Water Cycle

  • Created by: holly_u
  • Created on: 07-04-18 16:10

The Global Hydrological Cycle

  • The global hydrological cycle is a closed system, all water is circulated through the stores.
  • Works through inputs (precipitation), outputs (evaporation), stores (lakes) and flows.
  • It's driven by solar energy (evaporation) and gravitational potential (flow downhill)

Stores - resevoirs of water

Fluxes- they measure rate of flow between stores

Processes- physical mechanisms driving the fluxes.

Types of stores and size order:
- Oceans= 96.9% of total
- Cryosphere (ice)= 1.9% of total                  LONGEST RESIDENCE TIME (15,000 years)
- Groundwater= 1.1% of total        
- Blue water, visible (Rivers and lakes)= 0.01% of total
- Green water, invisible (vegetation) = 0.01% of total
- Atmosphere= 0.001% of total                   SHORTEST RESIDENCE TIME (10 days) 

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Water Budget

  • Only 2.5% of the worlds water is not saline (salty)
  • Most of this water (69%) is unavailable for human use as it's locked in the cryosphere (glaciers, snowflakes, ice sheets) which takes 15,000 years to become available
  • Also 30% is groundwater. Some is inaccessbile fossil water (ancient deep underground. It's non-renewable as no water source)
  • Current technology only allows us to access rivers (main source for humans(, lakes and soil moisture
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The Drainage Basin- Inputs

Drainage basin is a sub-system. It's an OPEN system as water levels vary over time (short and long term). Watershed = area water collects in rivers. Source= where the river starts. Confluence= two rivers meet. Tributary= smaller river running into large 

INPUTS- Precipitation (rain, snow, hail)

Types of rainfall:
Convectional- Ground is heated by the sun, air above heats up too, expands and rises, as it rises it cools and condenses, forms water which falls

Cyclonic- Warm air (lighter) is forced over cold air (dense), it loses it's ability to hold water. Rain forms

Orographic- air forced over a barrier e.g mountain it cools and condenses. Rain shadow created when one side of mountain doesn't recieve rain as mountains shelter it. 

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Impacts of precipitation:
Seasonality (winter = more rainful)

Rainfall amount (large amount= more in stores and so more processes)

Type of precipitation (snow= slows processes as has to melt)

Intensity of precipitation (more intense= harder to infiltrate so more on surface)

Distribution (If all focused on one area- flooding and 'drying up' in other places

Variability (Long term- climate change)

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Drainage Basin -Flows

INTERCEPTION- water is stored in vegetation by absorbtion and transpiration. Factors: type of vegetation e.g coniferous forests (needles) more than deciduous (lose leaves in autumn). 

INFILTRATION- the movement of water from the ground surface into soil. Factors affecting: infiltration capacity, rate, soil texture (sand/clay), type and amount of vegetation (more=more infiltration), slope angle and soil structure.

DIRECT RUN-OFF- water that flows over ground directly into streams/ rivers or lakes.

SATURATED OVERLAND FLOW- (surface run-off) saturated soil doesn't infiltrate and so causes movement of water unconfined by a channel across the ground. Water table rises.

THROUGHFLOW- movement of water down a slope in the soil via natural pipes. Slow.

PERCOLATION- deep infiltration of water into permeable bedrock beneath. 

GROUNDWATER FLOW- slow transfer of percolated water through porous rock.

CHANNEL FLOW- flow in river after overland, through and grounwater flow reaches it. 

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Drainage Basin - Outputs

EVAPORATION- physical process which soil moisture is lost to the atmosphere from the Sun's heat. The water vaporises from a liquid to a gas.

TRANSPIRATION- water is lost from plans to the atmousphere through tiny pores e.g in the leaves.

EVAPOTRANSPIRATION- combined effect of evaporation and transpiration. It accounts for nearly 100% of removal of precipitation. 

POTENTIAL EVAPOTRANSPIRATION- amount of water loss (evaporation & transpiration) that would occur if the sources were available. 

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Physical factors

The relative importance of inputs, flows and outputs is determined by physical factors.

CLIMATE- climate affects type and amount of precipitation  and type of vegetatione.g humid climate- convectional rainfall and rainforests. 

SOILS- determine amount of infiltration and throughflow

GEOLOGY- affects sub-surface processes e.g percolation and groundwater flow. If impermeable- less 

RELIEF- slopes affect amount of run-off. Steep = more run-off less infiltration.

VEGETATION- presence or absence of vegetation affects interception, infiltration and overland flow. 

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Human Factors

DEFORESTATION (impact on interception)- cutting down of trees to make space for farming, buildings etc. therefore reducing interception as less vegetation.
e.g Amazon Rainforest. 17% of vegetation lost in 50 years for cattle ranching.

LAND USE CHANGE (impact on infiltration) - human activities transform the land by a change in the way it's used. Converting to farmland reduces infiltration and causes soil compaction.

CLOUD SEEDING (impact on precipitation)- weather modification changing amount of precipitation by dispersing subtances in air. Increases rainfall in areas of drought.

WATER ABSTRACTION (impact on groundwater)- taking water from a ground source. 
e.g The Aral Sea, abstraction for the growth of cotton from rivers so less in sea.

NEW WATER STORAGE RESEVOIR- artificial lakes where water is stored, usually Dams
e.g 1960s Dam across Nile to reduce flooding. Caused less fertile soil.

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Local scale- Water Budgets

Water Budgets= the balance between precipitation, evaporation and run-off at global, regional and local scales.

  • Rivers flowing in regions with surplus water are vital for supplying zones in deficit e.g Nile supplying Egypts desert.
  • Human activity and climate change modify short and long term.

They provide a good indication of water supplies. 
If inputs and outputs are balanced water availability is low (deficiency) e.g Africa
If inputs are larger than outputs water availability is high (surplus) e.g South America

Water budget graph: (Southern England)
Winter= precipitation exceeds evaporation so available supply
Summer= evaporation exceeds precipitation so soil supply used. End of summer water deficit
Autumn= precipitation increased again

Climate type affects soil water availability: Temperate- warm summer, cold winter e.g UK
Tropical- all year 18 degrees or above. Wet and dry season. High deficit and surplus e.g 
Polar- cold all year round. Water stored in cryosphere e.g Arctic

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River Regimes

River regime= annual variation in discharge/ flow of a river at a particular point seasonally. 

  • Most from groundwater between periods of rain not immediate run-off. 
  • Groundwater masks fluctuations caused by precipitation. 
  • e.g River Kennet. Water flows over chalk. This masks fluctuations because of the chalks grounwater aquifers.

Influenced by: size of river,
the amount and type of precipitation,
temperatures e.g summer = more evaporation,
geology and overlying soils (permability, porosity),
amount of vegetation and human activities e.g Dams.

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Storm Hydrographs

Storm Hydrographs- show variation of discharge within short period of time e.g a storm

Before= main water supply is base-flow

1) once rainfall starts the rising limb shows how quickly it increases

2) Peak discharge is reached after peak rainfall because of lag time (time taken for water to move through system)

3) once storm stops falling limb shows reduced discharge

4) Eventually returning to normal base flow


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Factors affecting storm hydrographs

FLASHY RIVER- steep rising limb, high peak discharge, short lag time.
Intense storms or snowmelt from temperatures rising. 
Impermable rocks, so high surface run-off
Low infiltration of soils e.g clay
Steep relief, so more surface run-off
Small basin
Low vegetation levels so less interception
Urbanisation (human activitiy)

FLAT RIVER- long lag time, loq peak, gentle rising limb
Steady rainfall or slow snow melt
Permeable rocks so limited surface run-off
High infiltration, making movement through system slower
Gentle gradient slopes
Larger basin takes longer for water to travel
Dense, deciduous woodland so interception
Low population density

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Impacts of urbanisation

  • Buildings clear vegetation which exposes soil
  • High density of houses means rain falls down drains 
  • Drain and sewars drain water quickly
  • Bridges restrain discharge and act as dams for floods
  • Highly populated urban areas e.g London vulnerable in extreme weather events. Expensive properties effected. 
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Causes of Drought- meteorological

Drought= an extended period of deficit rainfall

METEOROLIGICAL DROUGHT- shortfalls in precipitation as a result of short-term variability or longer term trends, it varies between climates. 

El Nino, Pacific Ocean-
1) Weakened trade winds (tropical winds blowing to West in the Pacific)
2) So warm water towards the East
3) Warm water and moisture causes more precipitation and storms in South America
4) Reduced rainfall in West so drought e.g Asia and Australia

El Nina-
1) Strengthening of trade winds (pushing warm water westwards)
2) Low pressure and warm water heat atmosphere
3) Heavy rainfall in Asia, West
4) Drought in South America, East

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Causes of Drought- hydrological

HYDROLOGICAL DROUGHT- reduced streamflow and groundwater levels because of a lack of precipitation and high levels of evaporation. 

Effects: reduced storage in water resevoirs e.g lakes and threat to wetlands and wildlife, less water for urban supply

e.g Brazil- no permanent water supplies leading to ill health and reliance on water from road tankers

Agricultural Drought- rainfall deficiency from meteoroligical drought leads to deficiency in soil mositure so reduced plant growth. 

Famine Drought- Meteoroligical, Hydrological and Agricultural drought can lead to famine drought. This is the wide-spread failure of agricultural systems, food shortages devloping into famines. Population growth causes demand for water to grow.

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Human Activity

Human activitiy does not cause drought but acts as a positive feedback loop. 

  • Overgrazing
  • Deforestation for fuelwood
  • High population density
  • Over-extraction of groundwater supplies


Physical reasons for: 
Variable rainfall due to climate
El Nino- weakened trade winds, so less water moisture so drought
'The Big Dry' associated with climate change. 

Human reasons for:
Adelaide drew 40% of drinking water from River Murray. Over extraction has caused no water to run to mouth anymore.
Growing population with affluent lifestyles
Groundwater over-extracted for industrial and urban use

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Causes of desertification

Desertification= land degradation in arid and dry sub-humid regions due to climate change an human activities.

Physical factors- less rainfall- global warming (higher temperatures and increased evaporation
              effects- rivers/ resevoirs dry up, vegetation dies, reduced protection, soil exposed

Human factors- High birth rates, low death, Immigrants from wars etc, Increase in animals
             effects- farmers forced to change traditional methods of land use for food production,                                  .           increased demand for wood, 40% increase in livestock need water

So overgrazing (soil no nutrients), overcultivation, deforestation, vegetation removed and decrease in protective layer, soil exposed and eroded.             

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Impacts of drought on ecosystems

Impacts on ecosytems
WETLANDS (an area of marsh, peatland, bogs... with water either permanently or seasonally flowing or stored there)

Functions: temporary water stores, water filters, biological productivity and providers of resources e.g fish and fuelwood

Impacts: vegetation deteriorates so less protected surface, less percolation to groundwater stores, if wildfires occur even more destruction, water extraction schemes reducing water levels, 

Resilience: river-fed wetlands have higher levels of water so able to adapt to fluctuations, groundwater-fed have smaller levels of water supplies so more vulnerable to prolonged drought,
drought eliminates some species allowing others to thrive e.g aquatic birds leave so land-based can live e.g swallows

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Impacts of drought on ecosystems continued

FORESTS (a large area dominated by trees)

Function: stabilise the environment e.g water circulation, soil erosion, provide resources e.g timber, pollution control, recreation

Impacts: less interception so less photosynthesis, increased suceptability to pests/ diseases, higher likelihood of forest fires (dry wood), death of trees e.g California 1 billion trees impactes due to 4 year drought starting 2011. 

Resilience: Plants close stomata in leaves to reduce transpiration, if continued trees drop leaves to conserve water, but after this they will die. Not very resilient to drought conditions

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Environments at risk of flooding:
- Low lying flood plains or river estuaries - Low lying and urbanised. Affected by surface run-off and precipitation - Small basins subject to flash flooding 

Types of flooding:
- Flash flooding = flood with very short lag time- minutes to hours
- Groundwater flooding= flooding after the ground is saturated from heavy rainfall
- Surface water flooding= flooding occuring after intense rainfall has infiltrated into soil 

Causes of flooding:
-Meteorological (short-term weather events):

  • Prolonged heavy rainfall e.g UK, 
  • monsoonal rainfall e.g Bangladesh, Asia. 80% people exposed to flood risk,
  • Snow melt causes by rapid transition to summer and rainfall melting it e.g Bangladesh leads to Jokulhaup (rapid melting of glaciers)
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Flooding continued

Human factors

  • Urbanisation, using impermable materials e.g concrete. London 60% paved over front gardens and drainage systems e.g drains speed up lag time
  • Land use change e.g deforestation, overgrazing- upstream affects down stream,
  •  hard engineering e.g channel straightening Mississipi speeds up flow,
  • Climate change creating more precipitation

Physical factors

  •  temperature
  • relief of land- steeper means more surface run-off and reduced lag time
  • rock type - Impermable clay reduce lag times
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Impacts of flooding

Flood frequency= return period. Likelihood of a flood event to occur. The higher the flood magnitude the longer the return period.

+ recharge groundwater 
+ encourgae breeding and migration
- oversupplies of sediment so destruction of aquatic plants
- pollution from chemicals introduced into systems. Poisons wildlife in soil e.g worms

- developing countries people can't swim, particularly children and elderly
- water-borne diseases e.g cholera
- psychological stress
- agricultural land destroyed leading to food price escalation e.g 'Big Dry' Australia
- destruction of infrastructure e.g factories so loss of jobs
- loss of tourism

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Flooding Case Studies

BOSCASTLE FLOOD, 2004 (flash flood)
Causes= saturated ground, steep relief of drainage basin, confluence of 3 rivers, 200mm fell in 24 hours

short term- people left without houses/ belongings, 84 cars and caravans washed into sea, 2 million tonnes water flowed through in 1 day
long term- stress and anxiety, damage costing £60 million, river banks eroded, coastal pollution

Responses= helicopters rescued 100 people from rooftops & trees, rivers widened, car park raised, new bridge built

Causes= climate change causing more rainfall, 60 hours of continuous rain, 'tsunami like' wave

Impacts= 1/5 Pakistan submerged by floods, countryside became swamps, 14 million affected, $1.5 billion damage to agriculture

Responses= Government couldn't cope, so military saved 30,000 people in 72 hours

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Affects of Climate Change

Hard to measure climate change:
- difficult to distinguish between ENSO and global warming
- insufficient detail in some parts of the world (LICs)

How climate change modifies the hydrological cycle:

  • Greenhouse effect (radiation trapped by earths atmosphere, warms planets surface)
  • POSITIVE FEEDBACK. Enhanced greenhouse effect = Increased greenhouse gases e.g CO2 = more radiation = more heat.
  • Warms up oceans linked to ENSO
  • Warms up land so melting surface water e.g permafrost (land permanently frozen) so rising sea levels, more evaporation so arid land
  • Changes in climate. Drier in some parts e.g Africa and wetter in others e.g Indonesia
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Trends of water cycle components

Trends in water cycle components:
Precipitation input= increase in intense rainfall but overall amounts steady or decreasing. More falling as rain not snow as warmer temperatures

Evaporation & evapotranspiration =research suggests Asia and North America actual evaporation is increasing. Increasing transpiration due to increased precipitation

Soil Moisture= where precipitation increases so does soil moisture

Run-off and stream flow= there will be an increase in hydrological extremes (like climate extremes). Accelerated cyle will increase run-off and reduce infiltration

Groundwater flow= limited evidence, as human extraction is main effect

Resevoir storage= Regional changes in resevoirs linked to regional changes in climate e.g Lake Chad

Permafrost= increasing temperatures leading to degradation in northern area. It releases methane so positive feedback

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Snow = length of snow cover has decreased and spring melt occurred earlier

Ice = glaciers retreated due to temperature increase and more rain not snow. 

Oceans= Sea surface research lagged behind land. Areas of ocean warming increased evaporation, limited evidence for more cyclones

Impacts of short-term Climate change (ENSO):

  • More frequent cyclones & monsoon weather
  • Increased frequency and intensity of drought
  • Decreasing rainfall in some area
  • Evaporation of surface water
  • Loss of snow and glaciers - threatens communities in Himalayas
  • Depleted aquifers
  • Reduced water as forest stores
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Water supply and demand

Water insecurity= when present and future water supplies cannot be guarunteed for human well-being, socio-economic devlopment & protection against water-borne pollution
Water scarcity = when the demand for water exceeds to available amount during a certain period
Physical scarcity= when 75% of a countries blue water (visible) is being used
Economic scarcity= when the development of blue water sources are limited by a lack of capitol (money), technology and good governance
Water stress= the lack of ability to meet human and economic demands
Virtual water= hidden water when food/ commodities are traded e.g in food

Reasons for rising demand: population growth,
rising standards of living e.g China eat more meat so more water needed,
more affluent people require more water e.g swimming pools,
economic growth e.g fracking

Water availability gap: imbalance in water useage. Rich countries use 10x more. Future nearly half of world will be water vulnerable

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Causes of Water Insecurity

Climate variability- 
low pressure zones of mid-latitudes most water secure
short-term ENSO making water security worse

Geology- Permeable rock e.g chalk can store lots of water, so not evaporated
Salt water encroachment- Contamination of salt water into fresh water supplies e.g snow melt entering rivers. So lack of fresh water, storm surges, rising sea levels.

Pollution of surface&groundwater- e.g 20% Bangladesh wells contaminated with arsenic chemical leading to health problems. Common pollution = sewage causing cholera, chemical fertilisers, industrial waste= toxins 
Over abstraction- for domestic, agricultural and industrial purposes e.g new technologies such as Fracking. 

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Water Scarcity case studies

Coca-Cola factory built pumping out groundwater for industry
Farmers need water for agriculture, essential for living

Water privatisation= private coorperations buy/operate water utilities meaning people have to pay
Meant that citizens can't afford to pay for water, so drinking unsafe supplies

Water privatisation has meant communities live by water factories but cannot afford & their wells are empty meaning death, infection e.g diarrhoeia, social problems -no friends.

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Patterns of Physical & Economic Scarcity

Water scarcity linked to: availability (water distribution network), access (freedom to use), useage (entitlement to water useage)

Mainly determined by climate- concentrations in high-pressure latitudinal bands
Topography (features of the land e.g steep relief)
Continentality (interior of Asia)

e.g parts of CHINA

Lack of capitol and technology to exploit adequate supplies


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Price of Water

Determined by:
- physical costs of obtaining e.g how far it has to be piped e.g  LA gets water from Colarado
- demand for water, higher= more expensive
- who supplies the water e.g private water companies = high price

Water is now seen more of a commodity 
Neo-liberal view to privatise water- to conserve supplies & profits made

World Bank made SAPs making countries privatise water supplies "as existing systems were not working"

e.g Bolivia 1999-2000, local company Bechtel, given permission to privatise causing protests and civil war caused. They even privatised rain water supplies.

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Importance of Water Supply

Importance for economic development:
- energy industry requires water 
- provides jobs
- farmers rely on storing rainwater and groundwater
- indisutrial irrigation e.g along the Nile used to yield crops 
- production of biofuels e.g biodiesal require lots of water

shrinking since 1960s due to irrigation for cotton farming in an unproductive region.
Caused: fishing industry to die, drinking water polluted by fertilisers, infertile land and poorly built pipes allowed water to leak and evaporate out

Human well-being:
- unsafe drinking water spreads disease but water for personal hygiene use prevents this.
- water-related diseaseare linked to poor sanitation due to open defacation e.g south asia. causing: cholera, tyhpoid etc also parasites e.g mosquito's causing malaria
- diseases cause high morbidity so poverty cycle
- improved water means more people in school/ work so reduced poverty

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Within Countries:

  • Simple conflicts about NIMBY (Not In My Back Yard) concern over environmental and soci-economic impacts e.g dam and resevoir in Kielder, Northumberland. Mostly consists of protests.

International (Trans-boundary): 

  • More likely to develop into wars due to conflicting territory. Groundwater is underground so boundaries are unclear

Worlds largest river with 10 countries competing for water
Reasons for disputes: high evaporation due to ENSO cycles and arid areas, unfair allocation of water due to historical agreements giving Egypt and Sudan lots. Ethiopia has major production but little capture.
2010 treaty that all countries equal rights
Has the right conditions to turn into conflict, but Egypt and Sudan politically weak.
River Nile Initiative is a cooperative system

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Hard Engineering Schemes

Diversion of water from one basin to another- river riversion / large canals to carry from surplus to deficit e.g South-North China. 3 canals from water secure south, to drought stricken north e.g Beijing. funded 60% by government. Some fear disaster along waterways.
+ successful 
- damages fish stocks, spreads diseases and pollution, climate change can lead to scarcity in source area, promotes unsustainable use in recieving area

Able to hold 15% of global run-off. e.g China's FDI programme builds dams in Africa
+ Can provide HEP which is important due to global warming (renewable energy), flood control and a domestic supply.
- expensive, NIMBY

Removal of salts and minerals from seawater. e.g A plant on the Thames
+ Less intensive and easier to implement large scale than freshwater exploitation
- Expensive and ecological impact on marine life

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Sustainable Schemes

Water Conservation
Smart Irrigation- advanced technology makes drip technology more effective by measuring temperatures & if they reachs certain point given water, so less water used.

Recycling- re-using city water waste for agricultural use as they grey water (bath, sink or washing water) doesn't have to be drinking water quality. 

Education of farmers- strategies to combat water scarcity e.g which crops use least water but good profits

Rules/laws put in place- sprinkler ban, not flushing toilet, sharing bath e.g South Africa

Water Restoration (restoring damaged lakes, wetlands & rivers)
Locally- replanting vegetation, restoring meanders

Globally- e.g Aral Sea, Kazakhstan. Restoration of northern part using World Bank loans

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Water Security: IWRM

Integrated Water Resource Management (IWRM)-
Process where stakeholders come together to make group decisions about policies
River basin viewed holistically
Features: environmental protection, good governance, freedom of corruption, effective regulation
Involves: monitoring technology, groundwater managment (aquifer storage), waterway management
Challenges: everyone has to work together difficult with conflict
Successes: Water quality improved & less pollution

Recently drought due to climate change
Urbanisation, population growth & agricultural need for irrigation
Split into upper and lower sections for management
1956 comprehensive plan covering water rights etc
1990 Lower basin used their full allocation
States looking for alternative e.g irrigation restriction& desalinisation -California

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Water Security: Water Sharing Frameworks

Most water is across boundaries
e.g Nile Basin with Egypt & Sudan

Helsinki rules for water sharing:
- social & economic needs- population etc
- natural factors- amount of rainfall, climate change
- dependency- alternative sources e.g Egpy 95% reliant on Nile
- downstream impacts- pollution
- efficiency- avoiding waste

UNECE (United Nations Economic Commission for Europe) Water Convention: promotes joint management in Europe

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Water Security: Water Treaties

Most water is across boundaries
e.g Nile Basin with Egypt & Sudan

1) Helsinki rules for water sharing: (applied to whole basins)
- social & economic needs- population etc
- natural factors- amount of rainfall, climate change
- dependency- alternative sources e.g Egpy 95% reliant on Nile
- downstream impacts- pollution
- efficiency- avoiding waste
However these aren't compulsory so not completely successful

2) UNECE Water Convention: promotes joint management in Europe. Includes monitoring, research, alarm systems etc...Very successful 

3) Nile Basin Initiative- partnership along river to develop in cooperative manner and promote peace and security. Supported by the World Bank. Little was achieved

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Conflicts Continued

RIVER COLORADO (Trans-boundary)
Disputes between 7 states, due to river over-use. No longer reaches the sea
Electricity from hydroelectric dams, water for livestock, recreation e.g rafting, provides water for nearby dry areas e.g Las Vegas

Water problems due to more affluent people, irrigated farmlands, population growth
GAP scheme- aims to develop socially & economically through building 22 dams & irrigate more water. 
Conflict: Syria and Iraq as the river supplies them with water 
They released water for Syria but not Iraq

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God bless you, this is exactly what I needed to refresh my mind on the whole of water cycle!!!

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