The Earth has a finite amount of water. The water that is here today is the same water that will be here in 20 or even 20 million years’ time. So, if all living things use water, how is it that we don’t use up all our water? The answer is that water is constantly recycled through the Earth’s system through a process called the water cycle1.
Dynamic and complex: the global water cycle
Water in the Earth system is influencing all aspects of life on Earth. Pathways, storage, transfers and transformations have an effect on the global climate and human welfare. Within this interactive 4 scientists talk about some of the complex aspects of the water cycle.
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The water cycle encompasses a number of processes that circulate water through the Earth’s subsystems. Water evaporates from within soils and through vegetation2 and from bodies of water (such as rivers, lakes and oceans). This evaporated water accumulates as water vapour3 in clouds and returns to the Earth as rain or snow. The returning water falls directly back into the oceans, or onto land as snow or rain. It soaks into the soil to move into the groundwater4 or runs off the Earth’s surface in streams, rivers and lakes, which drain back into the oceans. The water may be taken up by plants and returned to the atmosphere5 through processes like transpiration6 and photosynthesis7. Water may also be returned to the atmosphere through the combustion8 of plants in fossil fuel9.
Water and the atmosphere
Water enters the atmosphere through evaporation10, transpiration, excretion11 and sublimation:
- Transpiration is the loss of water from plants (via their leaves).
- Animals excrete water by respiration12 and by passing urine.
- Sublimation is when ice or snow transforms directly into water vapour without going through a liquid phase (i.e. they do not melt).
Water commonly occurs in the atmosphere in the form of water vapour. If it cools down, it can condense, accumulating in clouds. As the clouds grow, they become heavier and can fall back to the Earth as precipitation13 (rain, snow, hail or sleet) or re-evaporate back into vapour.
Water and the biosphere, hydrosphere and geosphere
When water returns to Earth, it can either enter the hydrosphere14 or the geosphere15.
It can enter the hydrosphere by falling onto bodies of water or falling onto the ground. When it rains, water falling onto the ground can move in two ways – it can run off the surface of the ground and enter streams and rivers, or it can seep into the ground and enter the ground water. This second process is called infiltration – water moves through pore spaces between soil particles or rocks.
Once the water is in the hydrosphere or geosphere, it can be used by living things. Plants can take water from the soils, and animals can drink water from rivers and lakes or eat plants. Even microbes deep in the ground live in tiny films of water surrounding rocks. The water will then stay in the biosphere16 until released through evaporation, transpiration, excretion, decay, respiration and combustion and the whole process begins again.
It may stay in the hydrosphere or geosphere for a long time (such as in aquifers) or it may very quickly return to the atmosphere.
These processes that transform and transfer water within the Earth’s system occur continuously over time but at different rates in different places.
Activity idea
Water cycle models use simple materials to observe the interactions along the water cycle. It’s one of many activities featured in the interactive Learning about the water cycle.
- water cycle: The continuous movement of water on, above and below the surface of the Earth (also known as the hydrological cycle).
- vegetation: Plant life.
- water vapour: The gas phase of water.
- groundwater: Water located beneath the Earth’s surface in soil spaces and in fractures of rocks.
- atmosphere: 1. The layer of gas around the Earth. 2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.
- transpiration: The process by which water exits leaves in plants and enters the atmosphere in a gas phase.
- photosynthesis: A process that uses the energy from sunlight to convert carbon dioxide and water into carbohydrates, releasing oxygen as a byproduct. Photosynthesis occurs in the green parts of plants, in algae and in some microorganisms.
- combustion: A chemical reaction that involves the process of burning.
- fossil fuel: Materials such as coal, oil and natural gas formed from the fossilised remains of plants that lived many millions of years ago. Often burned as fuel – although this releases large amounts of CO2, which contributes to global warming. Fossil fuels are also not renewable – there is a limited amount.
- evaporation: The process by which a liquid is converted into a gas, without necessarily reaching the boiling point.
- excretion: A waste substance, such as urine, that is separated and eliminated from a living thing.
- respiration: Can mean either cellular respiration (the process by which cells create energy) or gas exchange (breathing).
- precipitation: 1. The formation of an insoluble solid (precipitate) from a given solution by altering either its temperature, concentration or chemical composition. 2. In meteorology, this term describes the formation of rain, hail, snow or ice in the atmosphere.
- hydrosphere: All the water on Earth, whether it is liquid, solid (ice and snow) or vapour (in the atmosphere).
- geosphere: The solid part of Earth – land and rock.
- biosphere: The parts of the land, sea and atmosphere in which organisms are able to live.
