The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above, and below the surface of the Earth. It is a complex system that includes many different paths and influences of water through Earth’s ecosystems, which are extremely complex and not completely understood. The water cycle is powered by the energy emitted by the sun, which heats water in the ocean and seas. Water evaporates as water vapor into the air, and some ice and snow sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. The water molecule H2O can exist in three states on Earth: solid, liquid, and gas. The water cycle is essential to life on Earth and plays a large role in the global climate and the ocean circulation.

The water cycle involves the following processes:

• Evaporation: The process by which water changes from a liquid to a gas or vapor. It occurs when water is heated and turns into water vapor, which rises into the atmosphere.
• Transpiration: The process by which water is taken up by plants and then released into the atmosphere as water vapor.
• Condensation: The process by which water vapor in the atmosphere cools and turns back into liquid water, forming clouds.
• Precipitation: The process by which water falls from the atmosphere to the Earths surface in the form of rain, snow, sleet, or hail.
• Infiltration: The process by which water on the ground surface enters the soil.
• Runoff: The process by which water flows over the ground surface into streams, rivers, and eventually into the ocean.

Changes in the water cycle can impact everyone through the economy, energy production and use, health, recreation, transportation, agriculture, and drinking water. Understanding the water cycle has become one of NOAA’s Grand Science Challenges, and NOAA is striving to expand understanding of the water cycle at global to local scales to improve our ability to forecast weather, climate, water resources, and ecosystem health.