The Earth's Water Cycle: A Closed Circuit

In the middle of the 18th century it was scientifically established that water, in one form or another, is continuously circulating throughout our ecosphere — by means of evaporation, condensation, precipitation, and penetration into the earth's surface.

The astronomer Edmund Halley (1656-1742), best known for predicting the elliptical path of the comet that bears his name, also correctly calculated that the volume of river effluence into the Mediterranean Sea is virtually equal to the amount of precipitation falling upon the same area.

It's astonishing to contemplate this natural cycle — that every drop of ocean water that turns into vapor eventually turns, once again, into ocean water. Whether it takes ten days or ten thousand years — as, for example, in the case of glaciers — the fact is that all water on earth is recirculated throughout the global ecosystem.

Our planet is often referred to as the "water planet."

Seawater accounts for 97.5% of the water on earth. With its high salinity content of about 35 grams per liter, seawater is unusable for human consumption or for agricultural purposes. But thanks to solar radiation, seawater evaporates into the air, turning into clouds or mist which eventually transform into rain, snow, or hail. This conversion is the process by which fresh water is produced.

Fresh water represents only 2.5% of all the water on earth. Although rivers, lakes and inland marshes are made up of fresh water, that water is in fact only temporarily extracted from the sea, since it, too, will eventually evaporate and ultimately return to the oceans.

Most of the fresh water on earth, however, is not liquid but locked in ice. Fresh water in a liquid state amounts to merely 0.01% of all the water on our planet.

The history of civilization is closely tied to water. Fresh water was a key factor in the emergence of communities, not only for direct human consumption but also for agriculture and animal husbandry. The growth of communities and their increasing dependence on a steady water supply led to the creation of primitive waterworks to ensure that supply.

The great civilizations of the ancient world all sprung up around abundant water supply. Rivers were vital to irrigation and the growth of commerce. Much later, industrialization and its expansion were also dependent on water supply and the use of waterways.

Mankind has been clever and creative with respect to the use of water, but also the cause of its contamination and shrinking availability. Worldwide, falling groundwater levels, disastrous droughts and calamitous floods are increasingly attributable to human activity, and deteriorating water quality is directly so.

Although every living organism on earth is dependent on water, humans use it far in excess of their survival needs. Fresh water from reservoirs and wells is used for countless purposes, and water consumption is constantly expanding. In the past century alone, total worldwide water consumption increased six times.

Water circulation in a home can be compared to blood circulation in the human body. The average person has about 5 liters of blood in his or her system, and that blood is continuously being purified by the kidneys at the rate of 1,500 liters per day. Thus, the total blood supply is renewed 300 times daily. When water becomes contaminated with household use, it needs something analogous to a kidney to restore its purity.

Blood Cliculation in the Human Body

• Average water intake per person/day is 0.7 gallon (2.5 liters), including water contained in food.
• Water makes up roughly 60% of the human body. Part of this water circulates around the body in blood and lymph (fluid).
• Blood is cleansed by the kidneys 300 times a day and recirculated throughout the body.
• Average water discharge per person/day is 0.7 gallon (2.5 liters), whether as waste, perspiration, water lost in breath, and so on.

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