How much water can we use?
About 97% of Earth's water is salty. About 2–3% is fresh, and most of that is locked in ice sheets and glaciers or deep underground. Less than 1% is easy-to-reach fresh water in aquifers, rivers and lakes.
Water is a renewable resource because the water cycle refills it. But it is renewable only at the rate nature refills it. Water in some deep aquifers is thousands of years old (fossil water) and is hardly refilled at all.
Unsustainable water use
Use is unsustainable when we take water faster than it is replaced, or damage it so it cannot be used.
- Over-abstraction of groundwater: pumping more than the recharge makes the water table (top of the saturated rock) fall. Wells dry up, land can sink (subsidence) and, near coasts, sea water seeps in (saline intrusion).
- Rivers diverted for irrigation: rivers may no longer reach the sea, and inland lakes can shrink dramatically, as happened to the Aral Sea in Central Asia.
- Wasteful irrigation: flood irrigation loses much water to evaporation, and can cause salinisation of soils.
- Pollution: sewage, farm fertilisers, industrial waste and plastics make water unsafe.
- Draining wetlands removes natural water stores and filters.
Sustainable water management
- Match use to recharge: monitor water tables and limit pumping with licences or quotas.
- Efficient irrigation: drip and sprinkler systems, and crops that need less water.
- Integrated catchment management: plan the whole river basin together, upstream and downstream, farms, towns and nature.
- Fair pricing and metering: people waste less when they pay for what they use, with a free basic amount for everyone.
- Protect wetlands and forests that store and clean water.
- Managed aquifer recharge: let rain and treated water soak back into the ground through ponds and check dams.
Thermohaline circulation
Thermo means heat and haline means salt. The density of sea water depends on both: colder and saltier water is denser (heavier).
- Warm surface currents carry heat from the tropics towards the poles.
- Near the poles the water cools. When sea ice forms, salt is left behind in the water, so it becomes even saltier.
- This cold, salty, dense water sinks to the deep ocean.
- It spreads slowly along the ocean floor towards the equator and other oceans.
- Over hundreds of years it rises (upwelling), warms and returns at the surface.
This global ocean conveyor belt moves heat, so it shapes climate; it also carries oxygen down and brings nutrients up. Melting ice adds fresh water, which makes surface water less dense and could slow the conveyor.
Treating contaminated water
- Screening: grids remove large objects (sticks, plastic).
- Sedimentation: water rests in tanks so heavy particles settle. A chemical (coagulant) can clump tiny particles so they settle faster.
- Filtration: water passes through sand and gravel beds that trap fine particles.
- Disinfection: chlorine, ozone or ultraviolet light kills harmful microbes.
Sewage treatment adds a biological stage where bacteria break down organic waste before the water returns to rivers. At home or in emergencies, boiling, cloth filters and solar disinfection (clear bottles in sunlight) help.
New water sources and saving water
- Desalination: removes salt from sea water, mainly by reverse osmosis (pushing water at high pressure through a membrane that blocks salt). It is reliable but uses a lot of energy and produces brine that can harm sea life.
- Water reuse: treated waste water for farms, industry or even drinking.
- Rainwater harvesting: collect roof run-off in tanks or let it recharge the ground.
- Inter-basin transfers: pipes or canals moving water between river basins; costly with environmental impacts.
- Saving water: fix leaks, low-flow taps and dual-flush toilets, reuse grey water for gardens, and water-saving crops.
Try it
For one day, write down every time you use water at home and estimate the litres (a bucket ≈ 10 L, a full bath ≈ 150 L, a 5-minute shower ≈ 40 L). Add them up. Then list three changes that would cut your total by 20%.
Key formulas and definitions
- Sustainable if: abstraction ≤ recharge
- Change in storage = recharge − abstraction
- Density of sea water rises when it is colder or saltier
- Treatment: screen → settle → filter → disinfect
- Reverse osmosis: pressure pushes water through a membrane, salt stays behind
Worked examples
1. An aquifer holds 600 million m³. Recharge is 20 million m³ per year and pumping is 50 million m³ per year. How long before it is empty if nothing changes?
Net loss = 50 − 20 = 30 million m³ per year. Time = 600 ÷ 30 = 20 years.
2. A city has 1 000 000 people. Each uses 150 L a day. Leak repairs and low-flow fittings cut use by 20%. How much water is saved per day?
Daily use = 1 000 000 × 150 = 150 000 000 L. Saving = 20% × 150 000 000 = 30 000 000 L, which is 30 000 m³ a day.
3. Explain why sea water sinks near the poles.
Cold air cools the surface water, and when sea ice forms it leaves salt behind. The water becomes colder and saltier, so its density rises above the water below, and it sinks. This drives the deep part of thermohaline circulation.
Common mistakes
- Thinking water is unlimited because it is renewable. It is renewable only at the recharge rate.
- Saying only temperature drives deep ocean currents. Salt (salinity) matters just as much: thermo + haline.
- Thinking filtration alone makes water safe. Disinfection is needed to kill microbes.
- Seeing desalination as a perfect fix. It needs much energy and makes brine waste.