Because the Earth is essentially a closed system containing all the water we will ever have, all of this water moves in a pattern we call the hydrologic or Water Cycle.
The form of water is always changing. Water moves from sky to Earth and back to the sky again. This is called the water cycle. Water falls to Earth as rain, snow, sleet, or hail. Some of the water soaks into the ground and is stored as groundwater. The rest flows into streams, lakes, rivers, and oceans. The area of land in which this takes place (from ridge to ridge, including the valley in between) is called a watershed. The sun warms surface water and changes some of it into water vapor. This process is called evaporation. Plants give off water vapor in a process called transpiration. The heated water vapor rises into the sky and forms clouds. When the vapor in the clouds condenses, it falls back to the Earth as rain or snow. The water cycle has then come full circle and begins again.
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