Hydrologic Cycle

The water cycle is a perpetual process in which water evaporates from the various water bodies , forms clouds , descends to the earth as rain or snow and finally reaches the oceans by means of the runoff mechanism (hydrologic cycle , 2002

Storage

Water cycle involves vast amounts of water . The earth contains nearly 340 million cu miles of water . Only three percent of this amount of water can be used for drinking . In other words nearly ninety seven percent of the oceans and therefore salty . However , the water that evaporates does not contain

salt and because of this rain water and snow are free of salt Salt free fresh water is located in glaciers , lakes and rivers Moreover , ground water is also fresh and free from salt . The earth contains nearly 8 .6 million cubic miles of fresh water . The atmosphere always contains at least 2 ,900 cubic miles of water in vapor . Storage of water in the rivers and lakes in the world is estimated at 29 ,000 cubic miles . The principle sources of fresh water are the great polar ice caps . Nearly 6 .7 million cubic miles of water is stored in the polar ice caps . Water stored in the ground is about 2 million cubic miles (Gedzelman , 2005

Evaporation

When a phase change occurs by which water gets transformed from a liquid to gas , it is known as evaporation . The quantity of water that evaporates everyday from the surface of the earth is about two hundred and ninety cubic miles . In the absence of evaporation , precipitation would have rendered the atmosphere dry within ten days

The quantity of water that undergoes evaporation is dependent upon various factors like the intensity of the sunlight , plant cover and ground moisture . Evaporation is indirectly proportional to the humidity of the atmosphere . Due to the different climatic zones on the surface of the earth , the rate of evaporation varies from near zero at the poles to about thirteen feet a year over the Gulf Stream . Evaporation of water is offset by precipitation and runoff (Perlman , 2006

Precipitation

A phase change that takes place in the atmosphere , whereby water vapor condenses to form clouds and falls on to the earth as rain is known as precipitation . This falling water can be either a liquid as in rain or it can be a solid like snow , ice etc . the daily average of precipitation that falls on to the earth is of the of seventy cubic miles . About a third of this precipitated water flows into the rivers and streams while the rest evaporates (Gedzelman , 2005

Runoff

Surface runoff occurs when water flows into rivers and streams . On an average about twenty four cubic miles of water is runoff every day . If snow and ice melt rapidly or if any rainfall occurs the result is an overland flow of water . It causes a rapid increase in the water level of a river and this could result in floods . On the other hand , the ground...