water cycle

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The continuous natural cycling of water from the atmosphere to the land and oceans and back. The basic structure of the global water cycle is quite simple. Water is evaporated from the oceans, seas, lakes, rivers, and vegetated land areas, and it becomes part of the atmospheric store of water vapour. Global and regional wind systems redistribute the water vapour across the Earth's surface. Condensation creates clouds and precipitation, and the latter brings water back to the surface where it enters the soil or groundwater, or flows directly into rivers or lakes. Rivers transport water (as runoff) from land surfaces into the oceans. Naturally, much of the precipitation falls directly over seas and oceans, effectively short‐circuiting the land phase of the water cycle. The total volume of water in the global cycle is estimated at about 1384 million cubic kilometres. Just over 2% of the total water in the global cycle is freshwater, and most of that is locked up as polar ice caps and in glaciers. If all of the ice were to melt it would release enough water to keep the world's rivers flowing at their normal rates for up to 1000 years. The major stores are the oceans (97.41%), ice caps and glaciers (1.9%), groundwater (0.5%), soil moisture (0.01), lakes and rivers (0.009%), and the atmosphere (0.0001%). Also known as the hydrogeological cycle or hydrological cycle.

Fig 21 Water cycle

Subjects: Environmental Science.

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