Drought

What is it? An extended period of below-average rainfall which impacts on the environment and/or people. Essentially, it is an extended water shortage. This can happen in humid environments eg the UK. Note Drought does NOT mean arid (ie area with low rainfall all the time). Drought does NOT mean desertification (ie degredation of land, often caused by people)

What causes droughts? Persistent sub-tropical high-pressure systems. Sinking air warms and clouds do not form. As a result there is reduced rainfall. El Nino & changes to ocean-surface temperatures. Lower temperatures reduce evaporation rates and change wind speed/direction. Changes in mid-latitude depression tracks eg Westerly winds moving southwards mean less rain at northerly latitudes. Anticyclones lasting weeks can block depressions and lead to calm (ie no wind), dry conditions. Changes in wind speed/direction may also reduce orographic and frontal rain.

Human causes of drought Deforestation – reduces water stored in soil and vegetation. This leads to drier air and reduced precipitation. Over-grazing – reduces vegetation cover which dries soils. Reduced evaporation means drier air and reduced precipitation.

How long do droughts last? Unlike most other hazards, droughts develop slowly and are often longer-term events (can last weeks, months, and in some cases, years).

Spatial extent

Spatial extent Semi-arid areas (often places with very unreliable rainfall) experience droughts more often. Examples include: The Sahel, Australia and Western USA. Feedback loops often exacerbate droughts eg over-grazing and deforestation reduces moisture levels which reduces rainfall.

How is drought measured? Due to the nature of drought, it is a difficult thing to quantify and measure. The Palmer Drought Severity Index was developed in 1965. It uses temperature and precipitation data to calculate water supply and demand.

How frequent are droughts? Have become more frequent in recent years in many places.

Can they be predicted? Satellite monitoring of weather systems can identify the onset of drought eg by analysing vegetation cover. Surveys of food production and consumption levels can also provide indications of drought.

What are the impacts? Reduced crop yields - food shortages & higher food costs. Overgrazing - land cannot support as many grazing animals. Damage to soil structure and soil erosion Added costs for farmers - drilling deeper wells Increase water abstraction from the ground can lead to subsidence

Impacts (continued) Increased cases of wildfires Reduced capacity at HEP schemes Water rationing Loss/damage to natural habitats Forced migrations Increased dust in the air - health problems