Drought
Drought is a long and continuous period of dry weather leading to extremely arid conditions. The definition of depends on the culture.
In the UK:
In the UK:
- Drought is 50% deficit ober three months, or 15% shortfall over two years.
- Absolute drought is often defined as a period of at least 15 days in a row with less than 0.2 millimetres of rainfall.
- Partial drought is period of at least 29 days in a row during which average daily. rainfall does not exceed 0.2 millimetres.
The most world's surfaces experienced dry conditions.
Semi-arid areas have rainfall less than 500 millimeters per a year.
Arid areas have rainfall less than 250 millimeters per a year.
Extremely arid areas have rainfall less than 125 millimeters per a year.
Semi-arid areas have rainfall less than 500 millimeters per a year.
Arid areas have rainfall less than 250 millimeters per a year.
Extremely arid areas have rainfall less than 125 millimeters per a year.
The causes of arid conditions:
- Global atmospheric ciruculation (dry air associated with subtropical high-pressure belt is the main cause of aridity around 20-30 degrees nord)
- Distance from sea (continentslity) limits the amount of water carried across by winds
- Cold offshore currents limit the amount of condensation into the overlying air (Atakama and Namib)
- Others are caused by intense rain-shadow effects, as air passes over mountains
- Human activity may also cause drought by deforestation leading to erosion, followed by the spread of desert conditions into areas previously fit for agriculture. This is called desertification and is an increasing problem.
There are 3 different types of drought:
- Meteorogical drought - when there is a prolonged period with less than average precipitation
- Agricultural drought - when there is insufficient moisture for average crop or range production
- Hydrologic drought - when the water reserves avaiable in sources such as aqufiers, lakes and reservoirs falls below the statistical average
- Western and midwest USA
- Central America and northeastern Brazil
- The sub-Saharan belt and the Horn of Africa
- Southern and central Africa and Madagascar
- Southern Spain and Portugal
- Central Asia, northwest India and northeast China
- Southeast Asia, Indonesia and southern Australia
About 38% of land worldwide has a level of drought exposure. This 38% contains around 70% of both the total population and the agricultural value produced. In 2006, approximately 280 000 farmers were disolaced from their villages into makeshift shelters as seasonal rivers switched from being dustbiwls into raging torrents.
Direct impact of drought include reduced cropping, rangeland and forest productivity, increased fire hazards and death rates among livestock and wildlife, the damage to wildlife and fish habitats.
Direct impact of drought include reduced cropping, rangeland and forest productivity, increased fire hazards and death rates among livestock and wildlife, the damage to wildlife and fish habitats.
The impacts of drought(direct and indirect):
Case study - Drought in Africa(Ethiopia)
In 2003, Ethiopia experinced the longest drought anyone had known. Because of sixth poor rsiny seasons in three years, 20 million people needed help. The situation was worst than the 1984 famine, when 10 million people lacked food.
There is 20 million people in Ethiopia under threat of famine in Africa.
A variety of hazards affect dry areas, such as :
There is 20 million people in Ethiopia under threat of famine in Africa.
A variety of hazards affect dry areas, such as :
- Declining water resources and food shortages
- Flooding of valleys, alluvial fans and plains
- Increased soil erosion
- Sedimentation of deposition of river sediments
- Weathering
Case study - The Australian drought 2006
Australia has the most variable rainfall climates in the world. The worst drought for 1000 years was in the 2006. Over the long term, about 3 good years of rain and 3 bad years of out of 10 can be expected. These fluctuations have many causes but the strongest is the climate phenomenon called the Southern Oscillation. This is a major air pressure shift between the Asia and east Pacific regions. Its the best - known extreme called El Niño.