Water continually moves among oceans, atmosphere, surface waters, soils, groundwater and living organisms. This movement is the hydrologic cycle.

Major processes include:

• evaporation: liquid water enters the atmosphere as vapor;
• transpiration: plants lose water vapor from their tissues;
• condensation: atmospheric water vapor forms liquid droplets or ice;
• precipitation: water returns to Earth's surface as rain, snow or other forms;
• infiltration: water enters soil and can recharge groundwater;
• runoff: water flows across land toward streams, lakes and oceans.

Organisms are part of this cycle rather than separate from it. Plants move large amounts of water from soil to atmosphere through transpiration, while animals and microorganisms return water through respiration, excretion and decomposition.

The hydrologic cycle also transports dissolved nutrients. Runoff can move nitrate or phosphate from soils into aquatic ecosystems, and groundwater can carry solutes over long distances.

Unlike carbon or nitrogen, water itself is not primarily cycled by changing into many different elemental forms. Its ecological movement is dominated by physical transport and phase changes.

The cycle can therefore be summarized as

$$\boxed{\text{storage reservoirs} + \text{phase changes} + \text{transport between reservoirs}.}$$

Climate and ecosystem structure strongly influence the rates of these fluxes.