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The Water Cycle

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The hydrosphere is the entire area where water occurs on a planet. Most of the Earth's water supply is contained in salt water oceans, where individual water molecules may remain for as long as 3,000 years, a measurement known as residence time. Water molecules in critical freshwater sources, such as lakes and streams, cycle much faster.

The cycle begins as the sun's energy warms and evaporates surface water from oceans, streams and lakes into the atmosphere, where it condenses to form clouds. Water then returns to the earth's surface as precipitation.

Now it can flow back into the ocean as runoff via streams and lakes, seep into the soil to be stored in aquifers, flow beneath the surface as groundwater, or evaporate again through transpiration by terrestrial plants.

Overview

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The Earth’s hydrosphere includes all of the areas where the storage and movement of water occurs. Since water is the basis of all living processes, the cycling of water is extremely important to ecosystem dynamics.

The water cycle begins as the sun warms surface water on the land and in oceans, causing it to evaporate and enter the atmosphere as vapor. The water vapor condenses into clouds and eventually falls as precipitation in the form of rain, snow or hail.

After falling back to the Earth, water may enter large bodies of water, evaporate again, remain on the surface as runoff, or seep into the soil, where it may be absorbed by plants and transpired (released from pores in the leaves and evaporated into the atmosphere) or become groundwater. Deep groundwater may form reservoirs, or aquifers, and shallow groundwater eventually reaches a body of water, where it can be evaporated as surface water to continue the cycle.

Long-Term Storage

Human cells are over 70% water, and almost all organisms on land require fresh water to survive. However, 97.5% of the water on Earth is saltwater, and less than 1% of freshwater is accessible through rivers and lakes. Most water on Earth exists as ice, groundwater, or saltwater in the oceans and seas, and is inaccessible to many plants, animals, and fungi, and unavailable for short-term cycling. In these forms, water is stored for extended periods of time—called residence times—before entering the water cycle.

Short-Term Water Cycle

Energy from the sun warms surface water on land and in the ocean, causing it to enter the atmosphere through evaporation, sublimation (ice vaporizing from a solid form), and transpiration (water evaporation from plants). The water vapor then condenses to form clouds and eventually falls to the Earth as precipitation (e.g., rain or snow). The water that returns to the Earth’s surface may: fill bodies of water, evaporate from the ground again, permeate the soil to be absorbed and transpired by plants, flow beneath the surface as groundwater, or be stored for prolonged periods in aquifers.

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