Water cycle

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.