Here's the thing that surprises most students: even though 71% of Earth's surface is covered in water, 97.5% of it is saline (salty) seawater — completely useless for drinking or farming without expensive treatment. That leaves just 2.5% as freshwater.

But don't get excited yet — most of that freshwater is locked away where humans can't easily reach it:

So the water humans can realistically use — rivers, accessible groundwater, lakes — is a tiny sliver of a tiny sliver. That scarcity is why water management matters so much.

This is one of the most testable ideas in this topic, so let's make sure it really sticks. Globally, water use splits 69% agriculture, 19% industry, 12% domestic. But that global average hides a massive difference depending on how developed a country is:

Why the flip? Think of it like this: in developing countries, more of the workforce is still tied to subsistence and small-scale farming, and hot climates with limited natural water supply push agricultural water use even higher. In developed countries, economies have shifted toward manufacturing, energy production and services — so industry dominates water demand instead. Leisure and tourism (golf courses, water parks) is also a growing water-guzzler, mostly in developed nations.

Demand = the amount of water required by users to meet their needs. Supply = the amount of water actually available. The gap between them is the water balance — a country can have a deficit (demand greater than supply) or a surplus (supply greater than demand).

Global water demand has absolutely rocketed: from around 1 trillion m³ in 1934 to 4 trillion m³ by 2014 — a fourfold increase in 80 years. Six overlapping causes drive this:

• Improving living standards — more appliances (washing machines, dishwashers), better sanitation, all using more water per person
• Leisure and tourism — water parks, golf courses, hotel pools
• Urbanisation — more people concentrated in cities needing piped supply
• Population growth — simply more people means more total demand
• Increasing industry — manufacturing and energy production are thirsty processes
• Increasing agriculture — feeding more people means more irrigation and livestock water

Three main natural sources: lakes and rivers, aquifers (underground water-bearing rock), and reservoirs (artificial lakes created by dams). The catch is that supply isn't spread evenly — and cruelly, many of the world's most densely populated regions are also among the driest (think of parts of the Middle East, India, and North Africa). The WHO estimates that in 2022, 73% of the world's population had access to safely managed drinking water — which, flipped around, means over a quarter did not.

A country's water balance depends on a combination of supply factors and demand factors:

Areas with the greatest deficit: Australia, North/East/South Africa, the Middle East, West USA, parts of South America, India.

Areas with a surplus: North-east Brazil (Amazon rainforest), Canada and northern USA, Russia.

Notice the pattern: deficit areas tend to be hot and dry with high evaporation and often high demand; surplus areas tend to be cold or tropical with high rainfall and lower populations.

"Potable" is a key exam word — it simply means safe to drink. Roughly a quarter of the world's population lacks access to clean, potable water, and unsafe water is estimated to cause 1.2 million deaths worldwide every year — mostly from waterborne diseases like typhoid, cholera, bilharzia and dysentery, which spread easily in stagnant water (water that isn't flowing or being refreshed, common in areas of low precipitation).

Globally, an estimated 80% of wastewater enters water systems without being treated — that's the scale of the pollution problem we're dealing with.

This is the section examiners love to test with "explain" questions, because it's easy to just list causes without linking them to actual impacts. Train yourself to always connect the two.

🌾 Agriculture

• Sources: wastewater from silage & slurry; fertilisers, herbicides and pesticides dissolving into rainwater; soil erosion; medicine residues from livestock treatment
• Impacts: dead fish/wildlife introduce bacteria into water; eutrophication (excess nutrients cause explosive algae growth, which then dies and decomposes, using up oxygen and suffocating aquatic life); poisoning of fish and wildlife; increased sediment raises turbidity (cloudiness), reducing light penetration and oxygen levels

🏭 Industry

• Sources: oil spillages; water used as a coolant then returned to rivers at higher temperatures (thermal pollution); toxic chemicals released directly into rivers
• Impacts: oil coats wildlife/plants on the surface so they can't eat, move or fly; higher water temperatures make survival impossible for temperature-sensitive species; toxins enter the food chain, harming both wildlife and the humans who eventually eat that wildlife or drink that water

🏠 Domestic

• Sources: untreated sewage; water from washing/bathing; chlorine-treated water from leisure use (e.g. swimming pools); medicine residues from human use
• Impacts: raised nitrogen levels and introduced bacteria; washing/bathing chemicals (bleach, soap, shower gel) harm wildlife; chlorine causes chemical burns to organisms; medicine residues are especially hard to remove during water treatment

Think of this as a supply chain, and like any supply chain, it's only as strong as its weakest link. Developing countries often break down at multiple stages, which is why access to clean water varies so dramatically by development level: over 90% access in developed countries vs. under 40% access in some developing countries (i.e. over 60% lack it).

1. Collection — Dams and reservoirs store surface water (and double up for flood control and hydroelectric power). Wells and boreholes access groundwater — almost 50% of the world's drinking water comes from groundwater, and it's especially vital in arid areas with little surface water. But here's the warning sign: groundwater abstraction is increasing so fast that levels in over 30% of the world's aquifers are dropping faster than they were in 1980. In places like California's San Joaquin Valley, this over-extraction has caused the ground itself to sink (subsidence) by nine metres over 50 years, because water is being pumped out faster than it can naturally recharge.

2. Pipelines — Water is transferred via pipeline infrastructure, which is far more extensive and reliable in developed countries because it's expensive to build and maintain. Large-scale water transfer projects move water from surplus regions to deficit regions — the Tagus-Segura Project (Spain) and the South-North Water Transfer Project (China) are the two named examples you need (covered in full as case studies below).

3. Treatment — Even groundwater contains some contaminants. In developed countries and urban areas of developing/emerging countries, water passes through treatment works involving filtration and disinfection, removing solid waste, bacteria, viruses, fungi, minerals and chemical pollutants. In rural areas of developing countries, treatment works often don't exist, so water may only be treated at the point of use.

4. Delivery — In developed countries, pipelines deliver treated water directly to homes and factories. In developing/emerging countries, urban areas often rely on shared standpipes, and rural areas often depend on untreated wells as the main source.

Desalination deserves its own mention — removing salt from seawater to make it drinkable. It currently supplies only about 1% of the world's water (2025) because it's extremely expensive and energy-intensive, which is why almost all desalination plants are located in wealthy, water-scarce nations.

Flooding happens when the capacity of a river channel is exceeded — the water simply has nowhere else to go, so it spills over the banks onto the surrounding land. It's usually the result of heavy or prolonged rainfall.

But the single most important concept here is lag time — the time taken for precipitation to reach the river channel after it falls. The shorter the lag time, the higher the flood risk, because river discharge rises faster. Picture two scenarios:

Both human and physical factors affect lag time — and almost all of them work by increasing (or decreasing) overland flow, which is water flowing across the surface rather than soaking in.

• Deforestation — fewer trees means less interception (leaves catching rain) and less infiltration (roots helping water soak in), so overland flow increases
• Urbanisation — impermeable concrete and tarmac increase overland flow; drains are specifically designed to move water to the river rapidly; bridges and their supports reduce a river's capacity
• Agriculture — bare soil and ploughing increase overland flow (nothing to intercept or hold the water back)
• Human-induced climate change — rising global temperatures may increase the frequency and intensity of storms, leading to heavier, more prolonged rainfall

• Relief — steep slopes reduce infiltration time and increase overland flow (water runs downhill fast before it can soak in)
• Rock type — impermeable rocks reduce percolation, increasing overland flow
• Soil — frozen, saturated or compacted soil reduces infiltration; clay soils in particular reduce infiltration and increase overland flow
• Weather — heavy/prolonged rainfall exceeds the infiltration rate; rapid snowmelt after rising temperatures also increases overland flow
• Seasonal variations — Northern Europe floods more in autumn/winter (more frequent rain); monsoon regions see huge rainfall concentrated in a few weeks, saturating the ground; spring snowmelt in mountains increases overland flow
• High drainage density — many tributaries feeding the main channel means a rapid increase in discharge
• Lack of vegetation — less interception, so more overland flow

Improvements in weather forecasting and computer modelling have made flood prediction increasingly accurate — particularly in developed countries. In the UK, the Flood Forecasting Centre is a partnership between the Met Office and the Environment Agency. Forecasting draws on:

• Rainfall amounts
• Hazard impact mapping (identifying the areas most at risk)
• Past storm hydrographs (to forecast the likely rate of discharge change)
• Drainage basin characteristics (rock type, vegetation amount, relief)

Flood alerts are then issued, informing people of current risk and the risk over the following five days.

Flood prediction is far more limited in emerging and developing countries, due to a lack of river monitoring infrastructure and the cost of setting up alert systems. The Bangladesh Water Development Board is a good named example of progress here — it has focused on improving flood warnings in recent years and can now inform people of flood risk up to 3 days in advance (still less warning time than the UK's 5 days).

This is a classic compare-and-contrast exam topic, so get the core distinction rock solid: hard engineering involves building structures or physically changing the river channel. Soft engineering works with natural processes of the river and surrounding environment instead of fighting them.

Soft engineering is becoming more popular because it has less environmental impact. It's also described as an example of mitigation — the schemes aim to minimise damage rather than to prevent flooding outright.

A third strategy that doesn't fit neatly into either category is flood-resistant building design: raising homes on stilts, replacing carpet with tiles, and placing power sockets above the likely flood level.

South-east Spain receives only around 365mm of precipitation a year (mostly in winter) — one of the driest parts of Europe. Agriculture uses 80% of available water, with 147,000 hectares of land requiring irrigation. Regular droughts squeeze supply further, creating a persistent water deficit. Tourism makes things worse: an average tourist uses 450–800 litres/day versus just 127 litres/day for an average Spaniard, and water parks/golf courses use huge quantities.

Tagus-Segura Project — completed in 1978, this is a 286km pipeline connecting four Spanish river basins (Tagus, Jucar, Segura, Guadiana). It transfers 60% of the water flowing into the Tagus, aiming to supply Alicante, Murcia and Cartagena in the deficit-hit south-east.

Issues: much of the transferred water went to tourism/leisure users rather than small-scale farmers; water consumption in the south-east actually increased because supply increased; an estimated 15% of transferred water is used illegally by leisure users like golf courses; large commercial farms benefit more than small-scale farmers.

Ebro Project — a second transfer scheme proposed in 2001 to move water from the River Ebro. It was abandoned due to the failures of the Tagus-Segura scheme, high cost, and the threat to the Ebro delta (the transfer would have disrupted sediment flow to the delta). Spain has since turned to desalination plants to meet demand instead.

North-east China receives only 200–400mm of precipitation a year, concentrated in a brief wet season, and average rainfall has steadily decreased since the 1950s. The main industrial and economic growth regions (North China Plain, around Tianjin and Beijing) rely heavily on groundwater. In Beijing alone, demand is 3.6 billion m³ against a supply of just 3 billion m³ — a significant deficit that has caused over-abstraction of groundwater and subsidence of up to 11cm in parts of the city.

South-North Water Transfer Project (SNWTP) — aims to transfer 12 trillion gallons of water a year from southern China to the Beijing area. The central section was completed in 2014; full completion is estimated for 2050. It links China's four major rivers: Yangtze, Yellow, Huaihe and Haihe.

Three Gorges Dam — a separate project on the Yangtze River, built for flood control, hydroelectric power, water storage and improved navigation. Constructed 1994–2012, it is the world's largest multipurpose dam: over 2km wide, 100m high, with a reservoir over 600km long.