Water cycle
Continuous movement of water across Earth's reservoirs.
The water cycle, also known as the hydrologic or hydrological cycle, is a biogeochemical cycle describing the continuous movement and change in form of water on, above, and below the Earth's surface. It is driven by solar energy and involves processes such as evaporation, condensation, precipitation, and runoff, moving water through reservoirs including oceans, ice, freshwater, and the atmosphere. The total mass of water on Earth remains fairly constant over time, but its distribution among major reservoirs—ice, fresh water, salt water, and atmospheric water—varies with climatic conditions. Water moves between these reservoirs through a set of physical and chemical processes: evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow. During these movements, water changes phase between liquid, solid (ice), and vapor. The ocean is a central component, holding 97% of the planet’s total water and supplying 86% of global evaporation. The cycle is powered by energy exchanges from heat transfers during phase changes; for instance, heat is absorbed as water evaporates (latent heat of vaporization) and released when it condenses or freezes. This energy transfer plays a critical role in moving heat from the tropics toward the poles via ocean circulation. The evaporative phase also purifies water by separating it from salts and particles, while condensation replenishes land with freshwater. Liquid water flow transports minerals and reshapes geological features through weathering, erosion, and deposition. Human actions, including deforestation, urbanization, and groundwater extraction, significantly alter the cycle, and climate change intensifies it, shifting precipitation patterns and increasing extreme weather events.
- type
- Biogeochemical cycle
- key_processes
- Evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, subsurface flow
- primary_energy_source
- Solar radiation
- ocean_evaporation_share
- 86% of global evaporation
- ocean_precipitation_share
- 78% of global precipitation
- atmospheric_residence_time
- About 9 days
Lore & Background
The water cycle is a continuous biogeochemical process in which water changes form across liquid, solid (ice), and vapor phases while moving between reservoirs on, above, and below Earth’s surface. The total mass of water on Earth remains fairly constant over time, but its distribution among major reservoirs—ice, fresh water, salt water, and atmospheric water—varies with climatic conditions. The ocean is the dominant reservoir, holding 97% of the planet’s water and supplying 86% of global evaporation. Water moves between reservoirs through processes including evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow. These movements are driven by energy exchanges: evaporation absorbs latent heat of vaporization, cooling the environment, while condensation and melting release heat, warming it. This heat transfer plays a critical role in moving energy from the tropics toward the poles via ocean circulation. The evaporative phase also purifies water by separating it from salts and other particles, and subsequent condensation in the atmosphere replenishes land with freshwater. The flow of liquid water transports minerals and reshapes geological features through weathering, erosion, and deposition. Human activities—such as deforestation, urbanization, and groundwater extraction—alter natural landscapes and affect the cycle. Climate change intensifies the cycle, shifting precipitation patterns, increasing extreme weather events, and altering rainfall timing and intensity, with consequences for ecosystems, water availability, agriculture, and societies.
Reader's Guide
The water cycle is essential for maintaining most life and ecosystems on Earth. It transports minerals across the globe and reshapes geological features through weathering, erosion, and deposition. Human actions such as deforestation, urbanization, and groundwater extraction alter natural landscapes and affect the cycle. Climate change intensifies the water cycle, shifting precipitation patterns, increasing extreme weather events, and changing rainfall timing and intensity. These changes impact ecosystems, water availability, agriculture, and human societies. The cycle's residence times vary greatly: groundwater can remain underground for over 10,000 years, while atmospheric water vapor averages about 9 days before precipitating.
Did You Know?
- The ocean holds 97% of the total water on the planet.
- 86% of global evaporation occurs over the ocean.
- 78% of global precipitation occurs over the ocean.
- Groundwater can spend over 10,000 years beneath Earth's surface before leaving.
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