The water cycle | Hydrionis

The water cycle is usually taught as a loop of four words: evaporation, condensation, precipitation, collection. That is true but not very useful. What makes the cycle informative is the rates — how much water moves along each pathway, and how long it stays in each store.

The water cycle

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

Evapotranspiration: Evaporation and transpiration together. Over land it returns roughly two thirds of precipitation to the atmosphere before it ever reaches a river.
Baseflow: The groundwater contribution to river flow. It is what keeps rivers running weeks after the last rain.
Residence time: Atmospheric water turns over in about nine days; deep groundwater can take tens of thousands of years.

The diagram shows pathways, not proportions. Ocean evaporation dwarfs every land flux shown, and the arrows are not drawn to scale.

The fluxes

Approximate annual global water fluxes

Flux	Approximate volume (km³/yr)	Note
Evaporation from the ocean	~413,000	About 86% of all evaporation
Precipitation onto the ocean	~373,000	Less than evaporates from it
Evapotranspiration from land	~73,000	Evaporation plus plant transpiration
Precipitation onto land	~113,000	More than evaporates from it
River discharge to the ocean	~40,000	Balances the ocean’s evaporation deficit

Estimates vary between compilations. The important relationship is structural: the ocean loses more to evaporation than it gains from rain, land gains more than it loses, and rivers carry the difference back.

That last row is the whole reason fresh water exists on land. The ocean evaporates about 40,000 km³ more each year than falls back onto it. That surplus is transported over land by the atmosphere, falls as rain and snow, and returns to the sea through rivers and groundwater. Everything humans use is drawn from that flow in transit.

Residence times

How long a water molecule stays in each store varies by ten orders of magnitude, and it is the single most useful thing to know about the cycle.

Approximate residence times

Store	Typical residence time
Atmosphere	~9 days
Rivers	2–6 months
Soil moisture	1–2 months
Seasonal snow cover	2–6 months
Lakes	~50–100 years
Shallow groundwater	Years to decades
Deep groundwater	Hundreds to tens of thousands of years
Ocean	~3,000 years
Antarctic ice sheet	Up to ~800,000 years

The processes in detail

Evaporation: Liquid water becoming vapour, driven by energy input and by the vapour pressure deficit of the air above.
Transpiration: Water drawn up by plants and released as vapour through leaf stomata. Over vegetated land it usually exceeds direct soil evaporation.
Evapotranspiration: The two combined. It is what is actually measured, because separating them in the field is difficult.
Sublimation: Ice becoming vapour directly, without melting. Significant in cold dry climates and on high glaciers.
Condensation: Vapour becoming liquid as air cools past its dew point, usually onto aerosol particles.
Precipitation: Water falling from cloud as rain, snow, sleet or hail.
Interception: Rainfall caught by leaves and evaporated before it reaches the ground. In dense forest this can be a quarter of total rainfall.
Infiltration: Water entering the soil surface. Its rate governs whether rain soaks in or runs off.
Percolation: Water moving downward through the unsaturated zone toward the water table.
Recharge: Water reaching the water table and adding to groundwater storage.
Runoff: Water flowing over the surface to a watercourse. Dominant where soil is saturated, frozen or paved.
Baseflow: The groundwater contribution to river flow. It sustains rivers between rainfall events.

Why winter rain matters more than summer rain

In temperate climates, summer rain largely evaporates or is taken up by growing plants. Very little reaches the water table. Winter rain falls onto cold soil with dormant vegetation and low evaporative demand, so a much larger fraction percolates down and recharges groundwater.

This is why UK water resource planning is dominated by winter rainfall and by soil moisture deficit. A wet summer after a dry winter does very little for aquifer levels. Two dry winters in succession is the condition that produces serious supply problems, regardless of what the summers do.

How a drought develops

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.

Why the lag matters: A wet month can end a meteorological drought while reservoirs and aquifers remain badly depleted. Headlines about rain "ending the drought" usually refer only to the first stage.
Winter rainfall: Groundwater recharges almost entirely in winter, when evaporation is low. Summer rain largely evaporates or is taken up by plants and does very little for aquifers.

Sources

United States Geological Survey — Water Science School — Earth’s water distribution. Open · US Government work — public domain
World Meteorological Organization — State of Global Water Resources. Public but restricted · CC BY-NC-SA 3.0 IGO
European Centre for Medium-Range Weather Forecasts / Copernicus Climate Change Service — ERA5 global reanalysis. Open — attribution required · Copernicus licence