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.

Where do YOU think the water cycle begins? Give us your opinion.

 A summary of the water cycle on a single Web page is also available:
  Complete summary  •  Text only  •  Quick summary

Streamflow    Surface runoff    Freshwater storage
Ground-water discharge    Ground-water storage    Infiltration
Precipitation    Snowmelt runoff to streams    Springs
Water in the atmosphere    Evaporation    Evapotranspiration
Condensation    Sublimation    Ice and snow    Oceans

---

Water-cycle home Water Science home