September 18, 2021   Read time 2 min
We “eat” an awful lot of water. The average calorie we put in our mouths takes a liter of water to produce. People around the world consume between 1,800 and 3,900 calories in food energy per day, on average, so those on affluent diets go through at least 3 tonnes (792 U.S. gallons) of water every day— around 1,240 tonnes (327,000 gallons).

This compares with the two or three liters (two- thirds of a U.S. gallon) a day we need for drinking. Since life expectancy in developed countries is now mostly around eighty years, this means that the average affluent person “consumes” nearly enough water in a lifetime in the form of food to float the USS Enterprise, a rather large aircraft carrier that displaces ninety thousand U.S. tons.6 Just over half of this water comes directly from rainfall. The rest comes from irrigation, drawn from limited resources such as rivers, lakes, and groundwater aquifers, which it is possible to exhaust by taking out water faster than it is naturally replenished by rainfall, runoff, or underground flows.

Most people are unaware of how much water goes into producing the food on their plates— yet this is likely to be a critical factor on which we will all base our food choices in the years ahead, as water becomes scarcer and more costly. Food uses so much water because the plants on which all our foods are based— including livestock products— take up and release a great deal of water as part of their internal functioning, just as we breathe air in and out. This pro cess is known as transpiration. The water embodied in our food is referred to as “virtual water.”

Superficially, the world has an abundance of freshwater— but it isn’t always located where the people are or where the food is grown, a lot of it is frozen, it isn’t always easy or cheap to extract, and often it is too polluted to use. We have already tapped most of the freshwater that is accessible and economic, and tapping more distant, dirty, or difficult supplies of water, like oil, is a great deal more costly. At the same time, we manage, conserve, and price water very badly.

World water figures should be regarded with some caution: for many countries the data are poor or out of date. The broad scientific estimate of world freshwater and where it goes, however, can be seen in table 2. Out of all this water, humanity actually extracts about 3,900 cubic kilometers (853 cubic miles) for all of its uses, of which 2,700 cubic kilometers (560 cubic miles) are employed to irrigate crops. This is what is meant by “available water,” the amount we can readily and affordably harvest from rainfall, rivers, lakes, and aquifers on an annual basis. This is the water that is in increasingly short supply because we have overextracted it from most rivers, and their flows are now no longer adequate for a healthy environment, either on land or in the sea.