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

According to the World Water Council, virtual water is the amount of water that is embedded in food or other products.

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 for its production.

For example, in North America one-third of the tap water used for drinking is used to brew coffee. Wasting only one fewer 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 exponentially 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 percent to 50 percent of our food and its virtual water is 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, a pound of potatoes 65 gallons and a pound of rice 650 gallons.

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 more than 1,300 gallons per day for a meat-based diet.

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

Nearly 10 percent of the water used in growing crops is traded internationally. An arid region can conserve 60 percent to 90 percent 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, because it takes two or three times as much water to produce a given crop in arid regions where evaporation is higher.

The international trade of virtual water creates geopolitical tensions between the water haves and water have-nots. The international food trade 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 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.

On the Net:

>> worldwatercouncil.org