Water Cycle
Water Cycle or Hydrologic Cycle, series of movements of water above, on, and below the surface of the earth. The water cycle consists of four distinct stages: storage, evaporation, precipitation, and runoff. Water may be stored temporarily in the ground; in oceans, lakes, and rivers; and in ice caps and glaciers. It evaporates from the earth’s surface, condenses in clouds, falls back to the earth as precipitation (rain or snow), and eventually either runs into the seas or reevaporates into the atmosphere. Almost all the water on the earth has passed through the water cycle countless times. Very little water has been created or lost over the past billion years.
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| Precipitation |
Precipitation occurs when water vapor in the atmosphere condenses into clouds and falls to the Earth. Precipitation can take a variety of forms, including rain, snow, ice pellets, and hail. About 300 cubic kilometers (about 70 cubic miles) of precipitation falls each day.
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| Storage |
The water from precipitation is stored on the Earth in both liquid and solid forms. Of the 1.4 billion cubic kilometers (340 million cubic miles) of water on Earth, slightly more than 97 percent is salt water stored in the oceans. Fresh water is found in glaciers, ice caps, lakes, and rivers. It is also stored as groundwater in the soil and rocks.
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| Run Off |
Water that flows down streams and rivers is called surface runoff. Every day about 100 cubic kilometers (about 24 cubic miles) of water flows into the seas from the world’s rivers. Runoff is not constant—it decreases during periods of drought or dry seasons and increases during rainy seasons, storms, and periods of rapid melting of snow and ice. Water reaches rivers in the form of either overland flow or groundwater flow; it then flows downstream. Overland flow occurs during and shortly after intense rainstorms or periods of rapid melting of snow and ice. It can raise river levels rapidly and may produce floods. Groundwater flow runs through rocks and soil. Precipitation and meltwater percolate into the ground and reach a level, known as the water table, at which the ground is saturated with water. Groundwater flows from areas where the water table is higher to areas where it is lower.
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| Evaporation and Transpiration |
Evaporation is the process by which water in the ocean and on land changes to water vapor and enters the atmosphere as a gas. Evaporation from plants is called transpiration. The evaporation rate increases with temperature, sunlight intensity, wind speed, plant cover, and ground moisture, and it decreases as the humidity of the air increases.
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| Condensation |
Water vapor cools as it rises, condensing into droplets of water to form clouds. Precipitation falls from the clouds and the water returns to Earth, continuing the hydrologic cycle. Almost all the water on Earth has passed through the water cycle countless times. Very little water has been created or lost over the past billion years.
Source: Encarta
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