We’ve all heard of virtual reality, but virtual water is the current buzz phrase among water conservationists.

According to the World Water Council (worldwatercouncil.org), “Virtual water is the amount of water that is embedded in food or other products needed for their production.”

The water is virtual because once a crop is grown or a material produced, the actual water used to produce it is no longer contained in it. A product’s virtual water footprint helps us realize how much water is needed to produce it.

For example, in North America one-third of the tap water used for drinking is used to brew coffee. Wasting one less cup of coffee a day could save enough water to supply drinking water to the 1.1 billion people across the globe who have no access to fresh water.

Although shorter showers and other overt water conservation efforts are important, the amounts saved are small by comparison. They save a few gallons, but we waste a hundredfold more when we throw food away.

The third most common refuse in landfills is uneaten food that represents wasted and nonrenewable virtual water.

On top of that, 30% to 50% of our food and its virtual water is already lost before it is consumed. The losses come from harvesting, production, processing, transportation and storage.

The size of the virtual water footprint of various common items is surprising.

A cup of coffee requires 37 gallons, an apple 19 gallons, a single banana 27 gallons, a slice of bread 10 gallons, a sheet of paper 3 gallons, a pound of beef 2,600 gallons, a pair of leather shoes 4,400 gallons, 65 gallons to grow a pound of potatoes and 650 gallons for a pound of rice.

Among nonfood agricultural products, cotton is a water-guzzler. One cotton shirt requires 650 gallons of water.

The per-capita virtual water footprint of our diet varies, from 265 gallons per day for a subsistence diet to 685 gallons per day for a vegetarian diet and over 1,300 gallons per day for an American-style meat-based diet.

Most fresh water is used for agriculture, not for drinking and bathing, accounting for 70% of all water use globally and up to 95% in some developing countries.

Nearly 10% of the water used in growing crops is traded internationally. An arid region can conserve 60% to 90% of its fresh water by importing foods with a large virtual water footprint from areas of high rainfall or large water supplies.

The actual savings can be even greater than the numbers suggest since it actually takes two or three times as much water to produce a given crop in arid regions where evaporation is higher.

International trade of virtual water has geopolitical consequences, as international food trade becomes a matter of commerce between the water haves and water have-nots. This generates dependencies between countries that can be either a stimulant for cooperation or a source of conflict.

A big first step in water conservation is becoming aware of the impact of our own virtual water footprint. To determine yours, go to waterfootprint.org.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.