The Earth’s water is always in circulation. It has been recycled for the last 3 billion years. This process is called the water cycle.

The cycle starts when the sun’s heat evaporates water from the oceans into the atmosphere to form clouds. When the conditions are just right, the clouds release water as rain or snow. Most of the rain falls in the oceans, but the rest falls onto land. Rivers and streams collect water from the ground and return it to the ocean so the whole cycle can start all over again. The water cycle never ends because the salty ocean water constantly supplies fresh water to the continents.

One process which tranfers water from the ground back to the atmosphere is evaporation. Evaporation is when water passes from a liquid phase to a gas phase. Rates of evaporation of water depend on things like the temperature, humidity, and wind.

Water that is held in lakes (Inland Water Bodies)and rivers evaporates directly into the atmosphere. Some of the water in the ground may also be returned to the atmosphere by way of evaporation through the soil surface. Of course, the ocean is the greatest source for water evaporated into the atmosphere.Transpiration is the process by which plants return water to the atmosphere. After absorbing water from the ground, plants release water through their leaves. Transpiration helps plants stay cool, in the same way perspiration keeps humans and animals cool.

source: http://www.windows.ucar.edu