DROUGHT
• Drought is an environmental extreme that
is characterized by an absence of
precipitation in the local and regional
water cycle as a consequence of
interactions of elements of the
atmosphere, hydrosphere, and lithosphere
Definition of Droughts
• Drought is a normal, recurrent feature of
climate, although many erroneously consider it
a rare and random event. It occurs in virtually
all climatic zones, but its characteristics vary
significantly from one region to another.
• Drought is a temporary aberration; it differs
from aridity, which is restricted to low rainfall
regions and is a permanent feature of climate.
The Nature of Droughts
• Drought is essentially an imbalance of the hydrologic
cycle.
• In this cycle, water vapour enters the atmosphere by
evaporation from oceans, lakes, and ground surfaces
and by transpiration from plants.
• Water is returned to the earth in the form of rain or
snow. Some of it recharges the soil moisture, some
accumulate in bodies of water, and some runs off to
the oceans.
• Thus drought can result simply from a deficiency in
precipitation over a period of time, or it maybe
caused by excessive evaporation and transpiration.
What Causes Droughts?
• Droughts are caused by a
lack of precipitation in an
area resulting from weak or
less frequent storms and
other weather systems than
normal.
• Most major droughts last for
months or years.
• What is considered a drought
in a rainy location may be
enough precipitation for
another region.
Causes of Drought
• Lack of a precipitation
• High temperature, strong wind, and low humidity all of which
increase the loss of moisture
• Shifting of normal cyclone (low pressure storm system) tracks
across a region
• High pressure systems in which cold, dense air sinks and is
warmed by compression
• Abnormally low sea surface temperature
• Human activities also contribute to the development of
drought conditions; poor cropping methods and improper soil
conservation techniques often contribute to create the drought.
CAUSES &
CONSEQUENCES
PROLONGED LACK OF
PRECIPITATION
LOSS OF SOIL MOSTURE
LOSS OF CROPS
DROUGHTS
DEPLETION/POLLUTION OF
GROUND WATER
LOSS OF VEGETATION
LOSS OF USE OF AG. LAND
• Drought can also reduce water quality, because lower water flows
reduce dilution of pollutants and increase contamination of remaining
water sources.
Common consequences of drought include:
 Diminished crop growth or yield productions and carrying capacity
for livestock
 Dust bowls(an area of land where vegetation has been lost and soil
reduced to dust and eroded, especially as a consequence of drought
or unsuitable farming practice., 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
 Hunger, drought provides too little water to support food crops.
 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
Where Do Droughts Occur?
• Areas that have frequent and severe droughts are shown on
the map as the darkest shaded areas, and less sever droughts
are the lightly shaded areas.
• According to the study, during 1870-1900s,
there were very few droughts, followed by
droughts once in three years
• during 1900-1930 and once in 15 years during
1930-1950. There were 10 drought years
during 1950-1990.
• Since 2000, there have been five drought
years: 2002, 2004, 2009, 2014, 2015
• They usually are most severe around the 30° latitudes, where
dry, cool air is falling.
• Asia, many African countries (i.e. Kenya, Ethiopia, Somalia),
are most often affected by droughts
• Humid areas are prone to short term droughts, whereas arid
regions are prone to long term droughts
Key Drought Indicators
•
•
•
•
•
•
•
•
Climate variables (e.g., precip., temp.)
Reservoir and lake levels
Soil moisture
Ground water
Snow pack
Stream flow
Vegetation
Forecasts
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: 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
EFFECTS OF DROUGHTS
PRIMARY EFFECTS:
• Primary effects of drought result from a lack of water. As a dry
period progresses and water supplies dwindle, existing water
supplies are overtaxed and finally dry up.
• The primary losses are loss of crops low of livestock and other
animals, and loss of water for hygienic and drinking
SECONDARY EFFECTS:
• As water supplies swindle and crops and fodder are depleted,
families begin to migrate in search of better grazing lands for
their herbs or move to cities to seek jobs and alternative sources
of income.
• The migration may, in itself, contribute to spreading the scope of
the disaster, especially if grazing animals are moved with people.
• If drought is long term, it may result in permanent changes of
settlement, social, and living patterns
• Major ecological changes, such as increased scrub growth,
increased flash flooding and increased wind wing erosion of
soils.
Destruction by Droughts
• Droughts cause major damage to crops and animals,
especially if they occur during the region’s growing
season.
• During a drought not enough water is provided to
support living things. Crops can decrease 10%-20%
during a drought.
• Droughts affect reservoirs, water supplies, water
tables, and topsoil. Water levels drop, and topsoil
becomes dry and less favorable to plants.
• Areas that are in a drought are more likely to have
wildfires than other regions.
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.
• Drought monitoring - Continuous observation of
rainfall levels and comparisons with current usage
levels can help prevent man-made drought. Careful
monitoring of moisture levels can also help predict
increased risk for wildfires
• Land use - Carefully planned crop rotation can help to
minimize erosion and allow farmers to plant less waterdependent crops in drier years.
• Outdoor water-use restriction - Regulating the use of
sprinklers, hoses or buckets on 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.
• Transvasement - Building canals or redirecting rivers
as massive attempts at irrigation in drought-prone areas