1.2- Global Water Cycle Flashcards
What are the processes involved in transferring water within global water cycle?
1- precipitation= transfers water from atmosphere back to Earth’s surface
2- evaporation= water transferred back to atmosphere
3- water may INFILTRATE the ground or PERCOLATE slowly through the rocks as GROUNDWATER FLOW
Water is stored within 4 physical systems:
- lithosphere (lands)
- hydrosphere (liquid water)
- cryosphere (frozen water- snow and ice)
- atmosphere (air)
Liquid water dominates with about 98% of water in liquid form, predominantly in oceans
Of all the water on plant Earth what % is oceans and what % is freshwater?
- oceans =97%
- freshwater= 3%
Of the 3% of freshwater on Earth, what % is ice caps and glaciers, ground water and % easily accessible freshwater?
- ice caps and glaciers= 79%
- groundwater= 20%
- easily accessible surface freshwater=1%
Of the 1% of easily accessible surface freshwater what % is lakes, soil moisture, atmospheric water vapour, rivers and water within living organisms?
- lakes = over 50%
- soil moisture= around 40%
- atmospheric water vapour=8%
- rivers=1%
- water within living organisms= less than 1%
Regarding global distribution and size, what are the major water stores?
- cryospheric water
- oceanic water
- terrestrial water
- atmospheric water
Regarding oceanic water, oceans dominante the amount of available water and covers approximately __% of the planet’s surface
72
Oceanic water tastes salty because
It contains dissolved salts that allow it to stay liquid water below 0 degrees Celsius- they are alkaline with a PH of 8.4 but PH steadily falling however due to increase in atmospheric carbon
Major oceans include:
- Atlantic
- Arctic
- Indian
- Pacific
- Southern
What are the 5 main locations of cryospheric water?
1- sea ice 2- ice sheet 3- ice caps 4- Alpine glaciers e.g. mer de Glace, France 5- permafrost e.g. Alaska North slope
Discuss sea ice
- much of Artic ocean= frozen and the amount of which grows in summer and shrinks in winter
- same is true of waters surrounding Antarctica
- sea ice forms when water in oceans is cooled below freezing
- sea ice does not raise sea level when it melts because it is frozen from ocean water
Discuss ice sheets
- mass of glacial ice extending more than 50,000km squared e.g. Antarctic and Greenland ice sheets
- ice sheets form in areas where snow falls in winter and doesn’t entirely melt in summer
- ice sheets contain vast amounts of frozen water= if Antarctic Ice sheet melted, scientists estimated sea levels would rise by 60m
Discuss ice caps
- thick layer of ice on land smaller than 50,000km squared
- usually found in mountainous areas such as Himalayas and Andes
- major source for many glaciers
Discuss alpine glaciers
- thick masses of ice found in deep valleys or in upland hollows
- many feed major rivers such as 15,000 Himalayan glaciers support rivers such as Indus and Ganges that are the lifeline of millions of people in south east Asia
Discuss permafrost
- permanently frozen group that remains at or below 0 degrees Celsius for at least 2 consecutive years
- covers 1/4 of Earth’s surface including Northern Canada, Alaska and Siberia
Terrestrial water falls into 4 classes:
1- surface water
2- ground water
3- soil water
4- biological water
Discuss surface water
- free-flowing water of rivers, lakes and ponds
- rivers= both transfer and store of water
- make up minute % of all global water
- Amazon= largest river by distance of water in the world and accounts for approx 1/5 of world’s total river flow
- lakes= mostly lie in Northern hemisphere at high latitudes- Finland and Canada have high number of lakes
- largest lake by area = Caspian Sea
Regarding surface water, also discuss wetlands
- wetlands= areas of marsh, fen or peatland whether there is a dominance of vegetation areas where water covers soil
- found on every continent minus Antarctica
- world’s largest wetland system = Pantanal of South America
- it is a complex system of marshlands and lagoons and provides economic benefits by being huge areas for water purification and water supply
- wetland= main ecosystem in Arctic and stores enormous quantities of greenhouse gases and critical for biodiversity
Discuss groundwater
- groundwater= water that collects underground in pore spaces of rock
- just over 20% of all freshwater= stored in rocks deep below ground surface forming vast underground reservoirs called aquifers
- these sources= crucial for sustaining civilisation across the world
- depth at which rock becomes completely saturated with water = water table
- level of water table rises in response to:
Groundwater flow, abstraction by people, or recharge - groundwater flow eventually flows to the surface and can form wetlands
- amount of available groundwater = rapidly reducing due to extensive extraction for use in irrigation agriculture land in dry areas
Discuss soil water
- affects run-off and water quality
Discuss biological water
- biological water= all water stored in all biomass and varies depending on type and amount of vegetation
- e.g. areas of dense rainforest store much more water than deserts
- if vegetation is destroyed, store of water in trunk and branches lost to the atmosphere
Discuss atmospheric water
- exists in all 3 states (gas, liquid, solid)
- most common atmospheric water exists as a gas: water vapour (WV)
- WV absorbs, reflects and scatters incoming solar radiation keeping the atmosphere at a temperature that can maintain life
- amount of WV that can be held in the air depends on its temperature- cold air cannot hold as much WV as warm air= poles quite dry whereas air over tropics= fairly humid
- small increase in WV will lead to increase in global temperatures leading to further rise in global WV level, thus further enhancing atmospheric warming = POSITIVE FEEDBACK
- clouds= visible mass of water droplets or ice crystals suspended in the atmosphere
- cloud formation is the result of air in lower atmosphere becoming saturated
- when cloud droplets grow they can eventually fall as rain
What are the key processes driving change in the magnitude of these stores over time and space?
1- evapo-transpiration
2- condensation
3- runoff generation
4- cryospheric processes
What is the approximate residence time for following stores: 1- groundwater 2- oceans 3- soil moisture 4- atmospheric moisture
1= approx 10,000 years 2= approx 4000 years 3= 2-50 weeks 4= 10 days
What is evaporation?
Transfer of water from a liquid state to a gaseous state (water vapour) and can occur from open water or wet surfaces- vast majority occurs from oceans-> atmosphere (makes sense as oceans cover approx 72% of planet’s surface)
What is transpiration?
The main force that ‘pulls’ water through the xylem vessels in the stem of a plant is the evaporation of water from the leaves through stomatal pores
Rate of evaporation depends upon several factors:
1- amount of solar energy (light intensity)
2- availability of water I.e. surface area e.g. more evaporation from a pond than a grassy field
3- humidity of air- closer the air is to saturation point, the slower the rate of evaporation
4- temperature of air- warmer air can hold more water vapour than cold air
As water evaporates, it uses energy in the form of latent heat and so
cools its surroundings
Globally, rates of evaporation over the ocean exceed terrestrial rates and this results in
A transfer of atmospheric moisture to the continents as moist air moves across the continents driven by global air mass circulation I.e. world-wide system of winds by which the necessary transport of heat from tropical to polar latitudes is achieved
What is condensation?
Conversion of a gaseous state to a liquid state
What is the dew point temperature?
As air cools it is able to hold less water vapour = if cooled sufficiently then it will get to a temperature at which it will become saturated- excess water in the atmosphere will then be converted to liquid water in the process of condensation
Water molecules require something to condense on e.g.
Minute particles of dust/smoke or of surfaces such as leaves, grass stems, windows etc that are below dew point temperature
What is precipitation?
Any product of the condensation of the atmospheric water vapour falling under the influence of gravity e.g. rain, sleet, snow and hail
Atmospheric moisture is returned to the terrestrial system through
Precipitation
Condensation is the direct cause of all forms of precipitation and takes place when either:
- temperature of the air is reduced to dew point but the volume remains constant
- volume of air increases but there is no addition of heat (adiabatic cooling)
temperature of the air is reduced to dew point but the volume remains constant occurs when:
warm moist air passes over cold surface
adiabatic cooling happens when air rises and expands in the lower pressure of the upper atmosphere occurring when:
a) masses of air of different temperatures and densities meet- less dense warm air rises over the dense cold air (frontal effect)
b) localised warm surfaces heat the air above- this expands, becomes less dense and rises (convectional effect)
Cloud formation and precipitation vary considerably
with time and space
The driving force behind global cloud formation and precipitation is the
Global atmospheric circulation model- 6 pressure cells circulating the Earth’s atmosphere
Summarise the global atmospheric circulation model
- at the equator where sun directly hits (not at angle/indirectly) high temperatures lead to high rates of evaporation
- warm, moist air rises, cools and condenses to form towering banks of cloud and heavy rainfall in a low pressure zone= ITCZ
- in mid latitudes, cloud formation is mostly driven by the convergence of warm air from the tropics and cold from the Arctic
- the boundary of these two distinct air masses- polar front- results in rising air and cloud (and rain) formation
- jet streams drive these unstable weather systems across mid latitudes, establishing the largely changeable conditions experienced in the UK
- cloud formation can occur on a more local scale- the formation of thunderstorms from intense convection activity is somewhat ‘hit and miss’, but it does clearly demonstrate the variations in both time and space of water cycle transfer process
Regarding run off, overland flow and river flow are relatively rapid whereas
Transit times to the ocean for deep groundwater can be thousands of years
Infiltration is a key process in determining how much water runs off and how much
Enters the soil as soil water storage/ soil through-flow and then percolates to bedrock to become groundwater flow where water can be stored up to 10,000 years
Surface run off is generated when
Rainfall intensity is greater than infiltration capacity or when rain falls on already saturated soils (where soil water store is full and the water table is at the surface)
Understanding groundwater flow, overland flow, infiltration and run off is essential to effective management of
Terrestrial water, including flood hazards
What are cryospheric processes?
Processes that affect the total mass of ice at any scale from local patches of frozen ground to global ice amounts- they include accumulation (build-up of ice mass) and ablation (loss of ice mass)
After oceanic water, the largest store of water on Earth takes form as
frozen water (ice)- 95% is locked up in two great ice sheets covering Antarctica and Greenland
While Earth’s ice masses may seem stable and lacking in change
This is far from the case- it is all a matter of timescale
Snow falling on glaciers and ice sheets becomes compressed and enters
Long-term storage, forming layers of glacial ice
There has thought to have been 5 major glacial periods in Earth’s history and during these sea levels
Were approx 120m lower than present and continental glaciers covered large parts of Europe, North America and Siberia
In between glacial periods are
Interglacials when global melting exceeds accumulation
On a shorter timescale, snow accumulated during the winter adds to the mass of a glacier or ice sheet- in summer, melting occurs or
Ice calves (breaks away)- most glaciers in the world are now shrinking and retreating
Explain how the melting of freshwater ice has a profound impact on sea levels
- total melting of all polar sheets could result in a 60m rise in sea level, adding a significant deal of water to the ocean store
- rising sea levels are a positive feedback on the rate of removal of glacial ice since they can destabilise glaciers leading to accelerated rates of iceberg calving
In contrast, by focusing on the processes operating on a local hill-slope, we can better see how a variety of
Human and physical factors can affect the water cycle
What are some examples of natural factors that affect the water cycle?
- if precipitation is heavy e.g. intense rainfall during a storm, the amount of water reaching the ground increases and so the magnitude of stores increases accordingly
- seasonal changes: in winter, snowfall and frozen ground interrupt the water transfers and thus the magnitude of stores
What are some examples of human-related factors that affect the water cycle?
- deforestation reduces interception and infiltration = increase in overland flow
- urbanisation occurs on slope = impermeable surface such as tarmac and concrete will drastically reduce infiltration- trees will probably be cut down too. Water will flow very quickly through drainage pipes to the nearest river channels
- farmers may use ditches to drain the land and so encourage water to flow more quickly to rivers- irrigation also increases the amount of water on the ground
One of the most influential processes operating on the hill-slope water cycle is
Infiltration- movement of water from the ground surface into the soil
Explain why infiltration is possibly the most influential process operating in the hill-slope water cycle
- water that is effectively trapped on the ground surface either because:
- the soil is saturated or frozen
- or because the rate of precipitation exceeds the capacity of the soil to absorb it
- will either be stored as surface storage, evaporate or start to flow downslope as overland flow; the rapid transfer of water overland is a major factor leading to flooding
- water that is able to infiltrate the soil may be stored for very long periods either within the soil or deep within the underlying bedrock
- soil water budget describes the changes in soil water store during the course of the year
Regarding soil water budget, what is soil moisture surplus?
- precipitation exceeds potential evapotranspiration rates
- Soil water store is full which gives a surplus of soil moisture for plant use and runoff into streams
- most likely Jan-March
Regarding soil water budget, what is soil moisture use?
- potential evapotranspiration rates exceed precipitation
- plants just rely on stored water which is gradually used up
- most likely April-June
Regarding soil water budget, what is soil moisture deficiency?
- plants must have adaptions to survive for long periods or land must be irrigated
- most likely July-September
Regarding soil moisture budget what is soil moisture recharge?
- precipitation rates exceed potential evapotranspiration rates
- soil water store starts to be recharged
- most likely October-December
Regarding soil moisture budget when is field capacity reached?
- When the soil water store is full after soil moisture recharge
- precipitation = potential evapotranspiration rates
- most likely December
Soil water budgets will vary considerably from place to place depending on:
- the type and depth of soil
- soil texture and permeability
- much is true for underlying bedrock, as its capacity to store and transfer water will depend upon its lithology and structure (permeability and porosity)
- e.g. water will move very rapidly through widely jointed limestone
