Managing water supplies

The distribution of the Earth's water

Almost all of the water on Earth is salt water in the oceans, which hold about 97% of the planet's water, and none of it can be drunk or used to irrigate crops without expensive treatment. Only about 2.5% is fresh water, and most of that is locked up where people cannot easily reach it: the largest store of fresh water is the ice sheets and glaciers, followed by groundwater. The surface fresh water you can see - lakes, rivers, swamps and marshes - is only a tiny fraction of the whole, and a smaller amount still is held in the soil as soil moisture, in permafrost, and in the atmosphere as water vapour.

The water cycle, also called the hydrological cycle, moves water between these stores. The Sun's energy evaporates water from the oceans and from the land, the vapour condenses into clouds and falls as precipitation, and gravity carries the water back to the sea through rivers and groundwater flow. The total amount of water on Earth stays roughly the same; what changes is where the water is, and whether it is fresh and clean enough for people to use.

The idea to hold onto is that fresh water is scarce and unevenly spread. Water-rich regions such as the wet tropics receive far more rainfall than water-poor regions such as the desert belts, and even within one country a wet basin and a dry basin can sit side by side. This uneven distribution is the starting point of every problem in the topic, because water security depends on where the water is relative to where the people are.

• Salt water in the oceans: about 97% of the Earth's water, too salty to drink or to use for irrigation without treatment.
• Surface fresh water: ice sheets and glaciers, then lakes, rivers, swamps, marshes and permafrost.
• Sub-surface fresh water: soil moisture and groundwater, which moves slowly through porous rock, plus permafrost.
• Atmospheric water: water vapour in the air, the smallest store, which drives precipitation and returns water to the land.

What water security means

Water security is the ability to access sufficient quantities of clean water to maintain adequate standards of food and of manufacturing of goods, adequate sanitation and sustainable health care. Notice that the definition is about access to clean water, not simply about how much water exists. A region can sit beside a huge river and still be water-insecure if the water is polluted, too far away, or too expensive to reach.

Three things must all hold for a supply to be secure. The water must be clean, so that drinking it does not cause disease; it must be available in a sufficient quantity all year round, not just in the wet season; and people must be able to access it, which means it has to be close enough, affordable and reliable. Remove any one of these and the people are water-insecure even though the water itself may still be there.

Security is also unequal within countries. Urban areas usually have piped, treated water because the population is dense and easy to connect, while rural areas often rely on wells, rivers or springs that may run dry or be contaminated. The same divide runs between water-rich and water-poor regions, and that inequality of availability is itself a cause of insecurity listed in the syllabus.

Causes of water insecurity: the natural and physical causes

The natural causes of water insecurity all reduce the amount of clean water available. Climate change alters rainfall patterns, making some regions drier while others suffer heavier, more destructive storms, and higher temperatures increase evaporation from reservoirs, rivers and bare soil. A place that could once rely on steady seasonal rain may now face longer dry spells and shorter, more intense wet seasons.

Natural disasters do the same thing in two opposite ways. Drought is a long period of below-average rainfall that dries up rivers, reservoirs and wells, while flooding overwhelms the land with water that is contaminated by sewage, silt and pollutants, so it cannot be safely used. Both disasters leave people short of clean water even though flooding is an excess of water, not a shortage of it.

Pollution events and inadequate sanitation work together. Industrial effluent, agricultural run-off of fertilisers and pesticides, and untreated sewage all enter rivers and groundwater and make them unsafe to drink. Where sanitation is poor, human waste is not removed or treated, so the water that people rely on becomes the vehicle that spreads disease back to them.

• Climate change: changes in rainfall, more severe drought and higher evaporation.
• Drought: a long period of below-average rainfall that dries up the stores of water.
• Flooding: excess water contaminated with sewage and pollutants, so it cannot be used.
• Pollution events and inadequate sanitation: effluent, run-off and untreated waste make water unsafe.

Causes of water insecurity: the human and management causes

Most causes of water insecurity are made by people. Population growth raises the demand for water for drinking, washing and food, and urbanisation concentrates that demand in cities while covering land with impermeable surfaces that stop rainwater soaking into the ground. Deforestation removes the trees that hold soil and regulate the flow of water, so rain runs off the bare land quickly, rivers flood and then run dry.

Competing demands pull a limited supply in different directions. The agricultural sector is usually the largest user of water, often taking 70% or more of a country's supply for irrigation, while the industrial, energy and domestic sectors each demand their own share. When these demands grow faster than the supply, someone has to go short, and it is usually the poorest who lose out.

Mismanagement of irrigation turns a short-term fix into a long-term problem. Watering crops by flooding the fields, or using irrigation water that carries dissolved salts, raises the water table and leaves salt behind as the water evaporates, a process called salinisation. The salt poisons the soil so that crops grow poorly or not at all. International competition adds a political dimension, because many rivers and aquifers cross borders, and one country's extraction reduces what the country downstream receives.

The impacts of water insecurity

Water insecurity harms people through the food they eat and the water they drink. Reduced crop yield and crop failure come first: when crops do not receive enough water they grow poorly, and in a drought the harvest can fail altogether. Livestock death follows for the same reason, as animals run out of drinking water and of the grass and fodder they eat.

The result of failed crops and dead livestock is food shortages, which lead to malnutrition and, in the worst cases, famine. These impacts cascade, because a malnourished person is less able to work or to fight off disease, so one bad harvest can trap a whole region in a downward spiral.

The second pathway is disease. When people are forced to drink contaminated water, the pathogens in it cause illness, and the syllabus limits you to two examples: diarrhoea and cholera. Cholera in particular spreads through water contaminated by human waste and can kill within hours if untreated, and it is most dangerous where water insecurity and poor sanitation meet.

• Reduced crop yield and crop failure when plants do not get enough water.
• Livestock death from lack of drinking water and fodder.
• Food shortages, malnutrition and famine as harvests fail.
• Illness from contaminated drinking water, limited to diarrhoea and cholera.

Managing water security: improving the supply

The first set of strategies improves the supply of water. A piped supply carries treated water from a source to homes and taps, and it is the backbone of water security in towns and cities because it keeps the water clean and delivers it on demand. Where groundwater is available, boreholes drilled into an aquifer reach water that is usually cleaner than surface water, and an artesian well is a well in which the water rises to the surface under its own pressure without pumping.

Gravity-fed schemes use the natural fall of the land to move water from a spring or stream high up a hillside down to villages below, with no pump and no fuel cost, which makes them cheap to run and easy to maintain. Reservoirs and dams store river water from the wet season so that it can be released through the dry season, smoothing out the supply and providing a reliable source for towns, farms and, often, hydroelectric power.

Every supply strategy has a price. Dams and reservoirs flood land, displace communities and lose water by evaporation from the open surface, while boreholes can lower the water table if too many are sunk and groundwater is extracted faster than it is recharged. The syllabus calls for sustainable water extraction, which means taking water at a rate the source can renew, so that the supply lasts.

Managing water security: reducing demand and supporting change

The second set of strategies reduces demand for water. Improved irrigation techniques, such as drip irrigation that delivers water slowly to the roots of each plant, use far less water than flooding a field and lose less to evaporation. Growing crops less dependent on high water supply matches the crop to the climate, so a dry region plants drought-tolerant crops instead of thirsty ones.

Recycling treats used water so that it can be used again, and rainwater catchment collects rain from roofs into tanks for drinking, washing and watering. Education on sustainable water use changes behaviour, teaching people to fix leaks, turn off taps and value water as a scarce resource. Poverty reduction supports all of this, because poor communities often cannot afford the pipes, pumps and tanks that make the other strategies possible.

International agreement and water-related aid matter where rivers and aquifers cross borders: neighbouring countries agree how to share the water, and aid pays for wells, pipes and training in poorer regions. Finally, rationing restricts how much water each household may use, usually in a crisis, as a short-term way to stretch a failing supply. Evaluating these strategies means weighing their benefits against their cost and side-effects, and recognising that most successful schemes combine several strategies rather than relying on one.