Water Cycle Key Terms

In science, each water cycle word names one particular change or movement. 'The water goes into the ground' could mean infiltration, percolation or groundwater recharge, and each means something slightly different. Using the precise term shows exactly what you mean, and it is what teachers and examiners look for.

This cheat sheet sorts the key terms into three groups: how water changes state, how water moves from place to place, and where water is stored.

Water exists as a solid (ice), a liquid and a gas (water vapour). The six changes of state form three pairs of opposites:

Change	Opposite
Evaporation: liquid to gas	Condensation: gas to liquid
Melting: solid to liquid	Freezing: liquid to solid
Sublimation: solid to gas	Deposition: gas to solid

The changes in the left-hand column take in heat energy, and their opposites give it out again. That is why evaporation from your skin cools you down, and why the Sun's energy is needed to keep the water cycle running. Sublimation and deposition skip the liquid stage completely. Frost on a cold morning is deposition; snow slowly vanishing on a freezing, sunny day without melting is sublimation.

• Evaporation, transpiration and evapotranspiration: evaporation comes from water surfaces and soil, transpiration comes from plant leaves, and evapotranspiration is the two added together.
• Condensation and precipitation: condensation makes cloud droplets; precipitation is when those droplets (or ice crystals) have grown heavy enough to fall.
• Infiltration and percolation: infiltration is water entering the soil at the surface; percolation is its slower journey deeper through gaps in soil and rock.
• Groundwater, water table and aquifer: groundwater is the water itself, the water table is the top of the fully soaked zone, and an aquifer is the rock or sediment layer that holds enough of it to supply wells and springs.
• Store and reservoir: scientists often call any place that holds water a store, pool or reservoir. In everyday language, a reservoir is a lake made by building a dam across a river.

Try following one water molecule using the precise words. It evaporates from the sea and is carried inland by the wind (advection). As the air rises and cools, it condenses onto a speck of dust and becomes part of a cloud. It grows, then falls as precipitation. On the ground it infiltrates the soil and percolates down past the water table into an aquifer.

There its residence time might be 100 to 200 years in shallow groundwater, or thousands of years if it sinks deep. One day it seeps into a river, flows back to the sea as runoff and is ready to evaporate again. Telling the story with these words is a good way to revise: if you can explain each step in one sentence, you know the cycle.