The water cycle (or hydrological cycle) is the continuous movement of water through Earth's systems — oceans, atmosphere, land, ice, and biosphere. Driven primarily by solar energy and gravity, water evaporates from oceans and land surfaces, is transported through the atmosphere as vapour and clouds, falls as precipitation, flows through rivers and groundwater, and eventually returns to the oceans. This cycle moves approximately 500,000 km³ of water annually.
The main components are: evaporation (ocean surfaces contribute ~86 % of atmospheric moisture), transpiration (plants release water vapour through their leaves, collectively called evapotranspiration when combined with surface evaporation), condensation (water vapour forms clouds when air reaches its dew point), precipitation (rain, snow, hail, drizzle), runoff (surface flow into streams and rivers), infiltration (water seeping into soil and groundwater), and storage (in oceans, ice caps, groundwater, lakes, and the atmosphere).
The water cycle is the fundamental link between meteorology, hydrology, and climate. Climate change is intensifying the cycle: a warmer atmosphere holds more water vapour (~7 % more per 1 °C of warming, per the Clausius-Clapeyron relation), leading to more intense precipitation events even as some regions become drier. Deforestation disrupts the cycle by reducing evapotranspiration and moisture recycling — the Amazon rainforest generates up to 50 % of its own rainfall through transpiration. Understanding the water cycle is essential for managing water resources, predicting floods and droughts, and assessing the impacts of land-use change on regional climate.