Water Cycle
The water cycle, also known as the hydrological cycle, is the continuous movement of water through Earth's systems — from the oceans to the atmosphere through evaporation, to the land through precipitation, and back to the oceans via runoff and groundwater flow. Driven primarily by solar energy and gravity, the water cycle regulates climate, freshwater availability, and supports all life on Earth. Key processes include evaporation, transpiration (evapotranspiration from plants), condensation, precipitation, infiltration, and surface runoff.
Major Processes of the Water Cycle
| Process | Description | Direction of Water Movement | Energy Source |
|---|---|---|---|
| Evaporation | Liquid water → water vapour from ocean/lake surfaces | Surface → Atmosphere | Solar energy |
| Transpiration | Water released from plant stomata as vapour | Land/Plants → Atmosphere | Solar energy |
| Condensation | Water vapour → liquid droplets forming clouds | Atmosphere (forms clouds) | Cooling |
| Precipitation | Water falls as rain, snow, sleet, hail | Atmosphere → Surface | Gravity |
| Infiltration | Water soaks into soil → groundwater | Surface → Underground | Gravity |
| Surface Runoff | Water flows over land into rivers and oceans | Land → Ocean | Gravity |
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Related Terms
Carbon Cycle
The carbon cycle is the biogeochemical process by which carbon atoms continuously move through the atmosphere, hydrosphere, lithosphere, and biosphere through processes such as photosynthesis, respiration, decomposition, combustion, and ocean absorption. Carbon exists in various forms — as CO₂ in the atmosphere, as carbonate in rocks, as organic molecules in living organisms, and dissolved in water. Human activities such as burning fossil fuels and deforestation have significantly accelerated the movement of carbon into the atmosphere, driving climate change.
Nitrogen Cycle
The nitrogen cycle is the biogeochemical process by which nitrogen is converted between its various chemical forms as it circulates through terrestrial and aquatic ecosystems, including the atmosphere. Because nitrogen is a key component of amino acids, proteins, and nucleic acids, all life depends on it, yet atmospheric nitrogen (N₂) is largely inaccessible to most organisms without fixation. The cycle includes key processes: nitrogen fixation (N₂ → NH₃), nitrification (NH₃ → NO₂⁻ → NO₃⁻), assimilation, ammonification, and denitrification (NO₃⁻ → N₂).
Biodiversity
Biodiversity refers to the variety of life on Earth at all its levels — from genes and species to ecosystems — and the ecological processes that support this variety. It is typically measured at three levels: genetic diversity (variation within species), species diversity (number and abundance of species in an area), and ecosystem diversity (variety of habitats, communities, and ecological processes). High biodiversity generally confers ecosystem stability, resilience to disturbance, and a wider range of ecosystem services such as food, medicine, and clean water for human societies.
Water from Old English wæter, from Proto-Germanic *watar; cycle from Greek kyklos, "circle or wheel". The scientific study of the water cycle (hydrology) developed formally in the 17th century with work by Pierre Perrault and Edme Mariotte.