Water resources and management

Water resources: the seven types and the global pattern

A water resource is any store or flow of water that people can use - for drinking, growing food, industry, power or recreation. The syllabus names seven: rivers, lakes and reservoirs, oceans, underground water, ice sheets, precipitation and recycled water. They are not interchangeable. Each has a different volume, a different cost of extraction, a different quality and a different rate at which it is replaced, and that is why a country can be desperately short of water while sitting next to an ocean.

The global pattern begins with one fact: about 97 per cent of the world's water is in the oceans and is too salty to drink or to irrigate with. Of the 2.5 per cent that is fresh, roughly two-thirds is locked in ice sheets and glaciers, mainly in Antarctica and Greenland, and most of the rest is groundwater deep below the surface. Lakes, rivers and reservoirs together hold well under 1 per cent of the world's fresh water, and that small, fast-cycling share is what humanity actually lives on. Precipitation is not a store at all but the input that refills the others.

Precipitation is very unevenly spread. The wettest belts are the equatorial zone, where convectional storms give rain almost daily, the tropical monsoon regions, and the windward coasts and uplands of the mid-latitudes, where moist air is forced to rise. The driest are the subtropical high-pressure belts at about 20 to 30 degrees of latitude, where sinking air suppresses rain and the great deserts lie, and the continental interiors, far from any sea. Relief adds a second layer: a mountain range takes the rain on one side and leaves a rain shadow on the other, so two places 50 km apart can differ by a metre of rainfall a year.

The pattern of water resources is therefore not the same as the pattern of rainfall, because the other six types redistribute it. A country with little rain but large aquifers, such as Libya or Saudi Arabia, has water underground; a country with heavy rain on steep, impermeable rock may lose almost all of it as runoff within a day. Ice sheets hold enormous volumes of fresh water that are effectively unavailable - frozen, remote, and once melted, gone from the store for good. Recycled water is the odd one on the list: it is not a natural store but water already used, treated and returned to supply, so its volume grows with investment rather than with rainfall.

• Fresh water is a tiny share of the total: about 2.5 per cent, and two-thirds of that is frozen.
• Groundwater is the largest store of liquid fresh water, but much of it recharges over decades or millennia.
• Rivers are a flow, not a store: they are renewed every year but vary hugely between seasons.
• Reservoirs are the main store people can actually control, and they lose water to evaporation and silt.
• Renewability is the test that matters: a shallow aquifer refills, a fossil aquifer does not, and an ice sheet is being mined.

The human water cycle: capture, management, use, disposal and reuse

The human water cycle takes water out of the natural cycle, uses it and returns it. Learn it as five stages: capture, management, use, disposal and reuse. Capture means taking water from a source - abstraction from a river or an aquifer, storage in a reservoir, collection of rain from a roof, or desalination of sea water. Management covers everything between capture and the tap: treatment to make the water safe, storage in tanks and service reservoirs, pumping and distribution through a pipe network, with metering and leak control to keep losses down.

Use is the stage students describe too vaguely. Water is used consumptively, where it is evaporated, transpired by a crop or built into a product and so never returns to the river; non-consumptively, where it is used and returned, as with cooling water at a power station or water used for washing; and in-stream, where it is not removed at all, as in navigation, fishing, hydropower and recreation. A city that returns most of its water to the river after treatment has a high withdrawal but a modest consumption, and that difference decides how much water a river really loses.

Disposal is the stage that sets quality. Used water leaves a house through a sewer or septic tank, a factory through an outfall pipe, and a field as drainage or return flow. If it is treated, the effluent can be clean enough for a town downstream to abstract again; if it is not, it carries sewage, nutrients, chemicals and plastics into the same river. This is why a river is usually cleanest near its source and worst immediately below a large city, and why water quality downstream is partly a decision made upstream.

Reuse closes the loop. Grey water - from baths, basins, showers and washing machines - can be used with little or no treatment for flushing toilets, washing cars and watering gardens. Treated wastewater can be returned to a river above an abstraction point, used to irrigate farmland, parks and golf courses, or, with advanced treatment, put back into the public supply. Reuse does not create water; it gets a second or third use out of the same litre before it is lost, which makes it the cheapest large new source for a city that already has sewers and treatment works.

Against this human cycle sits the natural one, which you met in Topic 1. A drainage basin is an open system: precipitation is the input; evaporation, transpiration and runoff are the outputs; water is held temporarily in interception, soil moisture, groundwater and the channel; and it moves between those stores by infiltration, percolation, throughflow and groundwater flow. Every human use changes at least one of those flows or stores, which is why the syllabus links the two topics so closely.

Modifying the water cycle: agriculture, forests, cities, industry, abstraction and flood management

Agriculture is the largest modification. Irrigation takes water from a river, reservoir or aquifer and spreads it over fields, which sharply increases evaporation and transpiration. In dry climates it also raises the water table until waterlogging and salinisation appear, as salt is drawn up and left behind as the water evaporates. Ploughing and the loss of soil organic matter reduce infiltration and increase overland flow and erosion, while paddy fields, terraces and farm ponds store water and slow it down. Farming is also the main source of diffuse pollution: nitrates and phosphates from fertiliser and slurry reach rivers and aquifers, so the water is still there but no longer usable without expensive treatment.

Deforestation removes interception and transpiration. Rain reaches the soil directly, machinery and grazing compact it, infiltration falls and overland flow rises, so the hydrograph becomes flashier - higher peaks, shorter lag times and lower dry-season baseflow, because less water has soaked down to the groundwater store. Afforestation does the opposite: trees intercept rain, roots improve infiltration and the trees transpire large volumes, so a planted catchment usually yields less total runoff but a more even flow. In several upland catchments, conifer planting has measurably reduced the water reaching reservoirs, which is why water companies now negotiate over new forests in their catchments.

Urbanisation is the most drastic change of all. Roofs, roads, pavements and car parks are impermeable, and the drains and sewers built to remove water quickly do exactly that, so infiltration and groundwater recharge collapse while surface runoff and flood peaks rise. A city also imports water from outside its own catchment and exports it as sewage and effluent, so the local water balance is rewritten: water that used to soak away now arrives in pipes and leaves in pipes. Runoff from streets and yards carries oil, metals, salt and litter into the river, so quality falls as well as quantity.

Industrialisation adds large abstractions for cooling, processing and washing, discharges that are warmer and chemically polluted, and hardstanding around factories, ports and industrial estates. Water abstraction is the modification that ties the whole topic together, because it is where human demand meets natural supply. Below the sustainable limit the system recovers each year; above it the water table falls, baseflow shrinks, wetlands dry out and, on a coast, salt water moves into the aquifer. Flood management pushes the other way: embankments and straightened channels speed water away, dams and storage basins hold it back, and floodplain zoning keeps people and assets off land that is meant to flood. Each protects people in one place by changing the flows and stores somewhere else.

Trends in consumption: stress, scarcity and the sectors that use the water

Global water use rose steeply from the mid-twentieth century, driven by four things acting together: population growth, rising incomes, the spread of irrigation and urbanisation. Use per person is not fixed. As a country gets richer its people eat more meat and dairy, which take far more water to produce than grain, live in houses with piped supply, washing machines, gardens and sometimes pools, and buy goods whose manufacture used water somewhere else. Industry and energy grow at the same time. Demand therefore climbs faster than population in middle-income countries, then tends to level off as economies shift towards services and appliances become more efficient.

Two terms must be kept apart. Water stress describes demand approaching the available supply: competition between users appears, rivers are fully allocated and groundwater starts to be drawn down, but the supply is not yet exhausted - there is simply no slack left. Water scarcity describes a shortage of water relative to demand, where users cannot get what they need. Scarcity comes in two forms. Physical water scarcity means there genuinely is not enough water: the resource is naturally small, or it has been used up or over-abstracted. Economic water scarcity means the water exists but people cannot get at it, because there is no money for wells, pipes, pumps, treatment works or the institutions that run them.

The distinction matters because the solutions are completely different. Physical scarcity has to be met by finding more water - transfers, desalination, reuse, storage - or by cutting demand. Economic scarcity is met by investment and infrastructure: a country with a great river and no pipes has an economic problem, not a physical one. Most of sub-Saharan Africa suffers economic scarcity; much of North Africa, the Middle East, central Asia and the south-western United States suffers physical scarcity. A country can suffer both at once, and can move from one to the other: Saudi Arabia has the money to desalinate, but it has emptied the aquifer beneath its former wheat fields.

About 70 per cent of global freshwater withdrawals go to agriculture, roughly a fifth to industry and about a tenth to domestic use, but the shares vary enormously with economic structure. In a low-income country agriculture usually takes the great majority; in a high-income industrial economy, industry and energy can take half. The figures describe withdrawal, not consumption: irrigated farming consumes most of what it takes, because the crop transpires it, while a power station returns nearly all its cooling water to the river, warmer but still there. That is why irrigated farming is the target of demand management even in countries where industry takes more out of the river.

Individual industrial sectors differ in both appetite and quality. Thermal power stations need very large volumes for cooling, can use poor-quality water and return most of it. Food and drink processing needs small volumes of very high quality and produces strong organic effluent. Textiles, dyeing, tanning and paper use moderate volumes and discharge heavily polluted, often coloured, effluent. Mining and metal processing use water to separate ore and produce acidic, metal-rich drainage. Electronics and pharmaceuticals need small volumes of ultrapure water. Domestic demand is small in volume but demands the highest quality of all, because it is drinking water, and it is the demand a government must meet first.

Supply: the physical and human factors that set volume and quality

Climate decides how much water arrives and how much leaves. The variables that matter are total precipitation, its seasonality (a monsoon that delivers a year's rain in three months is far harder to use than the same total spread evenly), its reliability from year to year, and evapotranspiration, which rises with temperature and removes water from soils, reservoirs and crops. A hot, dry climate can lose a large share of a reservoir to evaporation; a cool, wet one loses very little. Climate also sets the quality of raw water: heavy rain flushes sediment and nutrients into rivers, while low flows in a dry season concentrate whatever pollution is already there.

Geology decides where water is stored and whether it can be reached. Permeable rocks such as sandstone and chalk absorb water and hold it in pores and cracks, forming aquifers that can be pumped and that also filter the water as it passes through - which is why chalk groundwater is usually clear, hard and vulnerable to nitrate from the farmland above. Impermeable rocks such as granite, clay and slate absorb almost nothing, so rain runs off the surface, streams are flashy and there is little groundwater to fall back on in a drought. Karst limestone, dissolved into caves and swallow holes, can carry water underground so fast that streams vanish and reappear, and pollution can travel for kilometres without being filtered.

Drainage patterns decide whether a country can collect and store its water. A large basin with many tributaries gathers water from a wide area and offers good reservoir sites; a small, steep coastal basin sends its water to the sea within hours. Drainage density matters too: a dense network drains a catchment quickly, while a sparse network on permeable rock releases water slowly and keeps streams flowing in the dry season. A country's position in a basin is decisive - the upstream state controls the flow that the downstream state depends on, which is the root of most transboundary disputes. Relief shapes both: mountains capture orographic rainfall and act as water towers, but steep slopes shed water quickly and erode, while lowlands store water in floodplains and aquifers.

Human factors can raise the usable supply far above what the physical environment offers, or destroy it. Storage and water transfers move water in time and space: a reservoir holds the wet season's surplus for the dry season, and an inter-basin transfer moves water from a wet basin to a dry one. Both need capital, both damage the donor basin, and both create a dependence that is politically hard to undo. Water abstraction works in both directions: up to the sustainable limit it is simply how the resource is used, beyond it, it lowers the water table, reduces river flow and cuts future supply.

Salt removal (desalination) turns an unusable resource into a usable one, and it is the only option that does not depend on rainfall at all, which is why it dominates in the Gulf states, Israel, Malta, Cyprus, Spain, Australia and Singapore. Its limits are energy, which makes it expensive and carbon-intensive; brine, a concentrated salt stream that must be discharged without damaging the sea bed; and cost per cubic metre, which makes it affordable for drinking water but rarely for irrigation. Water treatment and recycling increase supply without any new water at all, by turning water of poor quality into water of usable quality - protecting a spring or treating effluent can be cheaper than a new dam.

Type of infrastructure decides whether any of this reaches people: treatment works, pumps, pipes, storage tanks, tankers, standpipes, meters, sewers and latrines. Where infrastructure is missing or leaking, the resource exists but is not a supply, which is exactly what economic water scarcity means. Infrastructure also fixes quality at the point of use: clean water delivered through a broken pipe, or a well drawing from contaminated groundwater, is not safe water, however good the source.

Demand: population, economic structure and season

The number of people is the first control on demand: more people need more drinking water, more food and more space, and a growing population on a fixed resource moves a country from stress towards scarcity. Income characteristics change what each person uses. A household with a piped supply, a washing machine, a dishwasher, a garden and a car uses many times the water of a household carrying it from a standpipe, and richer populations eat more meat and dairy, buy more manufactured goods and take more holidays. Because water is usually priced well below its real cost, rising income turns almost directly into rising use unless price or technology changes.

Economic structure decides how the water is used, and therefore how easy it is to manage. An economy based on irrigated agriculture puts most of its water into a sector that is politically protected, hard to meter and quick to protest at any cut. An economy based on manufacturing uses water through a smaller number of large, metered industrial users, which is easier to regulate but concentrated in cities that cannot function without it. Recreation and tourism add a highly seasonal, high-quality demand: hotels, pools, spas, golf courses and artificial snow all need treated water, often in the driest and most popular regions, and often within a few weeks of the year.

Seasonal demand is where supply and demand most often fail to match. In a Mediterranean climate demand peaks in July and August for irrigation, gardens, pools and tourism at exactly the moment rainfall, river flow and reservoir storage are at their lowest. In a monsoon climate irrigation demand peaks in the dry weeks before the rains break. In a cold climate summer garden watering and winter heating and industry create two different peaks. Peak demand, not average demand, is what the infrastructure must be designed for, and it is what triggers restrictions in a drought.

Two consequences follow. First, demand is not fixed: it responds to price, to the efficiency of the appliances and irrigation in use, and to habits and expectations, which is why demand management is possible at all. Second, the composition of demand decides which strategy will work. Pricing works where users are metered; education works where users pay their own bills; technology works where capital is available to install it. In a country where most water goes to unmetered irrigation, a hosepipe ban in the cities is a gesture; in a city where domestic use dominates, it is a real instrument.

Water security and water insecurity

Water security is the reliable availability of an acceptable quantity and quality of water for health, livelihoods, food production, industry and the environment, together with an acceptable level of risk from water-related hazards such as floods and droughts. It is not simply having enough water. A country is water-secure when supply is adequate, reliable, safe and affordable, when the infrastructure can deliver it to everyone, and when institutions can manage droughts, pollution and disputes. Water insecurity is the opposite: supply that is unreliable, unsafe, unaffordable or contested.

The global pattern is not a simple map of rainfall. Insecurity is highest in North Africa and the Middle East, where rainfall is low and populations and irrigation have grown fast; in South Asia, where water exists but is polluted, over-abstracted and shared between rival users; in sub-Saharan Africa, where the problem is largely economic - too few wells, pumps, pipes and mechanics rather than too little rain; and in parts of central Asia, northern China, the western United States and the Mediterranean coast, where over-abstraction and drought have made an adequate supply unreliable. Security is highest in northern Europe, Canada and New Zealand, where rainfall is dependable, populations are modest and infrastructure is complete.

The physical factors behind insecurity are low and unreliable rainfall, high evaporation, recurrent drought, small or highly seasonal river flow, limited aquifer storage, and salt water where aquifers meet the coast. Climate change adds pressure by making rainfall more erratic and droughts and floods more intense, which hurts most where there is little storage to buffer it. Physical factors rarely cause a crisis on their own; they set the baseline against which human factors act.

The human factors are population growth, rising demand per person, inefficient irrigation, over-abstraction, pollution, leaking infrastructure, weak institutions, conflict and poverty. Poverty is the most important, because it decides whether a household can pay for a connection, a well, a filter or a tanker, and whether a government can borrow to build treatment works. Pollution turns water that exists into water that cannot be used: sewage, industrial effluent, agricultural nitrate and pesticides, and arsenic or fluoride dissolved from the rock itself. Infrastructure decides whether the resource reaches the user at all, and a country can have plenty of water and still be insecure if the pipes do not work.

Water access issues follow from those three. Poverty keeps households off the network and forces them to buy water by the bucket from vendors at many times the piped price, or to send women and children to walk for it, which costs schooling and paid work. Pollution means the water people can reach carries disease: diarrhoeal illness remains one of the biggest causes of child death worldwide, and arsenic in groundwater in Bangladesh and West Bengal is a slow, widespread poisoning. Infrastructure decides reliability - intermittent supply, low pressure, leakage, long queues at standpipes. The impacts of insecurity and drought run through people and environment together: crop failure and livestock loss, food price rises, hunger and migration; illness and lost schooling; conflict at the well and between farmers and herders; wetlands and rivers collapsing; land subsidence and saline intrusion; and in the worst cases famine and displacement.

Increasing supply: dams, transfers, desalination, harvesting and small technology

Dams and reservoirs store water when it is plentiful and release it when it is not, which is the most direct answer to seasonal and year-to-year variability. They also generate hydropower, irrigate farmland, reduce floods and support fisheries and recreation, and a well-sited reservoir can transform a regional economy. The costs are equally real: land and forest are flooded and people are displaced; sediment that would have reached the delta is trapped, so deltas erode and channels downstream scour; fish migrations are blocked; evaporation from the lake surface is high in hot climates; the reservoir silts up, so capacity falls over decades; and dams are very expensive and fail catastrophically when they fail.

A multipurpose river scheme plans a whole basin's dams, barrages, canals and power stations together so that one investment serves flood control, irrigation, hydropower, navigation, water supply and industry. The standard examples are the Tennessee Valley Authority in the United States, which turned a flood-ravaged, eroded valley into a managed and electrified region, and the Aswan High Dam on the Nile, which gave Egypt year-round irrigation and hydroelectricity but trapped the silt that renewed the delta's soils and fed its coast. Such schemes are efficient because costs are shared, but they concentrate control of water in one authority, and the losers - downstream farmers, deltas, fisheries - are rarely the people who gain.

Water transfers move water from a basin with a surplus to one with a deficit through canals, pipelines and tunnels. Spain's Tagus-Segura transfer, China's South-North scheme and California's state water project are the standard examples. A transfer can supply a dry region for decades, but it is expensive to build and to pump, it loses water to evaporation and leakage, it takes water from a donor basin whose rivers, wetlands and farmers are damaged, and it leaves the receiving region permanently dependent on a political decision taken somewhere else.

Salt removal (desalination) is either thermal, distilling sea water by evaporation, which is energy-hungry but simple and long proven in the Gulf, or reverse osmosis, which forces water through a membrane at high pressure and now dominates new capacity. It is the only source independent of rainfall, which is why it underpins supply in the Gulf states, Israel, Malta, Cyprus, Spain, Australia and Singapore. It is limited by cost - the plant, the energy and the membranes make the water expensive, so it is normally used for domestic and industrial supply rather than irrigation; by brine, which must be discharged carefully or it smothers the sea bed; and by carbon emissions unless the power is renewable. A single large plant can also become a single point of failure for a whole city.

Rainwater harvesting collects rain from roofs, courtyards, rock surfaces and hillsides into tanks, cisterns, ponds, check dams and wells, and it is the cheapest way for a household or village to get water where no network reaches. It works best where rain falls in a few intense storms that can be caught and stored, and it has a long history in India, the Sahel, the Mediterranean and Australia. Its limits are that it depends on rain in the same year, needs storage and a clean catchment, can breed mosquitoes and become contaminated, and cannot supply a city. Grey water - from baths, basins and washing machines - can be diverted to flush toilets or water gardens, cutting potable demand by a useful fraction in a household; it needs separate plumbing, some treatment or careful use, and public acceptance, and its volume is limited by how much water the household uses in the first place.

For the many people without a piped, safe supply, the effective technologies are small and cheap: boreholes and tube wells with hand pumps, protected springs, sand and ceramic filters, chlorination, solar disinfection, rainwater tanks and latrines that break the faecal-oral route. They save lives at low cost and can be maintained locally, which is why they are the backbone of rural water programmes. Their weakness is sustainability: a pump that breaks with no spare part and no trained mechanic is worse than no pump, because the community has already abandoned its old source. And a well drilled into rock containing arsenic or fluoride gives clear water that is poisonous, which is why testing matters as much as drilling.

Managing demand: price, rationing, technology and persuasion

Increasing the price of water is the most powerful demand tool, because it reaches every metered user at once without an inspector. It works through volumetric tariffs, where the bill rises with the volume used, and increasing block tariffs, where the price per cubic metre rises at higher blocks, so a household that uses little pays little and one that fills a pool pays a lot. The weaknesses are that demand is inelastic in the short term - people still must drink, wash and flush - so a large price rise cuts use only modestly, and it hits poor households hardest, which is why most schemes protect a free or cheap minimum volume. In agriculture, where water is often free, subsidised and unmetered, price barely works until the subsidy is removed and the supply is measured, and removing it is politically explosive.

Rationing limits use directly: a hosepipe or sprinkler ban in a drought, a quota per household or farm, a rota in which different areas receive water on different days, a cut in pressure at certain hours, or an abstraction licence capping what a company or farmer may take from a river or aquifer. Rationing acts immediately, costs little to impose and can target the least essential uses. But it is unpopular, hard to enforce fairly, usually arrives only once the shortage is serious, and does not change behaviour in the long run - people return to their old use as soon as it is lifted.

Technology to reduce the amount of water needed is the least visible and most durable option. In agriculture that means drip irrigation, which delivers water to the root zone and can cut use sharply against flood irrigation, plus sprinklers, soil moisture sensors, mulching, drought-tolerant varieties and the lining of canals to stop seepage. In homes it means low-flush toilets, low-flow showers and taps, aerated fittings, efficient washing machines and dishwashers, and smart meters that show a household what it is using. In cities it means leak detection and pipe replacement, because in many systems a quarter or more of the treated water is lost before it reaches a customer; in industry it means closed-loop recycling and dry cooling. The limits are capital cost, the long life of existing equipment and the rebound effect: a farmer who irrigates more efficiently often uses the saving to irrigate more land rather than to use less water.

Publicity and education try to change behaviour rather than restrict it: drought campaigns, school programmes, efficiency labelling on appliances, bills that compare a household with its neighbours, and community management of village supplies. They are cheap, they build consent for restrictions and prices, and they work best when the message is specific and local - fix the dripping tap, water the garden at dusk, do not wash the car. Their weakness is that they are slow, their effect is hard to measure, and the moment a wet winter refills the reservoirs the message is forgotten.

The general judgement is that demand management is cheaper, faster and less damaging than new supply, and should come first, because water that is not wasted does not have to be dammed, transferred or desalinated. Its weakness is that it needs meters, tariffs, enforcement and public consent, and it cannot create water where demand genuinely exceeds the resource. The realistic answer is a mix: price and technology to hold demand down, some new supply to cover growth, and rationing as the emergency brake. That balance, and who pays for it, is the political question at the heart of this topic.

Managing water resources: variability, the six challenges, and Spain

Water resources vary in volume and quality from season to season and year to year, and management is mostly about coping with that variation rather than with averages. A wet year fills reservoirs and can flood; a dry year empties them and concentrates pollutants; a run of dry years is what produces a crisis, because storage is drawn down and aquifers are mined to cover the gap. The impacts are environmental (wetlands dry out, rivers stop flowing, salt water intrudes, deltas erode, species are lost), social (restrictions on domestic use, illness where quality fails, migration from failed farms), economic (irrigation losses, lower hydropower output, industry cutbacks, higher food and energy prices, insurance costs) and political (disputes between regions and countries, and governments blamed for restrictions).

Rates of abstraction beyond recharge are the root of most other challenges. Groundwater is pumped faster than it refills, the water table falls, rivers that relied on groundwater baseflow dry up, and the resource is being spent rather than used. Saltwater intrusion follows on the coast: over-abstraction lowers the fresh water table below sea level, and sea water moves inland through the aquifer, salting wells that may never recover. The inland version is salinisation, where irrigation raises a saline water table and evaporation leaves salt at the surface, and both take land out of production permanently.

Water conflicts arise wherever several users want the same water: farmers against cities, upstream against downstream, one region against another, one country against another. Transboundary issues are the hardest, because a river ignores borders: an upstream dam, irrigation scheme or diversion changes the flow a downstream country depends on, and the downstream country has little leverage beyond a treaty, money or politics. The Nile, the Mekong, the Indus, the Colorado and the Tagus all carry water claimed twice. Water pollution reduces the usable supply and raises treatment costs, and is usually diffuse, arriving from thousands of fields and streets rather than one pipe. Ground subsidence occurs when groundwater is pumped from fine-grained sediments that then compact: the land surface sinks permanently, and with it go foundations, sewers, flood defences and, in a coastal city, metres of safety above sea level.

Spain is the detailed example. Rainfall is strongly uneven: the north and north-west are wet, Atlantic and green, while the south-east around Almeria, Murcia and Alicante is semi-desert with very high summer evaporation. Demand is concentrated where the water is not - irrigation in the dry south-east, tourism along the Mediterranean Costas where the population swells in July and August, plus urban growth, golf courses and industry. The Tagus-Segura transfer has carried water from the wetter Tagus basin to the Segura since the late 1970s, and after a planned transfer from the Ebro was cancelled in 2004, a national desalination programme added large reverse-osmosis capacity along the coast.

Spain's challenges match the syllabus list exactly. Over-abstraction has drawn down aquifers in La Mancha and Almeria and damaged the Tablas de Daimiel wetlands, which have dried and whose peat has burned underground. Saltwater intrusion has affected coastal aquifers where tourist development sits above them. Pollution from intensive agriculture and livestock has loaded rivers and the Mar Menor lagoon with nutrients, causing eutrophication and mass fish deaths. The Ebro delta is sinking and retreating because its dams trap sediment and the delta sediments compact. And conflicts are constant: Castilla-La Mancha, upstream on the Tagus, objects to water being sent to Murcia and Valencia, while Portugal objects when Spain takes more than the Albufeira agreement allows from the rivers they share.

The strategies have had mixed success. The transfer secures irrigation in the south-east but ties it to a political decision upstream, has been cut in droughts and has damaged the donor basin. Desalination has made coastal urban supply independent of rainfall, but it is expensive and energy-hungry, so it supplies taps, hotels and industry rather than fields. Wastewater reuse now irrigates a large area of farmland, parks and golf courses and is one of the clearer successes where treatment and regulation exist. Modernising irrigation with drip systems and lined canals has saved water per hectare, but the saving has often been reinvested in irrigating more hectares, so total use has barely fallen - that rebound is the evaluation point examiners reward. Pricing, metering and awareness campaigns have held urban demand down and drought decrees impose restrictions when reservoirs fall, but both are politically difficult where farming has strong regional representation. Overall Spain has avoided a national crisis by combining transfers, desalination and reuse, but it has not solved the imbalance between where water falls and where it is wanted, and drought years still bring restrictions, tanker deliveries and litigation.