Environmental Protection Agency


March 11, 2011

The water cycle, also known as the hydrologic cycle or H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Water can change states among liquidvapour, and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go, in and out of the atmosphere. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and sub-surface flow. In so doing, the water goes through different phases of liquid, solid, and gas.
The hydrologic cycle also involves the exchange of heat energy, which leads to temperature changes. For example, in the process of evaporation, water takes up energy from the surroundings and cools the environment. Conversely, in the process of condensation, water releases energy to its surroundings, which warms the environment.
The water cycle is hugely important in the maintenance of life and our ecosystems on Earth. Some impacts are fairly obvious, for example water in each reservoir plays an important role.  However much more important is that the water cycle brings added significance to the presence of water on our planet. By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and transports minerals to different parts of the globe. The water cycle is also involved in reshaping the geological features of the Earth, through processes such as  erosion and sedimentation. In addition, as the water cycle involves heat exchange, it also exerts an influence on climate.
Human activity like burning fossil fuels has an effect on the overall increase of the Earth’s temperature. We would expect that raising the Earth’s temperature will mean an increase in evaporation, melting of ice, and other processes of the water cycle adversely affecting the Earth’s climate.  We see this happening now.  Global warming is raising temperatures, raising temperatures means more ice melting, less ice means an addition increase is the Earth’s temperatures.  We are in a dangerous cycle.

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