Water leaves a ‘footprint’ in our food; here’s how it works

It takes a lot of water to produce the food we eat. Knowing just how much is a significant matter for California in a time of drought, as the state produces almost half of all fruits, nuts and vegetables grown in the United States. One way to measure usage is the so-called water footprint.

What is a water footprint?

A water footprint is an indicator of how much water goes into a product, according to the Water Footprint Network, a research group that advocates for sustainable usage and management. Water footprints can be applied to a city, a business, an individual or, yes, our food. The concept, when applied to food, goes beyond direct water use -- say, how much water it takes to irrigate a patch of strawberries -- and takes into consideration the water used during a product's entire supply chain.

How is it calculated?

It depends on the type of food. Take beef, for example. It's the most water-hungry food product -- in the U.S., on average, it takes 106.3 gallons of water to produce 1 ounce of beef. About 98% of the water in beef's footprint is devoted to produce the feed that cows consume, such as grass, soybean and corn. That constitutes an indirect water footprint, said Mesfin Mekonnen, a water management researcher at the University of Twente in the Netherlands. Globally, water demand for animal products comes almost entirely from feed, he and his colleague Arjen Hoekstra report.

When examining crops such as peas, factors involving climate, the water cycle, soil type, irrigation, the rate at which nitrogen fertilizer is applied and crop yield are all considered, Mekonnen said. The underlying data of our food water footprint project considered multiple types of farming techniques for every country using weight (cubic meter of water per metric ton of food produced) as the standard.

We also thought that weight was an intuitive comparison for the typical consumer, so we converted the data to U.S. gallons per ounce of food product (or fluid ounce for beverages), with assistance from Mekonnen and Hoekstra, and focused solely on production in the United States.

Does nutrition play a part?

Adjusting for nutritional value can provide additional insight. Below you can see the U.S. water footprints of selected foods as measured by gallon of water per gram of protein produced or per calorie.

Water footprint by protein

Water footprint by calorie

"The water footprint of any animal product is larger than the water footprint of crop products with equivalent nutritional value," Mekonnen and Hoekstra write. "The average water footprint per calorie for beef is 20 times larger than for cereals and starchy roots," they note, referring to global averages, not U.S.-specific figures. "The water footprint per gram of protein for milk, eggs and chicken meat is 1.5 times larger than for pulses," a group of legumes that includes peas, beans and lentils.

So what is the right metric?

Both methods -- measuring by weight or by nutritional value -- are valuable ways to understand the effects of food on water supplies. Although the numbers vary between metrics, the overall takeaway is clear: Generally speaking, foods derived from animals have a significantly larger water footprint than do plant-based foods.

Follow @kyleykim on Twitter.