Реферат: Fresh Water Resources

The Earth's water cycle or hydrologic cycle is the continuous circulation of moisture and water on our planet. The time required for a water particle to pass through one or more phases of the hydrologic cycle varies from a few hours to months or even centuries. A water particle may be evaporated from the ocean and in a short period falls back as rain or snow on the same water surface. If a water particle falls, as snow or hail on a mountain height, it may remain there for months until it is melted and joined with other melted particles on the long journey overland or underground.

If a water particle has been evaporated from the ocean and carried into the polar regions, falling there as ice or snow, it may remain frozen for centuries before it may return to the ocean part of an iceberg, or as melted outflow from the glacier. Rain, hail, and snow are various forms of precipitation. Precipitation that falls upon areas is the source of all the fresh water supply. The people depend upon it to compensate the quantity that is being taken from lakes, streams for man's numerous uses.

Hydrologists completed a study of the world water balance. They estimated that the natural water cycle is far more intensive than had been supposed. Annual evaporation from the Earth's surface amounts to the colossal figure of half a million cubic kilometres of water while the atmospheric moisture is renewed every ten days on the average. River water has a 12-day replacement cycle. Glaciers proved to be the slowest: «accumulations of moisture». Their replacement cycle is 8500 years. These figures will make it possible to achieve a more accurate picture of the world's fresh water resources.