Learning objective: To be able to explain the causes and characteristics of droughts

Learning objective: To be able to explain the causes and characteristics of droughts

Regional distribution of disasters by type [1991 - 2005]

Describe the spatial distribution of droughts – where most prevalent?

Key terms •

Arid: Arid means extremely dry, lack of moisture.

Desert: An arid area with very low levels of precipitation, making it impossible to support large amounts of vegetation, animals or people. Antarctica is technically a desert because of its low precipitation rates. The Sahara Desert is the largest traditional desert.

Water stress: When the demand for water exceeds the supply of water and shortages exist.

Physical Water Scarcity: When the supply of rainfall is lower than the demand of water.

Economic Water Scarcity: When water supplies exist, but the local population can not access them because of pollution, lack of technology, etc.

Current drought conditions

• Click on the link for the current drought conditions

• Describe the distribution using longitude and latitude

• Who is affected?

What are the causes of drought?Create a mind map to summarise your notes on this

• ITCZ movement• El Nino / La Nina • Deforestation • Overabstration• Population growth • Economic development

Physical causes

• P 208 course companion

El Nino / La nina

Trade winds

Hazards and Problems

Famine: Drought can lead to crop failures and livestock deaths that can lead to undernourishment and famine (food shortages).

Biodiversity Loss: Drought can cause plants and animals to die. Animals that drink large quantities of water like elephants can be some of the first to die.

Conflict: As water supplies begin to reduce then conflict over the remaining sources intensifies. This can be particular severe if resources are shared between countries or tribes.

Refugees: Because of the shortage of water and potential famine, people are forced to leave their home and often country to try and find food and water.

Desertification: A shortage of water can cause the land to start to degrade. Eventually this may lead to desertification (the land turning to desert).

Economic Loss: If a country is having to support refugees while at the same time seeing a decline in its exports (primary agricultural products) then the economy may decline.

Dependency: It is often poor countries that suffer from drought and it is these countries that find it hardest to cope, so often become dependent upon charities and international aid.

Education: As people suffer from undernourishment they often become weaker and are unable to teach and or go to school. Also during times of drought people (students and teachers) have to walk further to find water. All of these things would lead to a decline in the quality and quantity of education.

Drought IndicesPercent of Normal• simplest measurements of rainfall for a location. • Actual precipitation divided by normal precipitation,

(30-year mean) multiplying by 100%. • This can be calculated for a variety of time scales.• From a single month to a group of months

representing a particular season, to an annual or water year.

• Normal precipitation for a specific location is considered to be 100%.

Advantages• Effective to compare

a small region or season

• Simple to use

Disadvantages• Assumes a normal

distribution• Average rainfall and

mean rainfall over a period can be significantly different and give a skewed impression

Palmer Drought Severity Index

• This measures soil moisture as a measure of drought conditions

• The PDSI is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil.

• From the inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer.

• Human impacts on the water balance, such as irrigation, are not considered.

Palmer Classifications4.0 or more extremely wet3.0 to 3.99 very wet2.0 to 2.99 moderately wet1.0 to 1.99 slightly wet0.5 to 0.99 incipient wet spell0.49 to -0.49 near normal-0.5 to -0.99 incipient dry spell-1.0 to -1.99 mild drought-2.0 to -2.99 moderate drought-3.0 to -3.99 severe drought-4.0 or less extreme drought

Palmer index

• Most effective in determining long term drought e.g. several months and is not as good with short-term forecasts (a matter of weeks).

• It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.

• The advantage of the Palmer Index is that it is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions.

SE Australia Drought

Where did it happen? Include a map of the drought and written description When did it happen? Date and duration, speed of onset and recovery Why did it happen? The human and physical causes Who was affected? Social, economic and environmental consequences What happened? Preparation, management, recovery, international aid