Where have these water molecules been?
Because of the unique properties of water, water molecules can cycle through almost anywhere on Earth. The water molecule found in your glass of water today could have erupted from a volcano early in Earth's history. In the intervening billions of years, the molecule probably spent time in a glacier or far below the ground. The molecule surely was high up in the atmosphere and maybe deep in the belly of a dinosaur. Where will that water molecule go next?
The Water Cycle
The movement of water around Earth’s surface is the hydrological (water) cycle (Figure below). Water inhabits reservoirs within the cycle, such as ponds, oceans, or the atmosphere. The molecules move between these reservoirs by certain processes, including condensation and precipitation. There are only so many water molecules and these molecules cycle around. If climate cools and glaciers and ice caps grow, there is less water for the oceans and sea level will fall. The reverse can also happen.
The following section looks at the reservoirs and the processes that move water between them.
Because it is a cycle, the water cycle has no beginning and no end.
Most of Earth’s water is stored in the oceans, where it can remain for hundreds or thousands of years.
Water changes from a liquid to a gas by evaporation to become water vapor. The Sun’s energy can evaporate water from the ocean surface or from lakes, streams, or puddles on land. Only the water molecules evaporate; the salts remain in the ocean or a fresh water reservoir.
The water vapor remains in the atmosphere until it undergoes condensation to become tiny droplets of liquid. The droplets gather in clouds, which are blown about the globe by wind. As the water droplets in the clouds collide and grow, they fall from the sky as precipitation. Precipitation can be rain, sleet, hail, or snow. Sometimes precipitation falls back into the ocean and sometimes it falls onto the land surface.
Streams and Lakes
When water falls from the sky as rain it may enter streams and rivers that flow downward to oceans and lakes. Water that falls as snow may sit on a mountain for several months. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Snow and ice may go directly back into the air by sublimation, the process in which a solid changes directly into a gas without first becoming a liquid. Although you probably have not seen water vapor undergoing sublimation from a glacier, you may have seen dry ice sublimate in air.
Snow and ice slowly melt over time to become liquid water, which provides a steady flow of fresh water to streams, rivers, and lakes below. A water droplet falling as rain could also become part of a stream or a lake. At the surface, the water may eventually evaporate and reenter the atmosphere.
A significant amount of water infiltrates into the ground. Soil moisture is an important reservoir for water (Figure below). Water trapped in soil is important for plants to grow.
The moisture content of soil in the United States varies greatly.
Water may seep through dirt and rock below the soil and then through pores infiltrating the ground to go into Earth’s groundwater system. Groundwater enters aquifers that may store fresh water for centuries. Alternatively, the water may come to the surface through springs or find its way back to the oceans.
Plants and animals depend on water to live. They also play a role in the water cycle. Plants take up water from the soil and release large amounts of water vapor into the air through their leaves (Figure below), a process known as transpiration.
Clouds form above the Amazon Rainforest even in the dry season because of moisture from plant transpiration.
People also depend on water as a natural resource. Not content to get water directly from streams or ponds, humans create canals, aqueducts, dams, and wells to collect water and direct it to where they want it (Figure below).
Pont du Gard in France is an ancient aqueduct and bridge that was part of of a well-developed system that supplied water around the Roman empire.
Science Friday: Forecasting the Meltdown: The Aerial Snow Observatory
75% of Southern California's water supply comes from the snowpack in the Sierra Nevada Mountain Range. This video byScience Friday explains how NASA uses specialized instrumentation in the Airborne Snow Observatory to carefully measure the water content.
- The water cycle describes all of the reservoirs of water and the processes that carry it between them.
- Water changes state by evaporation, condensation, and sublimation.
- Plants release water through their leaves by transpiration.
- What is transpiration?
- Describe when and how sublimation occurs.
- What is the role of the major reservoirs in the water cycle?
Use this resource to answer the questions that follow.
- How often is water added to the Earth system?
- How can the two parts of the water cycle be summarized?
- What are the major reservoirs for water?
- What is precipitation?
- Where does snow melt go?
- As rain falls onto land, what can happen to it?
- How long does water stay in groundwater?
- How does water get back into the atmosphere?
- How do plants engage in transpiration?