EQ2: What factors influence the hydrological system over short- and long-term timescales? Flashcards
How is drought described by the National Drought Mitigation Centre in the USA?
An “insidious hazard of nature”, implying that it has a slow onset with harmful impacts that vary geographically.
How do some people measure drought?
Through impacts such as reservoir levels or crop losses.
Some quantify the rainfall deficit over a period of time.
What is the UN’s general definition for drought?
An extended period - a season, a year, or several years - of deficient rainfall relative to the statistical multi-year average for a region.
What does socio-economic drought measure drought in?
In terms of supply and demand for human use (domestic, farming, industry)
What is positive feedback?
A cyclical sequence of events that amplifies or increases change.
What is negative feedback?
A cyclical sequence of events that damps down or neutralises the effects of a system.
5.4 Deficits within the hydrological cycle results from
physical processes but can have significant impacts.
What pattern does drought have?
A dispersed pattern.
Approximately how much of the world’s land area has experienced some level of drought exposure?
38%
How much of the land surface has experienced severe drought exposure?
10%
What can the physical causes of drought in regions of the world be largely explained by?
The global atmospheric circulation system.
What is meteorological drought?
Occurs when long-term precipitation is much lower than normal, but there is no consensus regarding the threshold of the deficit or the minimum duration of the lack of precipitation that turns a dry spell into an official drought. It is region-specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable between climate types.
The degree of dryness compared to “normal” precipitation.
What is agricultural drought?
Occurs when there is insufficient soil moistures to meet the needs of a particular crop at a particular time. It is caused by a number of factors such as precipitation shortages, differences between actual and potential evapotranspiration, soil water deficits and reduced groundwater or reservoir levels. A deficit of rainfall over cropped areas during critical periods of the growth cycle can result in crop failures or underdeveloped crops with greatly depleted yields.
When is agricultural drought typically evident?
After a meteorological drought but before a hydrological drought.
What is hydrological drought?
Occurs when there are deficiencies in surface and subsurface water supplies as measured in rivers, reservoirs, lakes and groundwater. It originates with a deficiency of precipitation but is usually out of phase with or after the occurrence of meteorological and agricultural droughts, as it takes longer for precipitation deficiencies to reach some of the components of the hydrological system such as soil moisture, stream flow and groundwater or reservoir levels.
What is socio-economic drought?
Occurs when the water demand for social and economic purposes (such as crop irrigation or hydro-electric power) exceeds water availability. This could be a result of a weather-related shortfall in water supply or overuse of the available water supplies.
How does socio-economic drought vary from the other types of drought?
Its occurrence depends on temporal and spatial variations in supply and demand.
What is the first step of the global atmospheric circulation system?
- Intense solar radiation at the equator warms the air, which rises and and starts convection. The air cools as it rises and water vapour condenses to form rain.
First 3 steps of the global atmospheric circulation system?
- Intense solar radiation at the equator warms the air, which rises and and starts convection. The air cools as it rises and water vapour condenses to form rain.
- The subtropical high-pressure zone is created where air that had risen at the Equator has cooled and so sinks to form a belt of high air pressure and hot, dry conditions.
- The air returns to ground level at the Equator, creating trade winds
Is warm air dense or less dense?
Less dense
At the equator warm less dense air rises and creates what weather conditions and climatic zones?
Clouds and rain. Intertropical Convergence Zone (ITCZ)
Steps 4 - 7 of the global atmospheric circulation system?
- The trade winds meet at the Intertropical Convergence Zone (ITCZ) where the warmed air rises. The position of the ITCZ moves with the seasons (it migrates)
- In the northern hemisphere summer (June-August), the ITCZ is north of the Equator. In December to February, the southern hemisphere is tilted towards the Sun and therefore the ITCZ is south of the Equator. This movement causes alternating wet and dry seasons in the tropics.
- The warm air moving from the subtropics to the mid-latitudes meets cold polar air at the polar front, where the warm, less dense air rises, causing condensation and rainfall.
7.The warmer air rises into the polar front jet stream and is transferred at high altitude towards the poles, where it cools and sinks. This creates a movement of air at ground level back towards the equator.
Is cold air dense or less dense?
Dense
Does warm air rise or sink?
Rise
When air is rising, what area of pressure is created?
Low pressure area - more rainfall. Rainforests.
When air is descending, what area of pressure is created?
High pressure area - clear skies and less rainfall. Desert regions.
The subtropical high pressure zone is created when cool air sinks between which cells?
Hadley and Ferrel
High pressure creates what type of weather?
Anticyclone - clear skies and less rain
What is the ITCZ?
Intertropical Convergence Zone - a belt of low atmospheric pressure located around the Equator where the trade winds of the Northern and Southern Hemispheres come together.
How does the ITCZ move around the Equator?
Seasonally, e.g it’s further north during the northern hemisphere summer.
The ITCZ moves north in the Northern hemisphere in which season?
Summer
Explains what happens in the ITCZ.
The trade winds of the northern and southern hemispheres come together here, which leads to the development of frequent thunderstorms and heavy rain.
The UK experienced a prolonged period of below average rainfall in the South East due to what?
Blocking anticyclones. The stability of anticyclones, with their sinking air and calm conditions, means that they can persist and block weather systems.
Sometimes, what can the subtropical high-pressure zone (associated with the descending part of the Hadley convection cell) do?
Block the high humidity, rain-bearing air masses associated with the ITCZ.
In the mid-latitudes, what system is frontal precipitation created in?
Low-pressure systems that form along the polar front
What direction do depressions move in the mid-latitudes?
From west to east, as a result of the Coriolis force (caused by the rotation of the Earth)
What is the polar front jet stream?
A very fast-moving, meandering belt of air in the upper troposphere.
What allows high-pressure areas (anti-cyclones) from the subtropics to move northwards?
The loops of the polar front jet stream occasionally stabilising, or breaking up
How can anticyclones cause drought?
They can persist and block weather systems from the West for up to two weeks. If this situation is repeated over the months, normal precipitation levels are greatly reduced.
What happened from 2010 - 2012 in central, eastern and southern England and Wales?
Experience a prolonged period of below-average rainfall due to blocking anticyclones.
What are jet streams?
Fast flowing currents of air.
To summarise, what are the 3 physical causes of drought?
- The Intertropical Convergence Zone
- Mid Latitude Anticyclone Blocking
- El Niño Southern Oscillation Cycles (ENSO)
What can droughts range from?
Short-term and localised precipitation deficits to longer-term trends that are part of climate change.
What is El Niño?
The warming of sea surface temperature that occurs every few years, typically concentrated in the central-east equatorial Pacific.
What is La Niña?
The term adopted for the opposite side of the fluctuation, which sees episodes of cooler than average sea surface temperature in the equatorial Pacific.
What is El Niño-Southern Oscillation (ENSO)?
A naturally occurring large mass of very warm seawater in the equatorial Pacific Ocean. Surface water temperatures increase and trade winds weaken.
What would happen in “normal” years in the Pacific basin?
Warm water normally located in the western Pacific is pushed by ocean currents, trade winds and the Walker circulation cell in the atmosphere. Air pushes water westwards (towards Australia) forming rainclouds in Australia.
What does ENSO trigger?
The occurrence of droughts and dry conditions.
Describe what happens during an El Niño event
Every few years the El Niño phenomenon takes place in the pacific ocean, around the equator. Pushing forces weaken, allowing the mass of warm water to move eastwards towards the West coasts of Central and South America.
Wherever this mass of warm water is located, evaporation rates are higher and precipitation greater, while areas of cooler water, such as the cold current that flows along the Peru-Chile coastline, bring drier weather. An El Niño event reduces precipitation in the western pacific (e.g Australia) and the affected countries experience drought e.g 1997-2009 Millennium Drought in Australia
What does an El Niño event reduce?
Precipitation in the western pacific.
What is the cold current that flows along the Peru-Chile coastline called?
The Humboldt Current.
Summarise El Niño
In El Niño, the trade winds weaken or reverse, leading to warmer surface water in the central Pacific, and less rainfall across Australia.
Often associated with dry conditions, drought, and severe bushfire seasons.
How often do El Niño events occur?
Every 2-7 years
How long do El Niño events last?
Usually 18 months
When does La Niña occur?
When normal conditions are exacerbated. It’s the flip side of oscillation, when the warm mass of water is pushed even further west than normal.
Summarise La Niña
Strong trade winds blow west across the Pacific Ocean. Warm surface water is pushed towards Asia and the seas north of Australia, resulting in higher rainfall than normal and flooding.
The weather event has the inverse effect across the Pacific, as drought plagues the coastal regions of Chile and Peru.
La Niña causes higher than normal rainfall in Australia and the Central Andes, which in turn causes catastrophic flooding in Bolivia.
Where can La Niña cause drought?
Chile
Peru
California
So what are the two phases of ENSO?
El Niño and La Niña
While it originates in the Pacific, what is ENSO now known to cause?
Global variations in rainfall patterns by changing the global atmospheric circulation, creating both droughts and floods in different areas of the world.
What severe drought was attributed to La Niña?
East African drought of 2011
What changes the direction of trade winds?
Coriolis effect
Global impact of El Niño?
Changes the odds of floods, droughts and can raise global temps.
In a “normal” year the East coast of Australia experiences what kind of weather?
Rain - evaporation of the ocean causes warm, moist air, which cools and condenses forming rain.
In an El Niño year, what kind of weather does Australia experience and why?
Drought - El Niño reduces precipitation in the Western pacific. Sinking air causes dry conditions, causing drought.
In La Niña years, what weather conditions does South America face?
Drought in Peru and California. Unusually high air pressure due to sinking air causing dry weather.
What kind of trade winds cause El Niño?
Weak trade winds.
When is El Niño declared?
When sea temperatures in the tropical eastern zone reach 0.5°C above the long-term average.
What can El Niño limit?
Development of tropical storms in the North Atlantic.
What does anthropogenic mean?
Human induced.
Albedo?
Reflective property.
Latent heat?
Related to changes in phase between liquids, gases and solids.
Sensible heat?
Related to changes in temperature of a gas or object with no change in phase.
In the UK in February 2020, why was there intense flooding and storms?
La Niña
Why do some people think droughts can no longer only be due to physical causes?
Society is not a passive victim of drought. Humans have indirectly affected and development of droughts by changing land uses and altering hydrological processes e.g deforestation and overgrazing reduce vegetation cover.
Human responses to water shortages influence water levels in reservoirs, aquifers, and rivers.
How have humans directly affected the development of droughts?
Abstracting water from rivers and groundwater.
How have humans directly reduced the downstream supply of water?
Building reservoirs and water transfers.
How can deforestation cause drought?
Reduces vegetation cover which reduces evapotranspiration rates, thereby reducing atmospheric moisture and precipitation. Higher sensible heat loss.
How does overgrazing cause drought?
Reduces vegetation cover which reduces evapotranspiration rates, thereby reducing atmospheric moisture and precipitation. Higher sensible heat loss.
What can surpluses within the hydrological cycle lead to?
Flooding.
What can surpluses within the hydrological cycle lead to?
Flooding.
Quick examples of causes of flooding
Intense storms
Unusually heavy or prolonged rainfall Extreme monsoonal rainfall
Rapid snowmelt during a particularly warm spring, as on the plains of Siberia.
Surface water flooding
Flooding the occurs when intense rainfall has insufficient time to infiltrate the soil, so flows overland.
Groundwater flooding
Flooding that occurs after the ground has become saturated from prolonged heavy rainfall.
Flash flooding
A flood with an exceptionally short lag time - often minutes or hours. Happen very quickly and often without warning
Jokulhlaup
A type of glacial outburst flood that occurs when the dam containing a glacial lake fails.
Example of flash flood
In November 2013 thunderstorms on the island of Sardinia caused flash floods resulting in at least 18 deaths and more than US$1.14 billion in damage.
What can prolonged rainfall be caused by?
A series of mid-latitude depressions passing over the same place.
What is much of the UK’s flooding caused by?
Mid-latitude depressions brought to the UK by the polar front.
List 6 physical causes of flooding
Amount and type of rain (e.g monsoon)
Relief
Temperature
Previous weather conditions
Snowmelt
Rock type
How does amount and type of rain (e.g monsoon rains) cause flooding?
Prolonged rainfall and intense rainfall leads to an increase in surface runoff (short lag time). Saturated ground becomes impermeable meaning little or no water can infiltrate/percolate into the soil rock below.
Summer monsoon rains in India and South East Asia are often heavy, as the ITCZ moves northwards and warm moist air follows behind. Brings humid climate and torrential rain.
What is a monsoon?
A seasonal change in the direction of prevailing winds of a world region. In India and South East Asia, the summer monsoon is associated with very heavy rainfall.
How does relief cause flooding?
Cumbria case study - the Cumbrian fells created orographic rainfall (occurs in areas where land increases height).
Orographic rainfall - warmer moist Atlantic air from the south-west is forced to rise as it reaches the western uplands. As it rises, it cools over the high ground producing heavy rain over western and northern parts of the UK.
In Cumbria - the moist air mass stayed over Cumbria for up to 48hrs - delivering record amounts of rain.
How does temperature cause flooding?
Convectional rainfall
Bigger differential in heat causes storms
How do previous weather conditions cause flooding?
If the ground is already saturated, water will be more likely to runoff along the surface, which quickly moves water to river channels and increases their discharge. Increases likelihood of flash flooding.
Drought - makes ground crack, so can’t hold water. Results in flooding.
How does snowmelt cause flooding?
Flooding can arise after winter snowfall, when a sudden rise in air temperatures in spring causes rapid snowmelt, especially in mountain environments. Results in increasing surface runoff and flash floods.
Examples of snowmelt causing flooding
Red River in North Dakota, USA
Norfolk 2013
How does rock type cause flooding?
Permeable - more infiltration
Impermeable e.g metamorphic - more surface runoff
Pervious
Rocks with faults - water can pass through
Porous rocks
Can allow water to pass through
Impervious surfaces
Doesn’t allow water to pass through
What are reasons the likelihood of flooding increases?
Low-lying areas with impervious surfaces, as in towns and cities
Where the ground surface is underlain by impermeable rocks
When ice dams suddenly melt and the waters in glacial lakes are released
Where volcanic activity generates meltwater beneath ice sheets that is suddenly released (jokulhlaups)
Where earthquakes cause the failure of dams or landslides that block rivers
Low pressure
Clouds and precipitation
What does the IPCC predict about climate change and the hydrological cycle?
Predicts considerable changes to the global hydrological cycle as a result of increasing greenhouse has concentrations
Describe flow diagram showing changes to flooding as a result of climate change
Increasing anthropogenic greenhouse gas emissions
↓
Increasing heat trapped in the atmosphere (enhanced global warming)
↓
Increasing average air temperatures and sea surface temperatures
↓
Increasing convection and evaporation
↓
Increased condensation and cloud cover
↓
Increased precipitation in low-pressure regions in the tropics and mid-latitudes
↓
INCREASING FLOOD RISK in these locations
Describe flow diagram showing changes to flooding as a result of climate change (snowmelt)
Increasing anthropogenic greenhouse gas emissions
↓
Increasing heat trapped in the atmosphere (enhanced global warming)
↓
Increasing average air temperatures and sea surface temperatures
↓
More snowmelt, decreased permafrost and ice cover. Initial increase in meltwater will increase river floods
Describe flow diagram showing changes to drought as a result of climate change
Increasing anthropogenic greenhouse gas emissions
↓
Increasing heat trapped in the atmosphere (enhanced global warming)
↓
Increasing average air temperatures and sea surface temperatures
↓
Increasing intense high pressure systems in certain seasons or high-pressure regions, e.g subtropics and drier soils
↓
Decreased humidity and precipitation in the locations
↓
INCREASING INCIDENCE AND SEVERITY OF DROUGHT
List some potential impacts of climate change on the UK water cycle by 2050
Rainfall events will be much more intense (heavy downpour)
More flooding expected
Coldwater species will decline, with increases in invasive species
Estuaries at increasing risk from floods
Sea level will rise between 0.4m and 1m by 2100
Urban areas will see more flash flooding
Higher demand for water
Climate change affects…
inputs and outputs within the hydrological cycle: trends in precipitation and evaporation
What does climate change to naturally occurring physical causes of drought and flooding? e.g ENSO cycles
Enhances and amplifies them
Summarise how climate change will affect the global hydrological cycle
Warmer temperatures will increase the atmosphere’s ability to hold water vapour, therefore rain will be more intense
Change in storm patterns
As temperature rises, more likely precipitation will fall as rain rather than snow, e.g in Western USA
Warmer temperatures will cause earlier snow melt, earlier melt means less water is available for human use
More flooding
Increased incidence of drought
What will climate change result in for the hydrological cycle?
an intensification, acceleration or enhancement of the global hydrological system.
How will climate change impacts on the hydrological cycle vary around the world?
Differential amounts of temperature increase and varying changes in totals and distribution of rainfall, there will be differential changes in the way the hydrological cycle operates within the world’s drainage basins.
Different impacts will be experienced in different climatic zones
What does climate change mean countries will have to cope with?
Countries will have to cope with changes in water budgets, which in turn will impact how climate change is managed to secure water in the future.
Summarise points of the article on climate change affecting hydrological cycle
https://earthobservatory.nasa.gov/Features/Water/page3.php
An increase in precipitation intensity: as the lower atmosphere becomes warmer, evaporation rates will increase, resulting in an increase in the amount of moisture circulating throughout the troposphere (lower atmosphere). An observed consequence of higher water vapor concentrations is the increased frequency of intense precipitation events, mainly over land areas. Furthermore, because of warmer temperatures, more precipitation is falling as rain rather than snow
In parts of the Northern Hemisphere, an earlier arrival of spring-like conditions is leading to earlier peaks in snowmelt and resulting river flows. As a consequence, seasons with the highest water demand, typically summer and fall, are being impacted by a reduced availability of fresh water.
Warmer temperatures have led to increased drying of the land surface in some areas, with the effect of an increased incidence and severity of drought.
What percentage of the world’s land area is severely affected by drought?
30%
This has doubled over the last 30 years
What areas are particularly affected by drought?
Southern Europe, parts of USA, parts of Asia, eastern Australia
What does the IPCC consistently predict about climate change and water cycle?
Precipitation will become more variable
Increased risk of droughts of floods
IPCC represent serious concerns over the security of water supplies in the future.
Why is modelling climate change trends difficult?
Climate dynamics – the way the atmosphere, ocean, terrestrial, cryosphere and biosphere interact is only partially understood
Difficult to distinguish between the impacts of oscillations such as ENSO and climate warming
Global records are very incomplete – no reliable evidence on trends in climate change in may parts of the world, making firm predictions hard
Impacts of droughts on ecosystem functioning -
Why are ecosystems vital for human wellbeing?
Provide us with innumerable and invaluable services, from tangible amenities such as clean air and water, food and fuel, to intangible amenities such as aesthetic and psychological benefits
Define ecosystems
Interconnected communities of living organisms within a defined physical environment
What does the term “ecosystem functioning” refer to?
The biological, chemical and physical processes that take place within the ecosystem
Why are ecosystems vulnerable to drought?
All ecosystem components and processes are dependent on supplies of water
Levels of ecosystem resilience to drought vary, what ecosystems are most resilient?
Desert and semi-arid ecosystems
What’s a wetland?
A land area saturated with water, either permanently or seasonally, including marshes, swamps, and fens.
Example of a large wetland
The Everglades in Florida
Impact of drought on wetland ecosystems? (Environment Agency research)
As the supply of water is reduced, areas of open water shrink or dry up altogether, resulting in progressive loss of habitat.
Soil moisture is reduced, and extended drying can lead to soil erosion and a reduced ability to store water in times of flood (potentially increasing downstream flood risk) or to release water in times of drought.
Organic soils may oxidise, releasing carbon in the atmosphere; as water availability diminishes, concentrations of dissolved nutrients of dissolved nutrients or pollutants may increase.
Why do birds, terrestrial vegetation and invertebrates show varying responses to drought?
Because it will depend on species’ resistance and resilience, competitive and predatory interactions, availability of food, and time and characteristic of drought.
How can drought alter communities?
By eliminating some species and creating gaps in food webs in which other species can establish themselves.
How will snipe and thrush be affected by drought?
Because the dry soil surface will be less penetrable, limiting their ability to feed on soil invertebrates.
Two types of wetlands which can experience drought?
River-fed wetlands
Groundwater-fed wetlands
How do river-fed wetlands cope with drought?
Because river-fed wetlands experience a wider range of water levels naturally, they support communities that may be more adapted to such fluctuations.
How do groundwater-fed wetlands cope with drought?
Because they experience a smaller range of water levels, supporting communities that may thrive in this environment are therefore more vulnerable to prolonged drought over several seasons.
Why are forests highly valued?
For their ecological functions and services, including water storage and regulation of hydrological processes, timber production, wildlife habitat, carbon sequestration and recreational opportunities.
How long to trees take to recover and resume normal growth rates following a period of drought?
Average 2 to 4 years.
What tree species has a low resilience to drought?
Pine - tends to keep using water at a high rate, even during a drought.
Impacts (long-term) of drought on trees and forests?
Foliage loss, impairing growth, increased accumulation of pests and diseases, reduced carbon sequestration, lasting damage to vascular tissues which impairs water transport.
Example of forest ecosystem affected by severe drought
Piñon pines, New Mexico
How are forests important for carbon sequestration?
Global Footprint Network (GFN) calculated that forests store an average of 0.73 tonnes of carbon per hectare per year.
