Drought Flashcards
Types of Drought
Meteorological drought - Defined by shortfalls in precipitation over a prolonger period.
This can then cause:
Hydrological drought: Defined by reduced stream flow and ground water levels
Agricultural drought - defined as shortages in soil moisture - impeding agricultural production.
Causes of drought
Short term changes in weather patterns
periodic climate cycles such as ENSO
long term climate trends driven by global warming
What is ENSO (El Niños and La Niñas)
The El Niño Southern oscillation describes the effects of periodic sea surface anomalies on climate. A positive anomaly occurs when sea surface temperatures are warmer than average whilst a negative anomaly occurs when sea surface temperatures are cooler than average.
El Niños are caused by a positive anomaly and occur when trade winds are reversed This means that trade winds blow from West to
East - from The Western Pacific (countries in Asia and Australasia) to the Eastern Pacific (countries in the Americas). Trade winds blow warmer surface winds towards South America. This water evaporates forming moist air which rises, creating a region of low pressure - causing heavy rainfall over the Americas. Cool dry air descends over the Western Pacific causing dry conditions leading to drought in Asia and Australasia.
la Niñas are caused by a negative anomaly causing normal trade winds which blow from East to West - From the Eastern Pacific (countries in the Americas) to the Western Pacific (countries in Asia and Australasia). trade winds blow warmer surface waters towards Asia and Australasia. This water evaporates forming moist air which rises, creating a region of low pressure - this results in heavy rainfall in Asia and Australasia. Cool, dry air descends over the Eastern Pacific causing drought in South America
. 0Drought and Desertification
- Rainfall patterns change, and rainfall becomes less reliable both seasonally and annually. This means the occasional drought can extend several years.
- Vegetation becomes stressed and begins to dies. This leaves soil bare.
- Bare soil is eroded by wind and occasional intense rainfall.
- When rain does fall, it is only for short, intense periods. This makes it difficult for the remaining soil to capture it. This leeches nutrients in the soil
Vegetation can no longer grow in the area - forming a desert.
Desertification in the Sahel
85% of annual rainfall in the Sahel Is concentrated in the Summer. there is also huge variability in rainfall from year to year.
The Sahel has had below-average rainfall for the last 50 years, occasionally broken by years of slightly above-average rainfall.
Human impacts accelerating climate change:
1. Burning fossil fuel - This is increasing global temperatures, increasing evaporation rates and increasing the risk of drought. (The general consensus is that dry places will continue to get drier whilst wet places will continue to get wetter)
- Population growth - leading to over-abstraction of groundwater stores. it also leads to overgrazing and over-cultivation
- Deforestation - removal of trees for fuel, housing and fencing decreases infiltration and increases soil erosion
- Over cultivation - Soil is quickly exhausted of nutrients and water
- Over grazing - Vegetation cover is destroyed by livestock.
What caused Australia’s Big Dry?
Began with the 1996 - 1998 El Niño years.
Relatively normal weather conditions up until 2001 which some parts of Western and Central Australia had above average rainfall whilst most of Eastern Australia had below average rainfall
2002 - 4th dries year since 1900
No above-average rainfall between 2002 and 2006
2007 and 2008 La Niña did not cause as much rain as much as they should’ve
Coupled with intense heat, this meant aquifers couldn’t be recharged
The drought officially ended in 2012 but reoccurred from 2016 to 2019
Impacts of Australia’s Big Dry
Australia’s agricultural sector lost $7.4bn in production and 70,000 jobs between 2002 and 2003
Australian sheep farmers lost half their flock
Dairy cow herds declined by 25%
Average farm was $412,000 in debt
$150,000 exit grants given to farmers to abandon their farms.
Less than 30% of Australian farmers expected their children to continue farm work
How was the Big Dry managed?
2003 restrictions in Sydney banned sprinkler usage once dam levels dropped below 40%. Also restrictions of car washing and garden work.
Household water usage declined 20% between 2004-2005 and 2008-2009
In 2007, Brisbane’s water usage decreased from 300 - 139 litres per person per day.
Western Corridor Recycled water scheme - $2.5 bn spent on building 3 water recycling plants. These generated 45bn litres of water in 3 years for Perth.
$13.2 bn spent on building desalination plants. During periods of intense rainfall, the desalinated water is no longer economically viable so can be stored until it is needed when levels drop.
$4.5 bn in transfer payments made to relieve debt
Impacts of drought on wetlands and forests.
Limited precipitation results in less interception as vegetation cover becomes scarce. This means a reduction in infiltration and percolation to groundwater stores causing water tables to fall. Evaporation rates may increase (only if there is water available to evaporate). transpiration rates decrease making wetlands less functional. Desiccation may also accelerate destruction by wildfires.
Drought in forests has been linked to increased susceptibility to fungal disease causing tree mortality to increase. This reduces the volume of CO” absorbed from the atmosphere. As trees die they also release lots of carbo stored as organic matter into the atmosphere.
The importance of wetlands.
Act as a temporary water stream, mitigating floods downstream.
High biological productivity
Protect the land from erosion
Provide humans with resources such as fish and peat for fuel
Recharge aquifers
High biodiversity
Peat is a valuable carbon sink
used for recreation
Chemically, they act as filters by trapping and recycling nutrient sand remove pollutants.
