The water cycle is the continuous circulation of water above, on, and beneath the surface of Earth. It connects oceans, the atmosphere, inland waters, soils, ice, and living organisms through transfers of water and changes between liquid, solid, and gaseous states. Rather than a single circular route with a fixed beginning, it is a network of interconnected stores and flows. Hydrology studies these movements, their physical controls, and their relationship to water quantity and quality. (usgs.gov)
Water stores and timescales
Earth’s hydrosphere includes water in many distinct stores, often called pools or reservoirs. The oceans contain approximately 96.5 percent of the planet’s total water. Most freshwater is stored in ice caps, glaciers, and groundwater: a widely used global inventory estimates that about 68.7 percent of freshwater is held in ice and permanent snow, and 30.1 percent underground. Lakes, rivers, soil moisture, atmospheric moisture, and organisms contain much smaller shares. These estimates describe stored quantities, not the rates at which water moves. (usgs.gov)
Storage duration varies greatly. Atmospheric water vapor remains in the atmosphere for about nine days on average, while some underground water follows pathways lasting thousands or even millions of years. Water can therefore circulate rapidly through one part of the system while remaining in another for a very long time. The cryosphere, including snow and land ice, provides storage that may be seasonal or persist over much longer periods. (science.nasa.gov)
Atmospheric transfers and phase changes
Energy from the Sun and the action of gravity are the principal natural drivers of the cycle. Solar heating supplies energy for evaporation, in which liquid water becomes vapor. This occurs over oceans, lakes, rivers, wet soil, and other surfaces. Ice and snow can also pass directly into vapor through sublimation, without first melting. Atmospheric circulation transports moisture away from its source, linking distant regions through water exchange. (usgs.gov)
Vegetation contributes through transpiration, the release of water vapor from plants. Together, evaporation and transpiration constitute evapotranspiration. This combined transfer connects water stored in soils and vegetation with atmospheric moisture; its magnitude depends partly on energy availability and the supply of water. Actual evapotranspiration can be limited when soils dry, even where conditions would otherwise favor rapid evaporation. (usgs.gov)
Condensation converts water vapor into liquid droplets and is central to cloud formation. It releases heat into the atmosphere, linking water transfers with atmospheric energy exchanges. Water returns to the surface through precipitation, including rain, snow, and hail. These processes do not require water to complete a prescribed sequence: moisture may return to the ocean, fall on land, or undergo further atmospheric transport before reaching the surface. (usgs.gov)
Surface and underground pathways
On land, precipitation may collect in surface waters, enter the soil, return to the atmosphere, or remain frozen. Surface runoff moves water across the ground, while a river carries water through a channel network. Snowmelt supplies an additional seasonal input. Surface flow and underground discharge connect these pathways, with lakes and rivers acting both as stores and as routes of transfer. (usgs.gov)
Infiltration is the entry of water from the surface into soil or rock. Some infiltrated water stays near the surface and becomes available to plants; some moves deeper and replenishes groundwater. An aquifer stores and transmits groundwater through pores and fractures. Groundwater can discharge into streams, lakes, springs, and the ocean, so surface water and groundwater are interacting parts of the same system rather than independent resources. (usgs.gov)
Water budgets
A water budget accounts for water entering, leaving, and accumulating within a defined area or volume. It can describe a soil column, lake, aquifer, or drainage basin. Its basic relationship is:
where is the change in stored water over the accounting period. For a simplified basin with negligible external groundwater exchange and no imported or exported water, this becomes , where is precipitation, is evapotranspiration, and is streamflow leaving the basin. More complex budgets include additional boundary flows and human transfers. (usgs.gov)
Budgets distinguish a large water store from a renewable supply: the amount present is not necessarily the amount replenished over a useful timescale. They also show how changes in one component affect others. For example, agricultural drainage and irrigation modify infiltration, runoff, evaporation, and transpiration, requiring these changes to be included in the accounting. (pubs.usgs.gov)
Human influence and climate change
Human activities alter water storage, pathways, timing, and quality. Dams create reservoirs and change downstream flows; withdrawals transfer water from rivers or aquifers to other uses. Urbanization replaces permeable surfaces with roads and buildings, commonly reducing infiltration and delivering runoff to streams more quickly. Used water returns through discharge, infiltration, or atmospheric transfer, often with altered quality. (usgs.gov)
Climate change modifies atmospheric moisture, precipitation, evaporation, and water storage. Near-surface atmospheric moisture capacity increases by approximately 7 percent per degree Celsius of warming, supporting more intense heavy precipitation. Global mean precipitation and evaporation increase more slowly because energy exchanges constrain them. Regional changes also depend on atmospheric circulation and land conditions; intensification of the global water cycle does not mean that every location becomes wetter or that all water moves through the system faster. (ipcc.ch)