The water cycle and water insecurity

WHAT ARE THE PROCESSES OPERATING WITHIN THE HYDROLOGICAL CYCLE FROM GLOBAL TO LOCAL SCALE?

• The global hydrological cycle is a closed system of linked processes so there are no external inputs or outputs. For this reason, the amount of global water is finite and constant.
• The proportions of global water held in each state (liquid, vapour, ice) vary over time with changes in climate.
• The power that drives the hydrological cycle comes from two sources:
• Solar energy: in the form of heat - heated by the sun, the water on the earth's surface evaporates into the atmosphere, while water is also drawn from the soil by plants and evaporated from leaves and stems by the process of evapotranspiration. When humid air rises, condensation occurs at the cooler temperatures, forming clouds, and this eventually leads to precipitation and water is returned back to the rivers, lakes and oceans on the Earth's surface
• Gravitational potential energy: causes rivers to flow downhill and precipitation to fall to the ground - on land, the gravitational potential energy (from the water being stored in clouds at height above the surface of the earth) is converted into kinetic energy as the 

water moves through the system by plant interception or over land as surface runoff. Water also flows through the soil by the processses of infiltration and throughflow.

• Stores are reservoirs where water is held. There are four main stores:
• 1. Oceans
• 2. Glaciers and ice sheets (cryosphere)
• 3. Surface runoff - rivers, lakes, groundwater and the moisture held in soils and vegetation
• 4. Atmosphere
• The oceans represent by far the largest store (96.9%), followed by the cryosphere (1.9%).
• Of these freshwater stores, the cryosphere is the largest, accounting for 69% of all the global freshwater, followed by groundwater (30%). Less than 1% is stored in the biosphere (vegetation and soil moisture).
• Flows are the transfers of water from one store to another. There are four main flows:
• 1. Precipitation
• 2. Evaporation
• 3. Transpiration
• 4. Vapour transport
• Fluxes are the rates of flow between stores. The greatest fluxes occur over the oceans.

• The global water budget takes into account all the water that is held in the stores and flows of the global hydrological cycle. The most significant feature of the budget is that only 2.5% of it is freshwater; the rest is in oceans. Only 1% of all freshwater is 'easily accessible surface freshwater'. Nearly 70% is locked up in glaciers and ice sheets.
• Residence time - the average time a molecule of water will spend in one of the stores. These vary from 10 days in the atmosphere to 3,600 years in the oceans and 15,000 years in an ice cap. 
• Fossil water - ancient, deep groundwater from pluvial (wetter) periods in the geological past.
• Cryosphere - made up of those areas of the world where water is frozen into snow or ice
• It is claimed that two water stores, fossil water and the cryosphere are non-renewable.

• Drainage basin - an area of land drained by a river and is tributaries, sometimes referred to as a river catchment. The boundary of the drainage basin is defined by the watershed.
• The drainage basin is a subsystem within the global hydrological cycle. It is an open system with external inputs and outputs. Since those inputs vary over time, so does the amount of water in the drainage basin.

• The main input is precipitation, which can vary in a number of different ways.
• Form: rain, snow or hail. With snow, entry of water into the drainage basin will be delayed
• Amount: this will affect the amount of water in the drainage basin and fluxes within it
• Intensity: the greater the intensity, the greater the likelihood of flooding
• Seasonality: this is likely to result in the drainage basin system operating at different flow levels at different times of the year
• Distribution: this is significant in very large basins, such as the Nile and the Ganges, where tributaries start in different climate zones

• There are at least seven flows that are important in transferring the precipitation that has fallen on the land into the drainage network:
• 1. Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation
• 2. Infiltration: the process by which water soaks into, or is absorbed by the soil
• 3. Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks

• 4. Throughflow: the lateral transfer of water downslope through the soil
• 5. Groundwater flow: the very slow transfer of percolated water through pervious or porous rocks
• 6. Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. 
• River or channel flow: takes over as soon as the water enters a river or stream; the flow is confined within a channel

• There are three main outputs of the drainage basin:
• 1. Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock
• 2. Transpiration: the biological process by which water is lost from plants through minute pores and transferred to the atmosphere
• 3. Discharge: (also known as channel flow) into another, larger drainage basin, a lake or the sea

Physical factors affecting drainage basin systems:

• Climate - climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation, i.e. the major inputs and outputs. Climate also has an impact on the vegetation type
• Soils - soils determine the amount of infiltration and throughflow and, indirectly, the type of vegetation
• Geology - geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers). Indirectly, geology affects soil formation
• Relief - relief can impact on the amount of precipitation. Slopes can affect the amount of runoff
• Vegetation - the presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurence of overland flow, as well as on transpiration rates

Impacts of human activities on drainage basin systems:

• River management - construction of storage reservoirs holds back river flows, abstraction of water for domestic and industrial use reduces river flows, abstraction of groundwater for irrigation lowers water tables

• Deforestation - clearance of trees reduces evapotranspiration, but increases infiltration and surface runoff
• Changing land use - agriculture - arable to pastoral: compaction of soil by livestock increases overland flow, pastoral to arable: ploughing increases infiltration by loosening and aerating the soil
• Changing land use - urbanisation - urban surfaces (tarmac, tiles, concrete) speed surface runoff by reducing percolation and infiltration, drains deliver rainfall more quickly to streams and rivers, increasing chances of flooding
• The components of the drainage basin system most affected by humans are: evaporation and evapotranspiration, interception, infiltration, groundwater and surface runoff

• A water budget is the annual balance between precipitation, evapotranspiration and runoff. It is calculated from the formula:
• P = E + R +- S
• Where P is precipitation, E is evapotranspiration, R is runoff and S represents change in storage over a period of time, usually one year

• Water budgets at a national or regional scale provide a useful indication of the amount of water that is available for human use (for agriculture, domestic consumption, etc).
• At a local scale, water budgets can inform about available soil water (the amount of water that can be stored in the soil and is available for growing crops). This is valuable to users, such as farmers, who can use it to identify when irrigation might be required, and how much.

• River regime - the annual variation in the discharge or flow of a river at a particular point, and is usually measured in cumecs.
• The character of a river's regime is influenced by a number of variable factors:
• The size of the river and where discharge measurements are taken along its course
• The amount, seasonality and intensity of the precipitation
• The temperatures, with possible meltwater and high rates of evaporation in summer
• The geology and soils, particularly their permeability and porosity; groundwater noted in permeable rocks is gradually released into the river as base flow
• The type of vegetation cover: wetlands can hold water and release it slowly into the river
• Human activities aimed at regulating a river's discharge

• Geology - where the underlying rock is largely impermeable, the river will have a variable regime which will reflect variations in precipitation. Where the underlying rock is mostly permeable, it acts as a reservoir for groundwater and usually maintains a steady flow. Porous or pervious rocks act as aquifers, i.e. groundwater storage, so water is released slowly through the system, leading to a very steady regime. Impermeable geology can lead to a very variable and quick response regime, with peaks following periods of heavy rain. Deep soils can also store water, again leading to a steady regime
• Human factors - e.g. reservoirs or abstraction of water may lead to unexplained changes in the regime. Dam building for energy or irrigation can regulate the flow
• Position of measuring station in relation to size and shape of basin. Clearly, where numerous tributaries meet, this causes a significant change in the amount of discharge
• Precipitation amounts and seasonality are important. In many areas autumn and winter are the main rainfall seasons and some river regimes reflect this.
• Regimes often reflect rainfall seasonal maxima or when the snow fields or glaciers melt. A spring maximum might be the result of melting snow in the upper part of a basin. Glaciers normally melt later, in early summer, and may cause a peak then
• Evaporation is usually greatest in summer at the highest temperature

• Whereas river regimes are usually graphed over the period of a year, storm hydrographs show discharge changes over a short period of time, often no more than a few days.
• The storm hydrograph plots two things: the occurrence of a short period of rain (maybe a heavy shower or storm) over a drainage basin and the subsequent discharge of the river.
• Features of a storm hydrograph:
• Once the rainfall starts, the discharge begins to rise, this is known as the rising limb
• Peak discharge is reached some time after the peak rainfall because the water takes time to move over and through the ground to reach the river
• The time interval between peak rainfall and peak discharge is known as the lag time
• Once the input of rainwater into the river starts to decrease, so does the discharge, this is shown by the falling or recessional limb
• Eventually the river's discharge returns to its normal level or base flow

• Some hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short time lag. These are often referred to as 'flashy' hydrographs.
• In contrast, there are some hydrographs with gently inclined limbs, a low peak discharge and a long lag time. These are often called 'delayed' or 'flat' hydrographs.

Urbanisation is an important factor affecting the character of storm hydrographs, particularly their 'flashiness' and has many effects on hydrological processes:

• Construction work leads to the removal of vegetation cover. This exposes the soil and increases overland flow.
• Bare soil is eventually replaced by a covering of concrete and tarmac, both of which are impermeable and increase surface runoff.
• The high density of buildings means that rain falls on roofs and is then swiftly fed into drains by gutters and pipes.
• Drains and sewers reduce the distance and time rainwater travels before reaching a stream or river channel.
• Urban rivers are often channelised with embankments to guard against flooding. When floods occur, they can be more devastating.
• Bridges can restrain the discharge of floodwaters and act as local dams, thus prompting upstream floods.
• The overall impact of urbanisation is to increase flood risk.

WHAT FACTORS INFLUENCE THE HYDROLOGICAL SYSTEM OVER SHORT- AND LONG-TERM TIMESCALES?

• Drought is defined in meteorological terms as a shortfall or deficiency of water over an extended period, usually at least a season.
• The UN provides a general definition: 'drought is defined as an extended period - a season, a year, or several years - of deficient rainfall relative to the statistical multi-year average for a region.
• Hydrological drought is characterised by reduced stream flow, lowered groundwater levels and reduced water stores
• Drought can hit agricultural productivity particularly hard, and can lead quickly to food shortages, famine and starvation

• Research suggests that sea surface temperature anomalies are an important causal factor in short-term precipitation deficits. This relates to how much temperatures of the sea surface, recorded at a particular time, differ from the long-term average. A positive anomaly occurs when the observed temperature is warmer than the average and a negative anomaly is when the observed temperature is cooler than the average.

El Nino-Southern Oscillation (ENSO)

• Temperature anomalies provide the key to the ENSO which, in turn, is thought to trigger the occurrence of droughts.
• The ENSO is a naturally occurring large mass of very warm seawater in the equatorial Pacific Ocean. This warm water is normally located in the western Pacific, where it is pushed by ocean currents, trade winds and the Walker circulation cell in the atmosphere.
• However, on average every 7 years these pushing forces weaken, and this allows the mass of warm water to move eastwards towards the coast of Central and South America.
• Wherever this mass of warm water is located, evaporation rates are higher and precipiration greater, while areas of cooler water, such as the cold current that flows along the Peru-Chile coastline, bring drier weather.
• An El Nino event reduces precipitation in the western Pacific, which leads to drought in Northern Australia and Indonesia. El Nino events usually occur every three to seven years and usually last for 18 months. El Nino events seem to trigger very dry conditions throughout the world. E.g. the monsoon rains in India and SE Asia often fail.
• La Nina episodes may follow an El Nino event. They involve the build up of cooler-than-usual subsurface water in the tropical part of the Pacific. This situation can also lead to severe drought conditions, particularly on the western coast of South America.

• People are not the cause of drought, but their actions can make droughts worse and more severe.
• Desertification - the process by which once-productive land gradually changes into a desert-like landscape. It usually takes place in semi-arid land on the edges of existing deserts. The process is not necessarily irreversible.
• The causes of desertification are essentially natural and set in motion a downward spiral:
• Changing rainfall patterns with rainfall becoming less reliable, seasonally and annually. The occassional drought year sometimes extends to several years
• The vegetation cover becomes stressed and begins to die, leaving bare soil
• The bare soil is eroded by wind and an occassional intense shower
• When rain does fall, it is often only for very short, intense periods. This makes it difficult for the remaining soil to capture and store it.

• Human factors do not cause drought but they act like a feedback loop. Humans enhance the impacts of droughts by the over-abstraction of surface water from rivers and ponds, and of groundwater from aquifers.

• Key human factors encouraging this are:
• Population growth - rapid population growth puts pressure on the land to grow more food.
• Overgrazing - too many goats, sheep and cattle destroy the vegetation cover
• Overcultivation - intense use of marginal land exhausts the soil and crops will not grow
• Deforestation - trees are cut down for fuel, fencing and housing. The roots no longer bind the soil and erosion ensues.
• Humans can be a direct cause of the development of droughts, e.g. over abstracting groundwater sources and by reducing the downstream supply of water by building reservoirs and water transfers.
• However, they can also indirectly affect the development of droughts through changing the land and altering hydrological processes. For example, deforestation and overgrazing reduce vegetation cover, so reducing evapotranspiration rates, and thereby redcuing atmospheric moisture and precipitation. The removal of vegetation also changes soil conditions through compaction and reduced organic matter and moisture retention; this reduces infiltration and increases surface runoff, which reduces soil moisture content and water storage.

• Wetlands perform a number of important functions: from acting as temporary water stores to the recharging of aquifers, from giant filters trapping pollutants to providing nurseries for fish and feeding areas for migrating birds.
• Drought can have a major impact on wetlands. With less precipitation there will be less interception (as vegetation becomes stressed), as well as less infiltration and percolation. Water tables will fall. Evaporation will also increase. This, together with the decrease in transpiration, will reduce valuable functions performed by wetlands.
• Forests have significant impacts on the hydrological cycle. They are responsible for much interception which, in turn, means reduced infiltration and overland flow. Forests are characterised by high levels of transpiration.
• Like wetlands, drought threatens forests, but it is people and deforestation that most threaten their survival. 
• In the coniferous forests, drought is not only causing direst physiological damage but is also increasing the susceptibility of pines and firs to fungal diseases. Tree mortality is rising. 
• The same applies to tropical rainforest, except that the increased mortality attributed to drought seems to be having a greater impact on large trees. Here there is added concern of what this increased tree mortality will eventually do to this important global carbon store

• The meteorological causes of flooding are:
• Intense storms which lead to flash flooding, as in semi-arid areas but more commonly in mountainous areas - flash floods happen very quickly, often without warning. They may be caused by intense heavy rainfall associated with severe thunderstorms or tropical storms.
• E.g. Sardinia hit by Cyclone Cleopatra and floods (November 2013)
• Prolonged, heavy rain, such as during the Asian monsoon and with the passage of deep depressions across the UK - In areas such as the UK the usual cause of flooding is the prolonged and heavy rain associated with the passage of mid-latitude low-pressure systems or depressions. Each depression usually brings two bands of rain - showers and rain with the warm front and then heavier rain with the cold front. Initially some of the ground may be able to absorb some of the rainfall, but when throughflow and groundwater flow cannot transfer the water away quickly enough, it becomes saturated.Once rain falls on saturated ground the only transfer is runoff, which quickly moves water to river channels and increases their discharge. Once the capacity of the river is exceeded, water will spill over the banks and spread over the floodplain
• A monsoon is a seasonal change in the direction of the prevailing winds of a world region. In India and SE Asia, the summer monsoon is associated with very heavy rainfall. 

• The meteorological causes of flooding are:
• Intense storms which lead to flash flooding, as in semi-arid areas but more commonly in mountainous areas - flash floods happen very quickly, often without warning. They may be caused by intense heavy rainfall associated with severe thunderstorms or tropical storms.
• E.g. Sardinia hit by Cyclone Cleopatra and floods (November 2013)
• Prolonged, heavy rain, such as during the Asian monsoon and with the passage of deep depressions across the UK - In areas such as the UK the usual cause of flooding is the prolonged and heavy rain associated with the passage of mid-latitude low-pressure systems or depressions. Each depression usually brings two bands of rain - showers and rain with the warm front and then heavier rain with the cold front. Initially some of the ground may be able to absorb some of the rainfall, but when throughflow and groundwater flow cannot transfer the water away quickly enough, it becomes saturated.Once rain falls on saturated ground the only transfer is runoff, which quickly moves water to river channels and increases their discharge. Once the capacity of the river is exceeded, water will spill over the banks and spread over the floodplain
• A monsoon is a seasonal change in the direction of the prevailing winds of a world region. In India and SE Asia, the summer monsoon is associated with very heavy rainfall. 

• It usually happens between April and September when warm, moist air from the south-west Indian Ocean blows towards India, Sri Lanka, Bangladesh and Myanmar, bringing a humid climate and torrential rain. This happens because the ITCZ moves northwards and the warm moist air follows behind it. 
• E.g. Storm Desmond, Storm Eva, Storm Frank in UK (2015-16)
• E.g. Pakistan in July 2010
• Rapid snowmelt during a particularly warm spring, as on the plains of Siberia - snow and ice are responsible for many flood events, usually in higher latitiudes or mountainous areas. Melting snow in late spring regularly causes extensive flooding in the continental interiors of Asia and America. The great north-flowing Siberian rivers, such as the Ob and Yenisei, cause vast annual flooding in the plains of Siberia. The quick transition from winter to spring upstream causes rapid snow melting, while their lower reaches remain frozen, with very limited infiltration. Flood water is help up by temporary ice dams. Sometimes rain falls on melting snow ice when a rapid thaw occurs and this combination can cause heavy flooding.
• E.g. in the Himalayas and in Iceland

• Storm surge - caused by very low air pressure which raises the height of the high-tide sea. Strong onshore winds then drive the 'raised' sea towards the coast, often breaching coastal defences and flooding large areas.
• The likelihood of flooding is increased by physical circumstances:
• In 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

Human activity and flood risk

• Urbanisation - expansion of impermeable surfaces increase the rate of surface runoff into rivers via the urban drainage system. Bridges and culverts often reduce river capacity and impede channel flow.
• Deforestation - reduces interception and evapotranspiration, resulting in higher volumes and rates of surface runoff, which ensures precipitation reaches river channels faster, creating flashy hydrographs. Exposes soil to higher rates of erosion, increasing river sediment load and deposition within channels - reduces capacity of river to carry water and increases likelihood of flooding. E.g. deforestation in Nepal and Tibet increasing floods in Bangladesh from Ganges and Brahmaputra rivers.
• Draining of wetlands - act as a natural buffer by absorbing water. Removal of storage area. Floodplain drainage reduces the natural storage capacity of the floodplain. The land may shrink as it dries out, getting lower and thus more susceptible to flooding.
• Management of rivers:
• Channelisation - an effective way of improving river discharge and reducing the flood risk. The pronlem is that it simply displaces that risk downstream. Some other location may be overwhelmed by increased discharge

• Dams - block the flow of sediment down a river so the reservoir gradually fills up with silt; downstream there is increased river bed erosion
• River embankments - designed to protect from floods of a given magnitude. They can fail when a flood exceeds their capacity. Inevitably, when this happens, the scale of the flooding is that much greater

Impacts of flooding

Socioeconomic:

• In many LICs many people have not learnt to swim. Worldwide children and old people are particularly vulnerable
• In LDCs post-flood morbidity is likely due to water borne diseases and secondary flood hazards
• Trauma
• Damage to property, particularly housing. Destruction of bridges
• In HDCs property values are severely impacted in flood prone areas. Issues also arise when coming to resell properties

• Income from tourism is disrupted
• Disruption of transport and communications
• Interruption of water and energy supplies
• Destruction of crops and loss of livestock
• Disturbance of everyday life, including work

Environmental:

• Intense flooding can lead to over supplies of sediment and nutrients, with possible eutrophication and destruction of aquatic plants
• Intense flooding can lead to pollution from nitrates, chemicals and heavy metals, all of which degrade aquatic habitats
• Floods can recharge groundwater systems, fill wetlands, increase connectivity between aquatic habitats, and move sediment and nutrients around the landscape
• Flooding can trigger breeding of species and encourage migration and dispersal
• Crops, livestock and agricultural infrastructure suffer major damage

HOW DOES WATER INSECURITY OCCUR AND WHY IS IT BECOMING SUCH A GLOBAL ISSUE FOR THE 21ST CENTURY?

• Water stress - when the demand for water exceeds the available amount during a certain period. The amount of water in a defined area is less than 1700m3 per person per year
• Water scarcity - water availability in a defined area is less than 1000m3 per person per year.
• Water insecurity - where present and future supplies of water cannot be guaranteed. A population no longer has sustainable access to adequate quantities of water of acceptable quality.
• There are a number of factors that reduce the amount of water available for human use:
• Evaporation and transpiration
• Discharge into the sea
• Saltwater encroachment at the coast
• Contamination of water by agricultural, industrial and domestic pollution
• Over-abstraction from rivers, lakes and aquifers and the acute need to replenish these dwindling stores

• The rising demand for water is driven by three main factors:
• Population growth 
• Economic development - increases the demand for water in almost all economic activities - agriculture, industry, energy and services. One of the biggest and fastest-growing consumers is irrigation
• Rising living standards - increase in the per capita consumption of water for drinking, cooking, bathing and cleaning. Added to this domestic consumption are water-extravagant things such as swimming pools, washing machines and dishwashers.

• Physical scarcity - occurs when more than 75% of a country's or region's blue water (water stored in rivers, streams, lakes and groundwater) flows are being used. E.g. countries in the Middle East and North Africa. Regions in north China, western USA and southeast Australia.
• Economic scarcity - occurs where the use of blue water sources is limited by lack of capital, technology and good governance. It is estimated that around 1 billion people are restricted from accessing blue water by high levels of poverty. Most of these people loive in Africa.

• In the developing world, supplying safe water in areas of physical water scarcity can be difficult, costly and well beyond the means of very poor people. This is where charities such as WaterAid provide such invaluable help. Their programmes are helping to reduce the extent of economic water scarcity.

Agriculture

• Producing crops under entirely rain-fed conditions, using green water in the soil, producing under fully irrigated conditions.
• In rain-fed agriculture, fields and grazing lands are entirely dependent on rainwater. Farmers focus on storing water (rainwater harvesting) to conserve supplies. Moving along the spectrum, more surface water or groundwater (blue water) is added to enhance crop production.
• Around 1/5 of the world's land is under full irrigation. 
• Around 30% of this irrigation is provided using dams. Much irrigated land becomes waterlogged, leading to salination of the soils
• The majority of irrigation is pumped up electrically from aquifers, leading to massive groundwater depletion, especially in India, the USA, China and Pakistan.

Industry and energy

• Just over 20% of all freshwater withdrawals worldwide are for industrial and energy production.
• Industries such as chemicals, electronics, paper, petroleum and steel are major consumers of water. Water pollution is a major problem associated with much of this industrial use of water.
• A major concern is the global shift in industrial production towards emerging nations such as China and South Korea. This rapid industrialisation, particularly in developing countries, has contaminated both rivers and groundwater, affecting the quality of water.
• Considerable progress has been made by many TNCs, such as Coca-Cola India, to reduce their consumption by efficient recycling and also to control effluents.
• Over half of the water used by energy production is either for generating HEP or as cooling water in thermal and nuclear power stations, so is returned to its source virtually unchanged.
• However, there is mouting concern about the growth of biofuels for the production of bioethanol and biodiesel. These crops are very thirsty.

Domestic use

• With economic development comes rising standards of living and an increasing per capita consumption of water. Safe water is a fundamental human need.
• Around 15% of the world's population still rely on unimproved water and around 2.5 billion people have no access to improved sanitation facilities.
• Water and disease interact in two ways: unsafe drinking water can spread disease, but water used for personal and domestic hygiene can prevent disease transmission.
• Water, particularly that polluted by lack of sanitation, is an effective medium for the breeding and transmission of a range of lethal diseases, such as typhoid, cholera and dysentery.
• Water is also a productive breeding ground for some disease vectors, such as mosquitoes, snails and parasitic worms. Malaria, dengue fever and bilharzia are debilitating vector diseases.
• Diseases related to a lack of clean water or lack of improved sanitation also lead to high levels of morbidity, as they affect people's ability to work and look after their family and, therefore, their ability to escape poverty.

Potential water conflicts

• When demand for water overtakes the available supply, and a number of stakeholders wish to use the same diminished resources, there is potential for conflict at all scales.
• Within countries, conflicts can arise between the competing demands of irrigation, energy, industry, domestic use and recreation. But it is when countries share the same river or drainage basin, as is the case with trans-boundary water sources, the 'normal' competition for water can be raised to a different level, namely one of international tensions and even open conflict.
• E.g. the Nile. 11 countries compete for water. 

• Hard-engineering schemes require high levels of capital and technology. 
• Water transfers - water transfer schemes involve the diversion of water from one drainage basin to another, either by diverting a river or constructing a large canal to carry water from one basin to another. E.g. China's South-North Transfer Project.

• Mega dams - nearly 60% of the world's major rivers are impeded by large dams, e.g. The Colorado, Nile and Yangtze. While the capital costs of such dams are immense, this hard-engineering solution to water shortages has other drawbacks, such as the high evaporation losses from the water surface, the disruption of the downstream transport of silt and the displacement of people.
• Desalinisation - the process by which dissolved solids in sea water are partially or completely removed to make it suitable for human use. Desalinisation has been undertaken on a small scale for centuries, but recently there have been technological advances in the process, most notably:
• development of the process of reverse osmosis
• pioneering work on carbon nanotube membranes
• Desalinisation is an expensive process, it requires inputs of advanced technology and energy. However, as the price of freshwater rises, so some countries will look increasingly to the seas for their water supplies. Some Middle Eastern states, such as Saudi Arabia, Kuwait and UAE, have already done so.
• It is a sustainable process, although it does have an ecological impact on marine life.

• Water sustainability is about ensuring that there are aqequate supplies of available water for the benefits of future generations. Water sustainability has 3 different aspects:
• Environmental - freedom from pollution and image, so available as safe water
• Economic - ensuring a secure water supply to all users at an affordable price; maximising efficiency of water usage and minimising wastage
• Socio-cultural - ensuring equitable distribution of water a) to poor disadvantaged groups, b) within and between countries
• There is a diversity of actions being taken today as steps towards those management aims:
• Smart irrigation - traditional sprinkler and surface flow systems are being replaced by modern automated spray technology and advanced drip irrigation systems. Drip systems allow water to drip slowly to plants' roots through a system of valves and pipes - reducing wastage and evaporation. E.g. China and Australia
• Hydroponics - growing crops in greenhouses that are carbon dioxide and temperature controlled in shallow trays where they are drip-fed nutrients and water; there is no soil
• Recycling of grey water - a low-cost option that produces water for agricultural use, but not human consumption
• Restoration - of damaged rivers, lakes and wetlands so that they can play their full and proper part in the hydrological cycle

• Rainwater harvesting - where people collect the rain falling on roofs of dwellings and store it in butts for various domestic purposes, such as flushing toilets and watering the garden. E.g. Uganda
• Filtration technology - this is now so effective that there is little dirty water that cannot be physically purified and recycled

• IWRM (Integrated Water Resources Management) was first advocated in the late 1990s. It emphasises the river basin as a logical geographical unit for the management of water resources. It is based on achieving a close cooperation between basin users and players. The river basin is treated holistically in order to ensure three things:
• The environmental quality of the rivers and catchment
• That water is used with maximum efficiency
• An equitable distribution of water among users
• Experience has shown that IWRM works well at a community level but not so well in larger river basins, especially if an international boundary is involved, as is the case with the Colorado River and the Nile.

Important international agreements include:

• The Helsinki rules - on the Uses of the Waters of International Rivers is an international guideline regulating how rivers and their connecting groundwaters that cross national boundaries may be used.
• The United Nations Economic Commission for Europe (UNECE) Water Convention - aims to protect and ensure the quantity, quality and sustainable use of trans-boundary water resources by helping with cooperation and resolving issues. 
• The EU Water Framework Directive (WFD) - agreed in 2000. Set targets to restore rivers, lakes, canals and coastal waters to 'good condition'. 
• National government agencies - e.g. the UK's Environement Agency, which checks compliance with EU Frameworks.