Clean Water: Water on Earth [c]

I N F I N I T E C Y C L E, F I N I T E R E S O U R C E

Wherever two reservoirs are in contact. Water can be exchanged between them. From one reservoir to the next, water moves around the Earth. See where it goes:

Each reservoir is a bank of water. Water moving outside the reservoir is a withdrawl and moving inside is a deposit. When the rates of withdrawl and deposits don't match, the supply either dwindles or grows in that reservoir.

During glaciation, the ice reservoir grows at the expense of the ocean. When we tap into groundwater faster than it replenishes, we drain groundwater at the expense of the living things we irrigate, or the stream runoff we create.

If we want an adequate supply of water in a particular reservoir, we need to keep close tabs on account activities.

Transfer processes
The transfers between reservoirs take place by the processes of evaporation, precipitation, evapotranspiration, and water flow. These processes are ultimately fueled by energy from the sun, whether it's evaporation or human muscle that carries water uphill.

Evaporation
Heated by the Sun, water moves from liquid water surfaces to the atmosphere: from the surfaces of the ocean, rivers, dams, and aqueducts. The warmer it is, the more water evaporates into the atmosphere and the more humid it becomes.

Evaporation is a natural purifier, a natural process of desalination. Evaporation selects mostly pure water molecules for entrance into the atmosphere reservoir, leaving behind salt and other chemical impurities, not to mention solids. Then the raindrops that condense from water vapor are clean. Clean rain falls into rivers and streams.

Fig. Water evaporates into the air, leaving mud cracks. Photo courtesy of ... Cornell University.

Raindrops can become dirty because they condense around very small particles in the atmosphere, and they pick up chemicals while they fall.

Condensation and precipitation
Water moves from the water vapor reservoir to any of the other reservoirs by condensing or sublimating, forming rain or snow. Rain quickly enters one of the other reservoirs - the ocean, streams, lakes, or groundwater.

Fig. Condensation on glass. Photo by Jayanta Bhadra.

Evapotranspiration
Plants both evaporate and utilize water. If you've ever left a large plant in your car overnight you'll find it very humid in the morning.

Fig. Stomata on the underside of a leaf. These are the pores that allow carbon dioxide intake and release water and oxygen. Photo courtesy of Maricopa College.

Water flow
Water flows by gravity. Both on land and underground water moves.

Flowing water in rivers moves from land to sea, usually downhill. The world's largest rivers collect water from many smaller rivers, carrying the products of rain and snow that fall on areas the size of small continents into the ocean reservoir.

Fig. Much of the rain that falls east of the Rockies drains into the Gulf of Mexico via the Mississippi River at New Orleans. Map courtesy of.

Groundwater also generally moves downhill. In many places, groundwater also meets seawater, underground. However, unlike surface flow, confinement underground allows ground water move in unexpected directions.

You might imagine that ground water moves like water through plumbing. The water in your home plumbing is moving generally downward. However, it might take a few unexpected upswings on the way down - up and around the trap underneath your sink for example. With all the bends, there is still a particular direction of flow. Also, water is confined to movement in a path of least resistance, through the pipe, and not everywhere.

Motion within a resevoir
The water in the ocean is not stale. Even though it looks a constant volume of water in the oceans, individual molecules of water are being exchanged. Before water molecules came to the ocean, they were somewhere else. Even the water at the bottom of the ocean has only arrived during the last 1,000 years or so.

Transfer processes mix up all the molecules of water on Earth. Water molecules are constantly exchanged between the reservoirs, mixing-up the countless molecules of water around the entire Earth.

However infinite the number of molecules may seem, there is only a fixed number. They just go round-and-round the Earth from reservoir-to-reservoir.

The last sip of water you took is used water. Maybe dinosaurs took a sip of one of those molecules.

Of the reservoirs of freshwater on Earth, only 0.003 percent is fit to drink. It's this body of water that we must be careful to preserve for people and other living things. The distillation process only works so quickly. The cycle passes on degraded water.

Part I - Water, Life and Earth
Section D
Water on Earth
• Intro - Blue Planet
• Stores of water
• Infinite cycle
• Sculpting the earth
• What's left to drink?
Quiz
Glossary
Resources
Teacher's guide
Information Sources
Contents

• Activity 2.1
Will the water cycle bring California drought?
Interview with Dan Cayan

• Activity 2.2
Condensation and the F-18
Unnatural cloud formation. Earthguide

• Activity 2.3
Just doing my job: The Mississippi Flood of 1993
Story of how Lee Blackmore uses satellite photos to help evacuation procedures in the Flood of 1993.