12.21: Water Cycle

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Could you be drinking the same water as George Washington?

Water is recycled constantly through the ecosystem. That means any water you drank today has been around for millions of years. You could be drinking water that was once drunk by George Washington, the first humans, or even the dinosaurs.

The Water Cycle

Whereas energy flows through an ecosystem, water and elements like carbon and nitrogen are recycled. Water and nutrients are constantly being recycled through the environment. This process through which water or a chemical element is continuously recycled in an ecosystem is called a biogeochemical cycle. This recycling process involves both the living organisms (biotic components) and nonliving things (abiotic factors) in the ecosystem. Through biogeochemical cycles, water and other chemical elements are constantly being passed through living organisms to non-living matter and back again, over and over. Three important biogeochemical cycles are the water cycle, carbon cycle, and nitrogen cycle.

The biogeochemical cycle that recycles water is the water cycle. The water cycle involves a series of interconnected pathways involving both the biotic and abiotic components of the biosphere. Water is obviously an extremely important aspect of every ecosystem. Life cannot exist without water. Many organisms contain a large amount of water in their bodies, and many live in water, so the water cycle is essential to life on Earth. Water continuously moves between living organisms, such as plants, and non-living things, such as clouds, rivers, and oceans (Figure below).

The water cycle does not have a real starting or ending point. It is an endless recycling process that involves the oceans, lakes and other bodies of water, as well as the land surfaces and the atmosphere. The steps in the water cycle are as follows, starting with the water in the oceans:

Water evaporates from the surface of the oceans, leaving behind salts. As the water vapor rises, it collects and is stored in clouds.
As water cools in the clouds, condensation occurs. Condensation is when gases turn back into liquids.
Condensation creates precipitation. Precipitation includes rain, snow, hail, and sleet. The precipitation allows the water to return again to the Earth’s surface.
When precipitation lands on land, the water can sink into the ground to become part of our underground water reserves, also known as groundwater. Much of this underground water is stored in aquifers, which are porous layers of rock that can hold water.

Run-off

Most precipitation that occurs over land, however, is not absorbed by the soil and is called runoff. This runoff collects in streams and rivers and eventually flows back into the ocean.

Transpiration

Water also moves through the living organisms in an ecosystem. Plants soak up large amounts of water through their roots. The water then moves up the plant and evaporates from the leaves in a process called transpiration. The process of transpiration, like evaporation, returns water back into the atmosphere.

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Summary

Chemical elements and water are constantly recycled in the ecosystem through biogeochemical cycles.
During the water cycle, water enters the atmosphere by evaporation and transpiration, and water returns to land by precipitation.

Explore More

Use the resource below to answer the questions that follow.

The Water Cycle at http://www.youtube.com/watch?v=IncMhop-4Jc (2:48)

What is a fundamental difference between the water cycle and other nutrient cycles?
What drives the water cycle? Where does this process primarily occur?
What happens to most of the water taken up by plants? How does this compare to most of the water taken up by animals?
How does water's role in photosynthesis explain increased biological productivity in areas of heavy precipitation?

Review

What is the water cycle?
What are two ways water returns to the atmosphere?
How does water get from the atmosphere back to land? What are the various forms of this process?
What is the relationship between groundwater and aquifers?