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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.