water Flashcards
what is meteorological drought?
shortfalls in precipitation due to short-term variability in long-term average.
common in arid/semi-arid regions e.g. The Sahel
what is hydrological drought?
deficiencies in stream flow and groundwater levels decreased, due to reduced precipitation and high evaporation.
what is agricultural drought?
lack of rainfall leads to soil moisture deficiency and water availability, therefore reducing plant growth and biomass. e.g. overgrazing
what is socio-economic drought?
water demand exceeds availability due to; lack of precipitation, overuse of water supplies.
dependent on spatial variations in supply and demand.
what areas are at high risk of drought?
India, Brazil, Middle East
what areas are at mid-risk of drought?
Indonesia, Nepal, South Africa
what areas experience low-risk drought?
South East Australia, South East England, Sweden
what are factors that can influence drought?
human activity, atmospheric circulation, topography
what is the definition of drought?
an extended period of deficient rainfall relative to the statistical multi-year average of a region
what is the InterTropical Convergence Zone? (ITCZ)
- belt of low atmospheric pressure around equator, moving N/S seasonally
- high evaporation rate due to solar radiation
- causes wet season on arrival/dry season when it leaves
- sub-tropical high pressure zones (descending hadley cell) can block high humidity/rain-bearing air masses, changing the ITCZ, preventing wet season and causing drought
what is El Niño? (ENSO cycles)
- warm seawater in equatorial Pacific.
- usually in western Pacific, pushed by currents, trade winds & Walker circulation cell. upwelling replaces warm water with cool from deep ocean.
- every 7 years ‘pushers’ weaken, allowing water to move east towards western Americas.
- warm water causes higher evaporation & precipitation, cool water causes drier weather
how do mid-latitude blocking anticyclones cause drought?
- in mid-latitudes, frontal precipitation creates low-pressure systems forming along polar front, where tropical air rises over polar.
- depressions move west to east in mid-latitudes due to coriolis, their track directed by polar front jet stream
- jet stream loops stabilise or break-up, so high-pressure areas (anticyclones) from subtropics move north, bringing stable weather, low precipitation & heatwaves to uk, causing drought (up to 2 weeks)
what is atmospheric circulation?
- equatorial solar radiation warms air, which rises and starts convection. cooling and condensing, it makes rain.
- subtropical high-pressure zone of warm air from equator that has cooled and sinks, creating belt of hot, dry conditons.
- air reaches ground level, creates trade winds.
- trade winds meet ITCZ.
- warm air from sub-tropics to mid-latitudes meets polar air at polar front, rising to cause condensation and rain.
- warm air rises into polar front jet stream, transferred at high altitude to poles, to cool and sink. creating movement of air at ground level back to equator.
what is the ITCZ position throughout the year?
- moves with seasons
- in Northern Hemisphere summer (June/August) ITCZ North of equator. December to February; Southern Hemisphere tilted to sun, so ITCZ is south of equator
what percentage of annual rainfall is in the summer in The Sahel?
85%
