High School Earth Science: Earth’s Fresh Water - Water on Earth — Flashcards

The Hydrologic Cycle    
Water moves continuously around Earth’s surface in the hydrologic (water) cycle. The sun provides the energy that drives the cycle. It supplies the energy needed for evaporation, in which liquid water changes to water vapor. The sun’s energy evaporates water from the oceans and from bodies of water on land. When water evaporates, only the water molecules evaporate; any salts in the water stay behind.    
Water vapor remains in the atmosphere until it undergoes condensation to become tiny droplets of liquid water. If the droplets gather in clouds and collide with other water droplets, they may grow large enough to fall from the clouds as precipitation. Precipitation may be liquid or frozen water.

Types of precipitation include rain, snow, sleet, and hail. Most precipitation falls into the oceans; the rest falls on land.    
Precipitation that falls on land as rain may runoff into streams, lakes, or the ocean; or it may infiltrate the ground and become soil moisture or groundwater. Water in soil is needed by plants to grow. Water that seeps more deeply into the ground may enter aquifers that store fresh water for centuries. Alternatively, groundwater may travel back to the surface through springs or find its way to the oceans. Precipitation that falls as snow may soon melt and follow one of the paths of liquid precipitation. When snow melts slowly, it provides a steady flow of fresh water to streams and  lakes. Alternatively, snow may sit on a mountain for several months, or it may become part of the ice in a glacier, where it may remain for hundreds or even thousands of years. A small amount of snow may change directly to water vapor by sublimation and re-enter the atmosphere.    
Plants and animals depend on water to live, and they also play roles in the water cycle. Animals take in liquid water in food and drink. They may release gaseous water when they exhale or liquid water when they sweat or urinate.    
Plants take up liquid water from the soil and release large amounts of water vapor into the air through their leaves.    
The process in which this occurs is called transpiration.