水の循環

The hydrosphere encompasses all areas where water is stored and moves on Earth. Since water is the basis of all living processes, the cycling of water is extremely important to ecosystem dynamics. Most organisms on land require fresh water to survive, making the hydrosphere essential for supporting life.

Energy from the sun warms surface water on land and in oceans, causing it to evaporate and enter the atmosphere as vapor. This solar energy also drives transpiration, where water evaporates from plant leaves. The sun's warmth is the primary force that initiates the continuous movement of water through the atmosphere and back to Earth.

After precipitation returns to Earth's surface, water may fill bodies of water, evaporate again from the ground, permeate soil to be absorbed and transpired by plants, flow beneath the surface as groundwater, or be stored for prolonged periods in aquifers. Each pathway determines how quickly water cycles through the environment.

Residence time measures how long water molecules remain in a particular location before cycling elsewhere. In salt water oceans, water molecules may remain for as long as 3,000 years. In contrast, water molecules in freshwater sources like lakes and streams cycle much faster, making these ecosystems more dynamic.

Although 97.5% of Earth's water is saltwater, less than 1% of freshwater is accessible through rivers and lakes. Most freshwater exists as ice, groundwater, or stored in aquifers for extended periods. This long-term storage makes the water unavailable for short-term cycling and inaccessible to many plants, animals, and fungi.

Plants absorb water from soil and release it through transpiration, where water evaporates from pores in leaves into the atmosphere. This process is a critical component of the water cycle, returning water vapor to the atmosphere where it condenses to form clouds and eventually falls as precipitation.

The water cycle is fundamental to ecosystem function and interconnects with other biogeochemical cycles. Water transport through soil, groundwater, and surface runoff carries nutrients and dissolved substances that drive what are biogeochemical cycles in the biosphere, enabling nutrient cycling and energy flow through ecosystems.