Where Earth's water is

If all the water in the air fell as rain at once, it would cover the globe only about 2.5 cm deep.

Count the water before you follow it

Pour a glass of water and ask where it has been. A few days ago it may have been part of a cloud; a few thousand years ago it may have been locked in a glacier. The water cycle is the name for this endless movement. Before following it, it helps to know where the water is sitting at any one moment, because the stores are wildly different in size.

The US Geological Survey publishes one widely used estimate. Oceans, seas and bays hold about 1,338,000,000 km³, which is 96.54% of all of Earth's water. Only about 2.5% is fresh. Of that fresh water, 68.7% is frozen in ice caps, glaciers and permanent snow, and another 30.1% is underground. That leaves only about 1.2% of the fresh water for lakes, rivers, swamps, soil moisture, ground ice and the air together.

Small stores, fast traffic

The atmosphere is one of the smallest stores of all, about 12,900 km³, or 0.001% of Earth's water. Rivers are smaller still, about 2,120 km³. Yet these two tiny stores carry most of the traffic that matters to us. The USGS calls the atmosphere a superhighway: water arrives as vapour, rides the winds and leaves as rain or snow. NASA estimates that a water vapour molecule stays in the air about nine days on average, so the air's small store is emptied and refilled roughly forty times a year.

Big stores are slow. Water can stay in a deep ocean basin or an ice sheet for thousands of years, and some groundwater has been underground for a very long time. When people say water is renewable, they usually mean the fast part of the cycle. Pumping old groundwater faster than rain can refill it uses up a store, just as spending savings does.

Pools and fluxes

NOAA describes the cycle with two words. Pools are the places water is stored: the ocean, ice, groundwater, lakes, the air, even living things. Fluxes are the ways water moves between pools: evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater flow. Water can also skip the liquid stage. Snow can turn straight into vapour (sublimation), and vapour can turn straight into ice, which is how frost grows on a cold night (deposition).

The usual diagram draws the cycle as one neat circle, and NOAA warns that the real thing is much messier. A raindrop can evaporate on the way down. River water can sink into the ground and come back up in a spring. Most water that evaporates from the ocean falls back on the ocean; only about 10% is carried over land, according to the USGS. In the model the dots follow a few typical paths, and each later unit opens one of them.