Climatic drought

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Text of Climatic drought

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    by By

    Prof. A. Balasubramanian

    Centre for Advanced Studies in Earth Science,

    University of Mysore, Mysore

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    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


    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


    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


    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


    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


    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


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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 Nio 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


    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


    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


    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


    Many grazing animals are forced to migrate due

    to the lack of water and feed to more fertile


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    4.6 El Nio:

    Drier and hotter weather occurs in parts of

    the Amazon River Basin, Colombia,

    and Central America during El Nio events.

    Winters during the El Nio 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


    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


    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 h