5.4

• Types of drought:

  Meteorological – when long term precipitation is lower than normal. Meteorological droughts are region specific as precipitation deficit is climate dependent.

  Agricultural – when there is insufficient soil moisture to meet the needs of a particular crop at a particular time. There may be several causes of this including lack of rainfall, differences between actual and potential evapotranspiration, soil water deficits and reduced groundwater levels. If crops have a lack of water they can fail or become undeveloped and so reduce yields.
• Types of Drought

  Hydrological – when there are deficiencies in surface water supplies such as in rivers, lakes, reservoirs and groundwater. It originates with a deficiency of precipitation but can happen sometime after as it takes time for water to move into some of the components e.g. groundwater.

  Socio-economic – when the demand for water exceeds the supply. This could be the result of a weather related fall in water supply or it could be caused by overuse of the available water supply.
• Physical causes of drought


  Global atmospheric circulation - intense solar radiation at the Equator causes equatorial low pressure forming clouds and rainfall

  This air then sinks and creates sub-tropical high pressure belts which causes clear skies and dry conditions

  Some of this descending air returns towards the Equator forming the trade winds, they form the ITCZ and this air rises and completes the Hadley Cell
• Physical causes of drought


  Global atmospheric circulation (2) - remaining air that descends at the sub-tropical high pressure belt moves poleward to the mid-latitudes where it meets cold air moving from the polar regions.

  Warmer air rises at the polar front to form the mid-latitude low pressure belts and frontal rain. Some of the rising air flows back towards the Equator to form the Ferrel Cell.

  Some moves towards the Polar regions where it sinks to form the Polar High pressure belts where the air is dry. It hits the ground and spreads towards the Equator to complete the Polar cell.
• Inter-tropical Convergence Zone (ITCZ)

  As the ITCZ moves location, the wet weather moves with it, however the areas where it leaves is replaced with the sub-tropical high pressure – this creates a series of wet and dry seasons e.g. the Savanna of Africa. Sometimes the sub-tropical high pressure blocks the high humidity, rain bearing air-masses that try to move towards the ITCZ and in doing so prevent the start of the wet season or even stop it completely – this can cause drought e.g Sahel region
• Mid-latitude blocking anticyclones

  In the mid-latitudes, frontal precipitation is created at the PF where the Polar and Ferrel cells meet. Depressions (areas of low pressure) are moved by the jet streams. This air can sometimes break up and allow areas of high pressure (anticyclones) from the sub-tropical high pressure belt to move into the mid-latitudes. The areas of low pressure are forced around this weather system which is associated with dry conditions. If they stay over an area for a period of time, precipitation levels will fall, if repeated over months then a drought may occur.
• El Nino Southern Oscillation (ENSO) cycles

  In the equatorial pacific, an area of warm water is pushed by ocean currents and the trade winds. However, about every 7 years these pushing forces weaken and this allows the mass of warm water to spread eastwards towards Central and South America. This warm water is associated with high evaporation rates and higher precipitation whereas the colder water around it are associated with drier conditions. So as El Nino moves east the wet weather also moves east and the western Pacific becomes drier leading to drought.
• Occasionally the area of warm water is pushed even further west by strong trade winds and ocean currents – La Nina.

  The western Pacific gets wetter but the eastern Pacific such as California and Peru get drier and experience severe droughts.

  Both El Nino and La Nina are large enough systems to disrupt global atmospheric circulation and cause droughts in different regions of the world.
• Causes of the Sahelian Drought

  Commonwealth Scientific and Industrial Research Organisation suggested droughts were caused by air pollution from Europe and North America - sulphur based aerosols cooled the atmosphere, weakened atmospheric circulation and so the rains associated with the ITCZ did not arrive.

  Climate models suggested that higher sea surface temperatures in the tropical Atlantic ocean caused by anthropogenic climate change caused the rain bearing winds that reach the Sahel to fail
• Causes of the Sahelian Drought

  The Sahel is one of the poorest regions in Africa and has one of the fastest population growth rates so the demand for food and fuel wood is accelerating so farmland is being over cultivated causing desertification

  Where the effects of reduced rainfall and human pressures are combined there is reduced vegetation cover and soil moisture which may create self-perpetuating severe droughts and increase human vulnerability to the drought hazard – positive feedback loop
• The Millennium Drought, South East Australia

  • Longest recorded period of below average rainfall in south east Australia.
  • El Nino events in 2002-3 + 2006-7
  • Strengthening of the sub-tropical ridge (STR) brought dry conditions
  • The STR blocked storm tracks so they moved towards higher latitudes.
  • Global warming may be reducing the temperature differential between the Equator and Polar regions and this will weaken the general atmospheric circulation and mid-latitude weather systems
• An ecosystem is an interconnected community of living organisms within a defined physical environment.

  Ecosystem functioning refers to the biological, chemical and physical processes that take place within an ecosystem.

  Ecosystem stress refers to constraints on the development or survival of ecosystems. The constraints can be physical (drought), chemical (pollution) and biological (disease).

  All ecosystems are vulnerable to drought. Ecosystems are vital for human wellbeing, providing us with services such as clean air and intangible amenities such as aesthetics and psychological benefits.
• Wetlands

  A land area saturated with water, permanently or seasonally. They include marshes (fresh and salt), swamps and bogs

  Functions of a wetland include:

  • a stores within the hydrological cycle
  • a giant filter by trapping and recycling nutrients
  • have high biological productivity
  • provide a range of valuable goods and services: Primary production, Nutrient cycling and Life support in the carbon cycle, Fisheries, Tourist attractions, Flood control, Groundwater recharge, Shoreline protection, Aesthetic value, Recreation
• Wetlands

  Groundwater-fed wetlands experience a smaller range of water levels but are more vulnerable to prolonged drought over several seasons.

  The Environment Agency have noted that drought causes the areas of open water to dry up resulting in progressive loss of habitat. As moisture is lost, extended drying can lead to soil erosion. Soil is damaged so that at times of flood they can no longer retain moisture and absorb carbon so release carbon into the atmosphere.

  Drought can damage food webs so that some species are replaced by others, or some species may die out.
• Forest Ecosystems

  Living trees take on average 2-4 yrs to recover following a period of drought. Drought stressed trees lose foliage, have growth impairment, are more vulnerable to pests + diseases and experience damage to vascular tissue which impairs water movement. Pine forests are particularly vulnerable. Severe drought and unusually high temperatures affected the forest of the Four Corners region of SW USA. Forests are highly valued ecosystems. They regulate hydrological processes, timber production, wildlife habitat, carbon sequestration