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 cycle.
The water cycle encompasses a number of processes that circulate water through the Earth’s subsystems. Water evaporates from within soils and through vegetation and from bodies of water (such as rivers, lakes and oceans). This evaporated water accumulates as water vapour 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 groundwater 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 atmosphere through processes like transpiration and photosynthesis. Water may also be returned to the atmosphere through the combustion of plants in fossil fuel.
Water and the atmosphere
Water enters the atmosphere through evaporation, transpiration, excretion and sublimation:
- Transpiration is the loss of water from plants (via their leaves).
- Animals excrete water by respiration 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 precipitation (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 hydrosphere or the geosphere.
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 biosphere 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.