Wasteful consumption depletes water sources too, recent data seem to show.
A report released this week by the Stockholm International Water Institute says that as much as 50 percent of the calories grown globally don't make it to the table. Given that crop production uses about 1,800 trillion gallons (1,700 cubic miles) of water a year, almost 40 percent of which comes from irrigation rather than rainwater, that loss represents a lot of water.
"About 1.2 to 1.4 billion people are living in areas where all of the water is committed," lead author Jan Lundqvist.said. "There is simply no more water to take. So if people want to have more water, they either have to take it from someone else, or we have to make more efficient use of it."
Agriculture is the biggest consumer of water -- up to 70 percent in California and most other areas where crops are grown, said Meena Palaniappan, an international water specialist at the Pacific Institute in Oakland, Calif. "When we're talking about ways to reduce or conserve water, obviously agriculture has to be the place where attention is paid."
In areas like Africa and many parts of Asia where upstream wastage dominates, Lundqvist said, waste could be reduced through better harvesting technology, storage and transport.
In the West, however, reducing household waste is key.
In the United States, up to 30 percent of food is tossed out each year, the report says, worth about $48.8 billion and equivalent to flushing 10 trillion gallons of water down the drain.
"There's a very low awareness about the size of these figures," said Jan Lundqvist. "I think most people don't realize that the loss and the wastage is at that level."
"It's not possible to reduce wastage and losses altogether, but we think a realistic target is to reduce losses and wastage by 50 percent by 2025," he added.
"The situation looks quite different in different parts of the world," Lundqvist said. "In the tropics, they have quite a big loss in the field from rodents, pests, mold. The farmers have poor storage, and the transport system is insufficient. But in our part of the world, we have comparatively much larger wastage at the end of the food chain."
These losses together make up more than half of the total. The remaining losses come from crops grown for animal feed that don't end up as calories in animals because of animals' inefficiencies in converting food into body mass, the report shows, says Jessica Marshall of the Discovery News.
Lundqvist pointed out that reducing meat consumption, and overeating in general, could also contribute. There are about 50 percent more people who are overweight or obese than who are undernourished, according to the report.
"This issue of efficiency of agricultural production is incredibly central to any environmental outcome over the next century, not just water," said Marshall Burke of the Program on Food Security and the Environment at Stanford University. "Wastage is one clear, clear inefficiency where, if you could reduce it, it would have really useful effects for water, for nitrogen use, and for land use."