Water cycle

The water cycle describes how water moves through Earth's atmosphere, surface, subsurface and living systems. Water occurs as liquid, ice and snow, and water vapour. It is stored in reservoirs—or pools—including oceans, lakes, rivers, wetlands, glaciers, snowpack, soil, groundwater and the atmosphere. Plants and animals also contain water. The term hydrologic cycle is another name for this continuous circulation.

Water moves between these stores through processes sometimes called fluxes. The Sun supplies much of the energy that drives evaporation, while gravity pulls precipitation downward and moves water downhill. The cycle is not a single, uniform loop: a water molecule can take different routes, and a store may retain water for a shorter or much longer time than another.

Evaporation changes liquid water at Earth's surface into water vapour. Plants release water vapour through transpiration; together with evaporation from land and water, this transfer is often described as evapotranspiration. As moist air cools, water vapour can condense into liquid droplets or ice crystals, forming clouds. Water then returns to Earth's surface as precipitation, including rain or snow.

After precipitation reaches land, some water flows over the surface as runoff and into streams, rivers, lakes and eventually the ocean. Some infiltrates the soil; part can percolate deeper and recharge groundwater. Groundwater moves through aquifers and may later discharge into springs, rivers or the sea. Snow and ice can melt and feed streams, or change directly to vapour by sublimation. These pathways connect surface water, the atmosphere and underground stores.

The water cycle links reservoirs that differ greatly in size and in how quickly water enters and leaves them. Oceans contain most of Earth's water, while atmospheric water vapour is a small store that is important for clouds, precipitation and weather. Water in ice, soil and aquifers is also part of the cycle, even when it is not moving visibly at the surface.

Residence time—the average time water remains in a reservoir—varies among stores. Atmospheric moisture may return as precipitation relatively quickly, whereas water in oceans, glaciers or some groundwater systems can remain much longer. These differences help explain why a simple diagram with one arrowed loop is only a teaching model: the real cycle comprises many connected routes operating at different rates and scales.

People depend on the cycle for freshwater, but also alter its pathways. Dams store water, river diversions move it, irrigation redistributes it across land, and pumping withdraws groundwater. Urban development, wetland drainage and pollution can change how water is stored, where it flows and whether it is clean. The U.S. Geological Survey notes that water availability depends not only on quantity but also on timing and quality.

Climate influences precipitation, snow and ice, evaporation and runoff, while a changing climate can affect the timing and pattern of these processes. The water cycle is therefore central to weather, ecosystems, water resources and the study of floods and droughts. Scientific diagrams and models help describe it, but the many interacting pathways remain more complex than the familiar three-step picture of evaporation, condensation and precipitation.