In simple terms
The water cycle is the constant movement of water around the planet: where it is stored, and how it travels between those stores.
The US Geological Survey calls it the hydrologic cycle and defines it as describing where water is stored on Earth and how it moves. Energy from the sun and the force of gravity keep it going.
How it works
Water is held in the atmosphere, on the land surface and underground. It shifts between liquid, solid and gas, and it moves at every scale, from whole watersheds and weather systems down to the inside of a single plant.
The ocean holds almost all of it. The USGS puts the planet’s total water at about 1,386 million cubic kilometers, of which roughly 1,338 million cubic kilometers is salt water in the oceans. Water covers about 75 percent of the surface.
The cycle has no start and no finish. Water that evaporates from the sea may fall as rain, soak into soil, travel underground and reach the ocean again through a river long afterward.
People are inside the cycle, not outside watching it. The USGS emphasizes that human activity affects where water is stored, how it moves and how clean it is, and it redrew its water cycle diagram in 2022 to show that.
Why it matters
The cycle is what makes fresh water available, and it is not behaving as it used to. The World Meteorological Organization reported that in 2024 only about one third of the world’s river basins saw normal conditions, the sixth year in a row of that imbalance. The rest had either too much water or too little.
2024 was also the third consecutive year of ice loss across every glaciated region, at 450 gigatonnes, which added about 1.2 millimeters to global sea level. Swings between drought and flood are the practical shape of a disrupted cycle.
Where you’ll see it
- School science diagrams of evaporation, condensation and precipitation
- Drought and flood forecasts
- Reservoir and water utility planning
- Climate reports tracking river flows and glacier loss
Example
Rain falls on a hillside. Some soaks into the soil, some runs off into a stream, and some evaporates straight back into the air before it reaches any river at all.
Often confused with
The water cycle is not the water supply. The cycle is the natural movement of all the water there is. Supply is the small share of it that people can reach, store and use.
Key facts
- The USGS defines the water cycle, or hydrologic cycle, as describing where water is stored on Earth and how it moves.1
- Energy from the sun and the force of gravity drive the continual movement of water on Earth.1
- Earth holds about 1,386 million cubic kilometers of water, of which about 1,338 million cubic kilometers is saline ocean water, and water covers about 75 percent of the surface.1
- Human activities affect where water is stored, how it moves and how clean it is.1
- In 2024 only about one third of global river basins had normal conditions, the sixth consecutive year of imbalance, and glaciers lost 450 gigatonnes of ice, adding about 1.2 millimeters to sea level.2
Related concepts
Quick checkWhat drives the water cycle?Show answer
Energy from the sun and the force of gravity.
Sources
- U.S. Geological Survey. What is the Earth's water cycle?. Undated (accessed 17 September 2026)
- World Meteorological Organization. From drought to deluge: WMO report highlights increasingly erratic water cycle. 18 September 2025 (accessed 17 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 17, 2026Researched and drafted with AI assistanceReport an issue