Process diagram: The water cycle

The diagram illustrates how water circulates continuously between the Earth's surface and the atmosphere in the water cycle.

Overall, the process is a closed cycle with eight stages and no real starting point. Water is carried up into the atmosphere by heat and plants, and it eventually returns to the ocean, from where it can evaporate again.

The cycle can be described as beginning with evaporation, when heat from the sun turns water from the sea and lakes into vapour. At the same time, water vapour is released by plants from their leaves, a process known as transpiration. The vapour then rises, cools and is condensed into clouds. Next, the water falls back to the ground as precipitation in the form of rain, snow or hail.

Once it has reached the ground, the water follows two routes. Some of it flows over the land as surface run-off and enters rivers, while some is absorbed into the soil and rocks through infiltration. This underground water then moves slowly towards the sea as groundwater flow. Finally, the water is collected in the ocean, as both rivers and groundwater return it there, and the cycle starts again.