precipitation in a given region, resulting in
prolonged shortages in its water supply.

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Drought refers to a prolonged period of
abnormally low rainfall, leading to a shortage of
water.
A drought can last for several months or years.
Sometimes, droughts are declared for a full
district for a few years.
It can have a substantial impact on
the ecosystem and agriculture of the affected
region and harm to the local life and economy.

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Factors Responsible :
There are several factors responsible for
drought.
The effects of drought will be disastrous.
This lesson highlights the characteristics, causes
and effects of droughts, methods of monitoring
and the methods of mitigation.

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1.0 Introduction:
Drought is a temporary aberration from normal
climatic conditions.
It can vary significantly from one region to
another region. Drought is different than aridity.
Aridity is a permanent feature of climate in
regions where low precipitation is the norm, as
seen in any desert.

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During dry and hot weather periods, it is
common to find dry and cracked earth surface
without even a single shed of water or wet
areas.
Lakes, rivers, and streams may as well run dry.
Well, these are the typical earth conditions that
define drought in layman terms.

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Annual dry seasons in the tropics significantly
increase the chances of a drought developing
and subsequent bush fires.
Periods of heat can significantly worsen drought
conditions by hastening evaporation of water
vapour.
Human factors, such as water demand and water
management, can exacerbate the impact that
drought has on a region.

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2.0 Characteristics of Droughts:
Drought is a normal, recurrent feature of
climate that occurs in virtually all climate
zones, from very wet to very dry.
Many plant species, like Cactaceae (or cacti),
have adaptations like reduced leaf area and
waxy cuticles to enhance their ability to tolerate
drought climate.

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Some others survive dry periods as buried
seeds.
Semi-permanent drought produces arid biomes
such as deserts and grasslands.
Prolonged droughts have caused mass
migrations and humanitarian crises.
Most arid ecosystems have inherently low
productivity.

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Like other hazards, droughts can be
characterized in terms of their severity, location,
duration and timing.
Droughts can arise from a range of
hydrometeorological processes that supress
precipitation and/or limit surface water or
groundwater availability, creating conditions
that are significantly drier than normal or
otherwise limiting moisture availability to a
potentially damaging extent.

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3.0 Drought is a disaster:
A drought is a natural event, caused by
other weather events like El Niño and high-
pressure systems.
Drought can also be triggered
by deforestation (people cutting down forests),
by global warming, and by diverting rivers or
emptying lakes.

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Drought is a disaster which usually takes place
slowly.
It is often difficult to decide when a drought
started and sometimes when it ends too. Its
effects often build up slowly over a long period
of time and may last from months to years after
rain resumes.

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4.0 Causes of drought
4.1 Precipitation deficiency:
Drought comes due to very low rainfall and due
to recurring failure of monsoons.
Drought can be triggered by a high level of
reflected sunlight and above average prevalence
of high pressure systems, winds carrying
continental, rather than oceanic air masses.

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Prolonged period of evapotranspiration can
worsen the situation and increase the drought
conditions.
4.2 Atmospheric Water Vapor:
Droughts commonly are referred to as "dry" in
the sense that not only does less precipitation
fall, but also the air is drier than usual.

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4.3 Atmospheric Circulation Patterns:
A drought is associated with persistent or
persistently recurring atmospheric circulation
patterns.
4.4 Sources of Moisture:
Heated air pulls moisture from the soil,
allowing it to form clouds and return to the

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Earth in the form of precipitation. If weather
patterns shift enough to create an area with little
rainfall over a period of time, there will not be
enough moisture in the soil to draw up into the
air to create clouds.
Moisture continues to be removed from the soil
on a daily basis, and no rain clouds will be able
to form to replace the moisture.

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4.5 Dry season :
The dry season greatly increases drought
occurrence, and is characterized by its low
humidity, with watering holes and rivers drying
up.
Many grazing animals are forced to migrate due
to the lack of water and feed to more fertile
spots.

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4.6 El Niño:
Drier and hotter weather occurs in parts of
the Amazon River Basin, Colombia,
and Central America during El Niño events.
Winters during the El Niño are warmer and
drier than average conditions in the Northwest.

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4.7 Climate change:
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.
Climate change affects a variety of factors that
are associated with the droughts.

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When considering the relationship of drought to
climate change, it is important to make the
distinction between weather and climate.
Weather is a description of atmospheric
conditions over a short period of time, while
climate is how the atmosphere behaves over
relatively long periods of time.
The individual drought periods can be
understood as discrete weather events.

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Climate changes occur over longer periods and
can be observed as changes in the patterns of
weather events.
4.8 Erosion and Human Activities:
Human activity can directly trigger
exacerbating factors such as over farming,
excessive irrigation, deforestation,
and erosion adversely impact the ability of the
land to capture and hold water.

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In arid climates, the main source of erosion is
wind. Erosion can be the result of material
movement by the wind.
5.0 Types of Droughts:
5.1 Meteorological drought:
Meteorological drought is defined usually on
the basis of the degree of dryness (in
comparison to some “normal” or average
amount) and the duration of the dry period.

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Meteorological drought is brought about when
there is a prolonged time with less than average
precipitation.
Meteorological drought usually precedes the
other kinds of drought.
Some definition about meteorological drought
identify periods of drought on the basis of the
number of days with precipitation less than
some specified threshold.

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This measure is only appropriate for regions
characterized by a year-round precipitation
regime such as a tropical rainforest, humid
subtropical climate, or humid mid-latitude
climate.
A drought in terms of meteorology takes into
account deficiencies in measured precipitation.

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Each year's measurements are then compared to
what is determined as a "normal" amount of
precipitation and drought is determined from
such analyses.
For hydrologists, droughts are monitored by
checking stream flow and lake, reservoir,
and aquifer water levels.
Precipitation is also considered here as it
contributes to the water levels.

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5.2 Agricultural drought:
Agricultural drought links various
characteristics of meteorological (or
hydrological) drought to agricultural impacts,
focusing on precipitation shortages, differences
between actual and potential evapotranspiration,
soil water deficits, reduced groundwater or
reservoir levels, and so forth.

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Plant water demand depends on prevailing
weather conditions, biological characteristics of
the specific plant, its stage of growth, and the
physical and biological properties of the soil.
Agricultural droughts are droughts that affect
crop production or the ecology of the range.

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This condition can also arise independently
from any change in precipitation levels
when soil conditions and erosion triggered by
poorly planned agricultural endeavors which
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.

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The agricultural droughts can impact crop
production. This further may cause changes to
the natural distribution of various species.
5.3 Hydrological Drought:
Hydrological drought is associated with the
effects of periods of precipitation (including
snowfall) shortfalls on surface or subsurface
water supply (i.e., streamflow, reservoir and
lake levels, groundwater).

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The frequency and severity of hydrological
drought is often defined on a watershed or river
basin scale.
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.

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Like an agricultural drought, hydrological
drought can be triggered by more than just a
loss of rainfall.
5.4 Ecological Drought:
Ecological drought can be defined as a
prolonged and widespread deficit in naturally
available water supplies that create multiple
stresses across ecosystems.

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As global temperatures continue to rise, the
intensity and frequency of such ecological
droughts in some areas are expected to increase
leading to a wide range of social and ecological
impacts.
6.0 Stages of Drought
Since many areas, regardless of their climatic
region, are prone to drought, different
definitions of the stages of drought have
developed.

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They are all somewhat similar however, usually
ranging from a drought warning or watch,
which is the least severe.
This stage is declared when a drought could be
approaching shortly.
The next stages are mostly called drought
emergency, disaster, or critical drought stage.

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This final stage begins after a drought has
occurred for a long period and water sources
begin to be depleted.
During this stage, public water use is limited
and oftentimes drought disaster plans are put
into place.
7.0 Impacts of drought:
The effects of droughts and water shortages can
be divided into three groups: environmental,
economic and social consequences.

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In the case of environmental effects:
a) lowering of surface and subterranean water
levels,
b) reduction in flow levels (with a decrease
below the minimum leading to direct danger for
amphibian life),
c) increased pollution of surface water, the
drying out of wetlands,
d) more and more fires,
e) higher deflation intensity,

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f) losing biodiversity,
g) worse health of trees and
h) the appearance of pests and den droid
diseases.
7.1 Economic losses:
Economic losses include
a) lower agricultural, forest, game and fishing
output,

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b) higher food production costs,
c) lower energy production levels in hydro
plants,
d) losses caused by depleted water tourism and
transport revenue,
e) problems with water supply for the energy
sector and technological processes in
metallurgy, mining, the chemical, paper, wood,
foodstuff industries etc.,

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f) disruption of water supplies for municipal
economies.
Drought can also reduce water quality, because
lower water flows reduce dilution of pollutants
and increase contamination of remaining water
sources.

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7.2 Environmental impact of droughts:
Plants, animals, climate, soils, rocks and many
others are all affected by drought conditions.
Some biotic and abiotic factors recover when
the droughts are over.
Others may not recover again.

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Soil moisture is key for the breakdown of
organic matter.
Droughts lower the quality of soils, because
there is less organic activity, more wind
erosion, and soil insects or organisms perish.
Water bodies (lakes, creeks, ponds, lagoon and
lakes) dry out, and water animals die. This is
called habitat destruction.

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When aquatic animals (and other wild life) die,
entire food chains and ecosystems are also
affected.
Desertification is when fertile lands (vegetation
lands) become bare and infertile, often as a
result of overgrazing, deforestation and other
economic activity.

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Droughts make this process even worse and
eliminate any chances of the land recovering.
The health and quality of Freshwater Biomes
such as lakes and ponds, rivers and streams,
wetlands are affected and living organism in
there are also endangered.
Animals (wildlife) migrate long distances in
search of water.

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They end up in new habitats, making them
vulnerable and endangered, whiles others face
new threats.
7.3 Common consequences of drought
include:
• Diminished crop growth or yield productions
and carrying capacity for livestock
• Dust storms, when drought hits an area
suffering from desertification and erosion

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• Famine due to lack of water for irrigation
• Habitat damage, affecting both terrestrial and
aquatic wildlife
• Hunger, drought provides too little water to
support food crops.
• Malnutrition, dehydration and related
diseases
• Mass migration, resulting in internal
displacement and international refugees

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• Reduced electricity production due to
reduced water flow through hydroelectric
dams
• Shortages of water for industrial users ;
Snake migration, which results in snakebites
• Social unrest; War over natural resources,
including water and food
• Wildfires, are more common during times
of drought and even death of people.

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• Exposure and oxidation of acid sulfate soils
due to falling surface and groundwater levels.
8.0 Drought index:
Drought Index is a numerical scale that
scientists use to describe the severity of a
drought.

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Scientists take many kinds of data (like
streamflow, rainfall, temperature, and
snowpack) and "blend" it into a single number,
called a drought index , to make it easier to
understand the drought conditions of a
particular area.
Drought indices are one type of drought
indicator.

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8.1 Drought indicator:
Indicators are variables or parameters used to
describe drought conditions.
Examples include precipitation, temperature,
streamflow, groundwater and reservoir levels,
soil moisture and snowpack. It may be a record
of a single measurement, such as rainfall at a
particular rain gauge.

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It may also be a complex index.
Drought indices (indexes) are a subset of
drought indicators.
8.3 Palmer drought index
The Palmer drought index is a measurement of
dryness based on recent precipitation and
temperature.

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It is sometimes called as Palmer drought
severity index.
It was developed by meteorologist Wayne
Palmer, who first published his method in the
1965 paper Meteorological Drought.
The Palmer index is widely used operationally.
The Palmer Drought Severity Index (PDSI) uses
readily available temperature and precipitation
data to estimate relative dryness.

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It is a standardized index that spans -10 (dry) to
+10 (wet).
It has been reasonably successful at quantifying
long-term drought.
As it uses temperature data and a physical water
balance model, it can capture the basic effect of
global warming on drought through changes in
potential evapotranspiration.

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8.4 Why is it important to monitor droughts?
Droughts are a normal part of the climate, and
they can occur in any climate regime around the
world, even deserts and rainforests.
Droughts are one of the more costly natural
hazards on a year-to-year basis;

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their impacts are significant and widespread,
affecting many economic sectors and people at
any one time.
The hazard footprints of (areas affected by)
droughts are typically larger than those for other
hazards, which are usually constrained to
floodplains, coastal regions, storm tracks or
fault zones.

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Perhaps no other hazard lends itself quite so
well to monitoring, because the slow onset of
droughts allows time to observe changes in
precipitation, temperature and the overall status
of surface water and groundwater supplies in a
region.
Drought indicators or indices are often used to
help track droughts, and these tools vary
depending on the region and the season.

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8.5 Planning to Minimise the Impact of
Droughts:
Unlike a hurricane or tropical storm, no one can
keep a watch on when a drought is coming and
when it will end.
However, there are many things an individual,
community or government can do to minimize
the impact of drought if they occur. The term
for taking precautions to minimize drought risk
is ‘drought mitigation’.

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8.6 Drought Mitigation Measures:
Because severe drought is often slow in its
development, it is relatively easy to tell when
one is coming and in areas that are capable,
there are several mitigation measures that can
be used to reduce the impacts felt by drought.
The most important steps in lessening the
effects of drought though are soil and water
conservation.

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By protecting the soil, it is better able to absorb
precipitation, but it can also help farmers to use
less water because it is absorbed and not as
much runs off.
It also creates less water pollution by the
pesticides and fertilizers present in most farm
runoff.

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8.7 Water Conservation
In water conservation, public use is often
regulated.
This mostly includes watering yards, washing
cars and outdoor fixtures such as patio tables,
and swimming pools.
Many have also implemented the use of
xeriscape landscaping to reduce the need to
water outdoor plants in dry environments.

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In addition, Water conservation devices like
low-flow toilets, shower heads, and washing
machines can be required for use inside the
home.
Finally, desalination of seawater, water
recycling, and rainwater harvesting are all
things that are currently under development to
build on existing water supplies and further
reduce the impacts of drought in dry climates.

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Whatever method is used however, extensive
monitoring of precipitation and water usage are
the best way to prepare for a drought, inform
the public on the problem, and implement
conservation strategies.

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8.8 Strategies for drought protection,
mitigation or relief include:
Dams - many dams and their associated
reservoirs supply additional water in times of
drought.
Cloud seeding - a form of intentional weather
modification to induce rainfall.
Desalination - of sea water for irrigation or
consumption.

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Drought monitoring - Continuous
observation of rainfall levels and
comparisons with current usage levels can
help prevent man-made drought.
Land use - Carefully planned crop rotation
can help to minimize erosion and allow
farmers to plant less water-dependent crops
in drier years.
Outdoor water-use restriction - Regulating
the use of sprinklers, hoses or buckets on

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outdoor plants, filling pools, and other 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.

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9.0 Conclusion:
Fluctuations in rainfall, consequent upon shift
in temperature and pressure are also influenced
by changes in the sun's position and heat.
Deforestation, cultivation and land clearance for
constructional work may be responsible for
failure of rain.

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The immediate cause of drought is the
predominant sinking motion of air (subsidence)
that results in compressional warming or high
pressure, which inhibits cloud formation and
results in lower relative humidity and less
precipitation.
People must be prepared for an increased risk of
more frequent and severe drought conditions, in
many developing countries.

Climatic drought

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