Diagram library

Water moves continuously between ocean, atmosphere, land and ground. Nothing is created or destroyed — the cycle redistributes a fixed global stock.

Evaporation is a natural distillation. It lifts water molecules and leaves dissolved salts behind — which is both why rain is fresh and why the sea is salty.

Groundwater sits in the pore spaces and fractures of rock, not in underground lakes. An aquifer is a rock formation that holds and transmits usable quantities of water.

Pumping lowers the water table around a well into a cone. If pumping exceeds recharge, the cone deepens and widens indefinitely.

Fresh groundwater floats on denser sea water. Over-abstraction lowers the freshwater head, and the salt interface moves inland — usually irreversibly on human timescales.

A catchment is the area of land from which all rainfall drains to a single point. It is the natural unit for managing water — and rarely matches any administrative boundary.

A reservoir is an accounting problem: inflows minus outflows equals the change in storage. Every drought is that sum turning negative for long enough.

Drought propagates through the water system in stages, each lagging the last. A rainfall deficit takes months to become a groundwater deficit — and years to recover from one.

Snowpack and glaciers act as natural reservoirs, storing winter precipitation and releasing it through the dry season — which is when it is most needed.

The classic surface water treatment train. Each stage is a barrier, and the design principle is that no single stage is relied upon alone.

Sewage treatment removes solids, then organic matter, then — where required — nutrients, before returning the water to a river or the sea.

In dry weather everything goes to treatment. In heavy rain, flow exceeds what the sewer can pass forward, and the excess spills over a weir to a watercourse.

Osmosis moves water toward higher salt concentration. Applying pressure greater than the osmotic pressure reverses it, pushing pure water out and leaving salts behind.

Two ways to separate salt from water: boil the water off, or push it through a membrane. Membranes won on energy, which is why almost all new capacity is reverse osmosis.

A softener does not remove hardness so much as trade it. Calcium and magnesium are swapped for sodium on a resin bed, which is then regenerated with brine.

Carbon removes organic molecules by adsorption onto an enormous internal pore surface. Capacity is finite, and an exhausted filter looks exactly like a fresh one.

Where household water actually goes. Toilets, showers and washing are the bulk of it, and almost all of it returns to the sewer within seconds of use.

From catchment to tap and back again. Every stage has its own regulator, its own failure modes and its own cost.

These are not the same thing. Non-revenue water is everything not billed; leakage is only the part that physically escapes from pipes.

Cities break the natural cycle. Paving stops infiltration, so rain that would have soaked away becomes runoff that arrives all at once.

Almost everything unusual about water follows from one fact: the molecule is bent, so its charge is not evenly distributed.

The chemistry linking atmospheric carbon dioxide, rock weathering, water pH and limescale. It is the single most important equilibrium in natural waters.

pH is logarithmic, so each unit is a tenfold change. That is why small-looking differences in a water report can matter a great deal.

Hardness is geology written into water. Rain is soft everywhere; what it flows over decides what it becomes.

Limescale is calcium carbonate coming back out of solution. Heating is what does it, which is why kettles and heating elements scale and cold pipes generally do not.

Contamination enters at four distinct points, and each has a different owner, a different detection method and a different fix.

Nutrients and pesticides reach water by several routes with very different timescales — which is why land management changes take decades to show up in groundwater.

Nutrient enrichment triggers a chain that ends in oxygen collapse. The killing step is not the algae but their decomposition.

Fire changes a catchment’s hydrology as much as its ecology. The water quality problem usually arrives with the first heavy rain after the fire, not during it.

El Niño shifts where the rain falls. The same event brings flooding to some regions and drought to others, and the pattern is broadly repeatable.

Roughly 55 to 60% of an adult body is water, distributed between cells, the space around them, and blood plasma — and the proportions vary a lot between people.