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What is the Water Cycle?

A water molecule may exists as a liquid, gas, or solid.
A plant absorbing water, which is part of the water cycle.
Fog is water vapor.
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  • Written By: Niki Foster
  • Edited By: Bronwyn Harris
  • Last Modified Date: 02 April 2014
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The water cycle, or hydrologic cycle, is the movement of the Earth's water. Water is in constant movement, and undergoes a number of processes and property changes as it runs through the water cycle. At any given time, a water molecule may exist as liquid, vapor, or ice.

The water cycle is constant and has no real starting or ending point. The sun heats bodies of water on the earth, causing some of the water to evaporate, or change from liquid to gas form. Plants also help water change from liquid to gas through transpiration.

Water vapor returns to liquid form when it cools in a process known as condensation, resulting in clouds and fog. When the condensed water becomes heavy enough, it falls back to the earth in precipitation. Most precipitation is rain, but there are other forms as well, including hail, snow, and sleet.

Below freezing temperature, water takes the solid form of ice or snow. This water can change state through melting, becoming liquid water, or through sublimation, passing directly from a solid to a gaseous state with no intermediate liquid phase. Liquid water moving over the earth's surface, including melted snow or ice, is called runoff. Any movement of water that does not involve a change in material state is called advection. The movement of water from the earth's surface into the soil is called infiltration, and the movement of groundwater beneath the earth's surface is termed subsurface flow.

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Some processes of the water cycle move very quickly, such as precipitation, while others can take millions of years, such as changes in lakes or glaciers. Any place that water can be stored during the water cycle is termed a reservoir. Some reservoirs are very short term, such as the atmosphere, in which water molecules remain for only nine days on average. Other reservoirs are extremely tenacious. Oceans have an average residence time of 3,200 years, and deep ground water can remain in the earth for over 10,000 years.

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Fiorite
Post 2

@ GiraffEars- You are absolutely right. When teaching the water cycle, it is not uncommon for it to be taught in relation to the rock cycle and the tectonic cycle along with the common energy inputs that power these cycles. The water cycle, or hydrologic cycle as it is often referred too, is dependent on solar energy for it to flow between the various geologic reservoirs. These reservoirs include oceans, plants, animals, the atmosphere, and the lithosphere where it accumulates as ground water and surface water.

This last reservoir is where the water cycle interacts with the rock cycle. Surface and ground water aids in sedimentation, weathering, and erosion. This has the effect of both creating rock, and breaking rock apart. The sedimentary rock and regolith then travels through the rock cycle where it is compressed into metamorphic rock or melted into igneous rock. This is the point in the rock and tectonic cycles where the internal energy of the Earth is input into these geologic systems.

GiraffeEars
Post 1

How does th water cycle interact with the other cycles of the earth? I understand how the water cycle works, but I thought the water cycle was part of an interconnected web of geologic cycles that interact with each other. How does this work? Anyone?

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