Water Sources and Supply

The water cycle: stores and transfers

The water cycle (hydrological cycle) is the continuous movement of water between the oceans, the land, the atmosphere and living things. It is a closed system: essentially no water is added to it or lost from it, so the total supply on Earth is fixed. What changes is where the water is stored and how quickly it moves between those stores, and that is what decides whether people in a region have water when they need it.

A store is a place where water is held for a period of time. The largest store by far is the ocean, which holds salt water - about 97 per cent of all the water on Earth. Less than 3 per cent is fresh water, and most of that is frozen in ice sheets and glaciers in Antarctica and Greenland. Of the fresh water that is liquid, most is groundwater, held in the pores and cracks of rock below the surface. The atmosphere holds water vapour and cloud droplets, and lakes and rivers hold only a tiny share. That is the first fact to hold on to: fresh water is scarce, and most of it is either frozen or underground.

A transfer (flow) is a process that moves water from one store to another. Precipitation - rain, snow and hail - brings water from the atmosphere to the land and the sea, and some of it is caught by leaves and evaporates before it reaches the ground (interception). The water that reaches the ground either flows over the surface as surface run-off into streams and rivers, or soaks into the soil (infiltration) and then moves sideways through the soil and rock (through-flow) or downwards to the water table to join the groundwater (groundwater flow). Water returns to the atmosphere by evaporation from open water and wet surfaces and by transpiration from plants. Condensation turns that vapour back into droplets, which form clouds and eventually fall as precipitation again.

Interpreting the cycle for a real region means asking which store is large and which transfers are fast. A rainforest has heavy rainfall and rapid transfers, so water spends little time on land before running back to the sea. A hot desert has small stores, little precipitation and high evaporation, so the same rainfall supports far less life. A city that draws its water from a river or an aquifer can read the same diagram to see where its supply comes from and how quickly a dry year would reach it.

• Stores: salt water in the oceans; fresh water in ice sheets and glaciers, groundwater, the atmosphere, and lakes and rivers.
• Transfers: precipitation, interception, surface run-off, infiltration, through-flow, groundwater flow, transpiration, evaporation and condensation.
• The cycle is closed, so the global supply of water is fixed; only the distribution between stores changes.
• Precipitation that reaches the ground either runs off over the surface or infiltrates into the soil and rock.

Where the fresh water people use comes from

People use fresh water from four places. Rain and snow are the input to all the others: rainfall is collected from roofs and from rock catchments and stored in tanks and small dams, which is the oldest form of water supply and still the main one in many rural areas. Surface water - rivers, lakes and reservoirs - is what most large cities depend on, because it is easy to reach and can be moved in large volumes through pipes and canals.

Groundwater is water that has infiltrated and filled the pores and cracks of a permeable rock. An aquifer is a body of rock that stores water and allows it to flow, and the water table is the top of the saturated zone below which the pores are full. Water is taken from an aquifer by a well or a borehole, usually with a pump. Groundwater is attractive because it is not exposed to evaporation or to the pollution that collects on the surface, and because it can be pumped during a drought when a river is low - but it is renewed only by the slow recharge of water seeping down from above, which may take years or centuries.

Some aquifers hold water that fell as rain in a much wetter past climate and is not being replaced at all; those stores are worked rather like a mineral deposit. The oceans are the world's largest store, and every continent has a coastline, so seawater is available even where rain is not. The problem is that it is saline - it contains dissolved salts, mainly sodium chloride - which makes it undrinkable and useless for irrigation without treatment. Removing those salts is desalination. The five oceans are the Pacific, the Atlantic, the Indian, the Southern and the Arctic; the Pacific is the largest and the deepest.

Choosing between these sources is a management decision, and it is usually made on three grounds: how much water the source can give, how reliable it is through the year, and what it costs to treat, move and store. A reliable source that is 200 km from the city may be worse than an unreliable one next door, once the pipeline is paid for.

• Atmosphere: rain and snow, collected from roofs and catchments and stored in tanks and small dams.
• Surface water: rivers, lakes and reservoirs, the source most large cities rely on.
• Groundwater: aquifers and wells, protected from evaporation but renewed only slowly by recharge.
• Oceans: Pacific, Atlantic, Indian, Southern and Arctic; seawater must be desalinated before use.

Desalination: distillation and reverse osmosis

Desalination is the removal of dissolved salts from seawater, or from brackish groundwater, to produce fresh water. Two processes are examinable. In distillation, the water is heated until it boils, the salt stays behind in the liquid, the steam is pure water, and the steam is then cooled and condensed to form potable water. Distillation works with any quality of source water and can be driven by waste heat from a power station, but the energy needed to boil large volumes of water is enormous.

In reverse osmosis, the water is put under high pressure and forced through a membrane that allows water molecules to pass through but stops most ions and other molecules. The salts are left behind in the concentrated solution on the pressurised side. Reverse osmosis uses far less energy than distillation and is the process used in most large modern plants, but the membranes are expensive, they clog with sediment, algae and scale, and they must be cleaned and replaced. The plant also needs a reliable electricity supply and skilled operators, and it produces only as much water as it is given power for.

The benefits are real, especially for a dry coastal region with money and energy available. Desalination gives a supply that does not depend on rainfall, so a drought cannot empty the taps; it can be built on any coastline; it produces water of a consistent quality; and it can be built in stages as demand grows. Israel has a chain of reverse osmosis plants along the Mediterranean coast, including the large Sorek plant, which together supply a large share of the country's domestic water.

The limitations are cost, energy and waste, and they are what the marks are for. Desalinated water costs more per cubic metre than water taken from a river or an aquifer, because the plant has to be built, powered and maintained, and that cost is passed on to the customer. The brine left behind - a concentrated salt solution, sometimes about twice as salty as the sea - must be disposed of, and releasing it into shallow coastal water can harm seabed organisms and the plants and animals that live there. The energy demand is large, so the carbon footprint depends on how the electricity is generated, and the whole scheme is vulnerable to the price of that energy.

Making water potable: screening, sedimentation, filtration, chlorination

Potable water is water that is safe to drink. That means it is free of the bacteria, viruses and parasites that cause disease, free of harmful concentrations of dissolved substances such as nitrates, heavy metals and pesticides, and acceptable in appearance, taste and smell. Water can look perfectly clear and still be dangerous, which is a favourite trap: clarity is not the same as safety, and pathogens cannot be seen at all.

Water taken from a river, lake or reservoir goes through four stages. Screening removes large objects - leaves, sticks, fish, litter - by passing the water through a coarse metal grid or screen. Sedimentation is the settling stage: the water is held in a large tank so that sand, silt and other suspended particles sink to the bottom and can be removed as sludge, and a coagulant such as aluminium sulphate may be added so that fine particles clump together and settle faster.

Filtration passes the settled water through beds of sand and gravel, which trap the remaining fine particles and a large proportion of the microorganisms. Chlorination is the disinfection stage: chlorine, or a chlorine compound such as sodium hypochlorite, is added in a controlled dose to kill the bacteria and viruses that survive filtration. Chlorine also leaves a small residual concentration in the water, so the water stays protected as it travels through the pipes to the houses.

After treatment the water is stored in a covered service reservoir and distributed under pressure through a network of mains. Two things can undo good treatment: leaks, which allow contaminated groundwater into the mains when the pressure drops, and broken or cross-connected sewers, which allow sewage into the network. That is why the pressure, the chlorine residual and the bacterial quality of the water are monitored along the distribution system and not only at the works. Where there is no piped supply at all, the same principles are applied at the household: the water is filtered or left to settle, then boiled or dosed with chlorine tablets.

• Screening: removes large objects such as leaves, sticks and litter.
• Sedimentation: suspended particles sink as sludge; a coagulant may be added to speed this up.
• Filtration: water passes through beds of sand and gravel, which trap fine particles and many microorganisms.
• Chlorination: chlorine is added to kill the remaining bacteria and viruses, and a residual protects the water in the pipes.

Who gets safe water: water-rich and water-poor regions, cities and villages

Rainfall is not spread evenly over the world, and neither are the money, the pipes and the treatment works needed to turn it into potable water. Water-rich regions such as the Amazon basin, the Congo basin, northern Europe and Canada receive more water than they use, although the water may still be polluted. Water-poor regions such as the Sahel, the Arabian Peninsula, the interior of Australia and the Atacama in Chile receive little rain, lose much of it to evaporation, and may depend on groundwater that recharges very slowly. Water scarcity is not simply an absence of rain: a region is short of water when the demand from people, farms and industry exceeds the reliable supply.

The difference between urban and rural supply is largely one of infrastructure. A city of several million people can justify a large treatment works, a reservoir or a desalination plant and a piped network with continuous chlorination, because the cost is shared over a large connected population. In many rural areas the same service is not economic: households depend on a hand pump, a protected well or an unprotected stream, and water is carried home in containers. Boiling and household chlorination then have to do the work of a treatment works.

The sharpest global inequality is in sewage treatment. Where sewers and treatment works exist, human waste is kept away from drinking water sources and water-related disease is rare. Where they do not, sewage enters the same rivers, lakes and shallow groundwater that people drink from, so the water that is available is unsafe. Many people still live without a safely managed sanitation service, and rapid urban growth in informal settlements often runs ahead of the pipe network, so a household may have a piped supply in theory but not at the tap.

These differences change what a sensible solution looks like. In a wealthy dry city, desalination, metering and reuse make sense. In a poor rural area the priorities are usually a protected source, simple treatment at the point of use, latrines sited so that they do not contaminate the well, and bringing the supply closer to the home so that water collection does not take hours of the day.

• Water-rich regions: high, reliable rainfall (Amazon and Congo basins, northern Europe, Canada), but the water may still be polluted.
• Water-poor regions: low rainfall and high evaporation (the Sahel, the Arabian Peninsula, central Australia, the Atacama), often dependent on slowly recharged groundwater.
• Cities: a large connected population can fund treatment works, reservoirs and piped networks with chlorination and continuous monitoring.
• Rural areas: households may rely on hand pumps, wells or streams, with boiling or household chlorination as the treatment.
• Global inequality in sewage treatment: without sewers and treatment works, human waste re-enters the water people drink.

Multipurpose dams: eight uses in one structure

A multipurpose dam is a dam built across a river to do several jobs at once. The reservoir behind it stores water, and that store can be released when it is needed. The eight uses you must know are flood control, hydro-electric power, irrigation, storage of water, transport, recreation, tourism and fish farming.

Flood control: by holding back water in the wet season and releasing it slowly, the dam cuts the peak flow downstream, so towns and farmland are protected. Hydro-electric power: water released through turbines generates electricity without burning fuel, giving a renewable supply that can be switched on quickly as demand changes. Irrigation: stored water is released into canals in the dry season, so farmers can grow a second or third crop where rainfall alone would allow only one. Storage of water: the reservoir holds water through the dry season for domestic and industrial supply, evening out the difference between wet and dry months.

Transport: a navigable reservoir and a regulated downstream flow let barges and boats carry heavy loads at lower cost and with less fuel than lorries. Recreation and tourism: a large lake attracts fishing, boating, walking and hotels, creating local jobs and income. Fish farming: cages and pens in the reservoir allow fish to be farmed for food and for sale without going to sea.

The same dam also carries serious costs. Building the reservoir floods land upstream, so villages, farmland, forest and sometimes towns and archaeological sites are lost and the people who lived there must be resettled, often with compensation they consider inadequate. The dam traps sediment that used to travel downstream, so the river below erodes its bed and banks, floodplain soils lose the silt that kept them fertile, and deltas far downstream starve and shrink. Migratory fish such as salmon cannot reach their breeding grounds, so their numbers fall even where fish ladders are built. Water is lost by evaporation from the reservoir surface, which matters most in a hot, dry climate where the surface area is large. On an international river the dam changes the flow, the sediment and the fisheries in the next country, so a basin-wide agreement is needed and the upstream state holds the advantage. Weighing the eight benefits against these costs, and asking who gains and who loses, is exactly what a 'discuss' question wants.

Supplying more and using less: reservoirs, transfers, reuse and metering

When demand outgrows the local supply, managers either find more water or use less of it. On the supply side, water is stored in reservoirs to carry wet-season water into the dry season; it is moved long distances by water transfer schemes of canals, tunnels and pipelines from a wet basin to a dry one; sewage and industrial water is reclaimed and reused for irrigation, cooling and flushing, which frees higher-quality water for drinking; and rainwater harvesting collects roof and catchment run-off in tanks and small dams close to where it is used. On the coast, desalination adds a supply that does not depend on rainfall at all.

On the demand side, the aim is to get more useful work out of every cubic metre. Metering and charging by volume make households and factories pay for what they use, which encourages them to repair leaks and use water carefully. Leak detection and mains replacement recover water that has already been treated and paid for: in many old cities a substantial share of the treated supply never reaches a customer, which makes leak repair the cheapest new source of all. Drip irrigation delivers water to the base of each plant instead of spraying a whole field, cutting losses by evaporation and run-off. Reuse of greywater for flushing and gardens, low-flush toilets and efficient appliances, growing crops suited to the local rainfall, and public information campaigns all cut demand without building anything new.

Every strategy has a limitation, and that is where the marks are. Transfers take water from a donor basin, so the region that loses the water loses fisheries, wetlands and the flow that downstream users depended on. Reclaimed water can only be used where the second pipe network exists to deliver it, and public concern about using treated sewage can block a scheme. Reservoirs lose water by evaporation and trap sediment. Metering is unpopular, costly to install and can hit the poorest households hardest, and it only saves water where a piped supply exists to be measured: telling a villager who carries water from a hand pump to fit a low-flow shower changes nothing.

The judgement usually reaches the same conclusion. Supply-side schemes such as dams and desalination deliver a large, dependable volume but cost a great deal and carry environmental and social costs; demand-side measures are cheaper, quicker and less damaging, but there is a ceiling to how much a city can save and they cannot create water where none exists. The countries that manage water scarcity best, such as Israel with its combination of desalination, drip irrigation, reuse and metering, use both, and choose whichever is cheapest for the next cubic metre.

• Store the wet season: reservoirs and catchment tanks carry water into the dry season.
• Move it: transfers by canal, tunnel and pipeline from a wet basin to a dry one.
• Reuse it: reclaimed sewage and industrial water for irrigation, cooling and flushing.
• Collect it: rainwater harvesting from roofs and catchments into tanks and small dams.
• Make it: desalination on the coast, independent of rainfall but expensive.
• Save it: metering and charging by volume, leak repair, drip irrigation, efficient appliances and reuse of greywater.