Earth's water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle, with these processes happening in the blink of an eye and over millions of years.
Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry. The water in the apple you ate yesterday may have fallen as rain half-way around the world last year or could have been used 100 million years ago by Mama Dinosaur to give her baby a bath.
To explore the water cycle, choose a topic from the diagram or text links below.
Streamflow: The U.S. Geological Survey (USGS) uses the term "streamflow" to refer to the amount of water flowing in a river. Although USGS usually uses the term "stream" when discussing flowing water bodies, in these pages we'll use "rivers" more often, since that is probably what you are more familiar with.
Surface run of: The oceans are kept full by precipitation and also by runoff and discharge from rivers and the ground. Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans.
Freshwater storage:Surface water includes the streams (of all sizes, from large rivers to small creeks), ponds, lakes, reservoirs and canals (man-made lakes and streams), and freshwater wetlands.
Ground-water discharge:There are also vast amounts of water that are unseen—water existing in the ground. And even though ground water is unseen, it is moving below your feet right now. As part of the water cycle, ground water is a major contributor to flow in many streams and rivers and has a strong influence on river and wetland habitats for plants and animals.
Ground-water storage: Drawing showing how surface water infiltrates into the ground to be stored in aquifers. Large amounts of water are stored in the ground. The water is still moving, possibly very slowly, and it is still part of the water cycle.
Infiltration:Anywhere in the world, a portion of the water that falls as rain and snow infiltrates into the subsurface soil and rock. How much infiltrates depends greatly on a number of factors. Infiltration of precipitation falling on the ice cap of Greenland might be very small, whereas, as this picture of a stream disappearing into a cave in southern Georgia
Precipitation:Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
Snow melt runoff to streams:in the world-wide scheme of the water cycle, runoff from snowmelt is a major component of the global movement of water. Of course, the importance of snowmelt varies greatly geographically, and in warmer climates it does not directly play a part in water availability.
Springs:A spring is a water resource formed when the side of a hill, a valley bottom or other excavation intersects a flowing body of ground water at or below the local water table, below which the subsurface material is saturated with water.
Water in the atmosphere:The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe.
Evaporation:Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor.
Evapotranspiration:In general, evapotranspiration is the sum of evaporation and transpiration. Some definitions include evaporation from surface-water bodies, even the oceans. But, since we have a Web page just about evaporation, our definition of evapotranspiration will not include evaporation from surface water.
Condensation:Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation.
Sublimation:Sublimation is the conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. For those of us interested in the water cycle, sublimation is most often used to describe the process of snow and ice changing into water vapor in the air without first melting into water.
Ice and Snow:The water cycle describes how water moves above, on, and through the Earth. But, in fact, much more water is "in storage" at any one time than is actually moving through the cycle. By storage, we mean water that is locked up in its present state for a relatively long period of time.
Ocean: The water cycle sounds like it is describing how water moves above, on, and through the Earth ... and it does. But, in fact, much more water is "in storage" for long periods of time than is actually moving through the cycle.
Surface run of: The oceans are kept full by precipitation and also by runoff and discharge from rivers and the ground. Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans.
Freshwater storage:Surface water includes the streams (of all sizes, from large rivers to small creeks), ponds, lakes, reservoirs and canals (man-made lakes and streams), and freshwater wetlands.
Ground-water discharge:There are also vast amounts of water that are unseen—water existing in the ground. And even though ground water is unseen, it is moving below your feet right now. As part of the water cycle, ground water is a major contributor to flow in many streams and rivers and has a strong influence on river and wetland habitats for plants and animals.
Ground-water storage: Drawing showing how surface water infiltrates into the ground to be stored in aquifers. Large amounts of water are stored in the ground. The water is still moving, possibly very slowly, and it is still part of the water cycle.
Infiltration:Anywhere in the world, a portion of the water that falls as rain and snow infiltrates into the subsurface soil and rock. How much infiltrates depends greatly on a number of factors. Infiltration of precipitation falling on the ice cap of Greenland might be very small, whereas, as this picture of a stream disappearing into a cave in southern Georgia
Precipitation:Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
Snow melt runoff to streams:in the world-wide scheme of the water cycle, runoff from snowmelt is a major component of the global movement of water. Of course, the importance of snowmelt varies greatly geographically, and in warmer climates it does not directly play a part in water availability.
Springs:A spring is a water resource formed when the side of a hill, a valley bottom or other excavation intersects a flowing body of ground water at or below the local water table, below which the subsurface material is saturated with water.
Water in the atmosphere:The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe.
Evaporation:Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor.
Evapotranspiration:In general, evapotranspiration is the sum of evaporation and transpiration. Some definitions include evaporation from surface-water bodies, even the oceans. But, since we have a Web page just about evaporation, our definition of evapotranspiration will not include evaporation from surface water.
Condensation:Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation.
Sublimation:Sublimation is the conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. For those of us interested in the water cycle, sublimation is most often used to describe the process of snow and ice changing into water vapor in the air without first melting into water.
Ice and Snow:The water cycle describes how water moves above, on, and through the Earth. But, in fact, much more water is "in storage" at any one time than is actually moving through the cycle. By storage, we mean water that is locked up in its present state for a relatively long period of time.
Ocean: The water cycle sounds like it is describing how water moves above, on, and through the Earth ... and it does. But, in fact, much more water is "in storage" for long periods of time than is actually moving through the cycle.