National Weather Service United States Department of Commerce
Graphic indicating the water cycle.
Water moves around our planet in multiple ways. This is called the water cycle. The water cycle shapes the land by transporting materials and is essential to most life on Earth. Particularly important to hydrologists are the water cycle components precipitation, infiltration, and runoff.

Water Cycle Components


Precipitation is water that falls from the atmosphere to the Earth’s surface.
Infiltration is precipitation that soaks into the soil.
Runoff is precipitation that does not soak into the soil but instead moves on the Earth’s surface toward streams.
Streamflow is water moving across the Earth's surface in streams.
Evaporation refers to water that changes from a liquid to a gas and moves from the Earth's surface back into the atmosphere.
Transpiration refers to water that is pulled out of the soil and released into the atmosphere by plants.


Click a water cycle component below for more information.

Graphic showing the water cycle and it's components. Precipitation falls to the surface turning into runoff or infiltration, streamflow moves toward lakes and oceans, evaporation and transpiration transitions water into water vapor in the amtmosphere, water vapor condenses to form clouds, precipitation falls from clouds.


River Drainage Basins


A drainage basin, also known as a watershed or catchment, is the area from which water flows to form a stream (below, left). A basin is defined by its outlet. All precipitation (rain and snow) that falls within a drainage basin eventually flows to the outlet point, unless it is first removed by evaporation and transpiration. The drainage basin boundary marks the spot where runoff from precipitation splits - water on one side of the line moves toward the basin outlet while water on the other side of the line moves into streams of a different basin. There can be large changes to the basin with small changes to the location of the outlet, and there can be a near infinite number of smaller basins within a larger basin (below, right).

Graphic showing a drainage basin defined by a basin outlet, and runoff from precipitation splitting at the basin boundary.   Graphic showing smaller drainage basins within the larger drainage basin. Each drainage basin is defined by the basin outlet.


The water cycle can even be applied to drainage basins. Precipitation that lands within a basin is removed either by evaporation, tranpiration, and streamflow. For river basins in our area, more precipitation that falls on river basins is removed through evaporation and transpiration than through streamflow.

Curious where water comes from to get to your location? Curious where precipitation goes after it reaches the ground? The US Geological Survey's Stream Stats tool allows users create a drainage basin map for almost any location in the United States.


More Information

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Link to define evaporation Link to define transpiration Link to define precipitation Link to define infiltration Link to define streamflow Link to define condensation Link to define runoff Link to define groundwater