"Water footprinting", like carbon footprinting, can serve as a useful tool for the farming and food industry, but it is unlikely to be understood by customers, according to Warwick Crop Centre senior research fellow Dr Rob Lillywhite.
"It's still evolving and there are many different approaches," he said. "A 'volumetric' figure, which you see most often, tells you the total amount of water involved but won't tell you about the impact it is having."
By contrast, two "stress-weighted" approaches attempt to rectify this by scaling values by a factor that reflects local water scarcity and other variables. "Water use has to be considered in the context of the country you're growing in," he said.
Growers in Israel or Morocco show much higher water footprints from this approach, he explained. "But they still reckon it's worth doing. But considered this way, domestic production is far better than imports (from these countries)."
So far, however, Lillywhite added that he is aware of only one product, a Finnish breakfast cereal, that declares a water footprint on its packs. "There is no context to tell you what that figure means."
Lillywhite will present his Defra-funded research at a conference in France next month. It follows on from his earlier studies of carbon footprinting, from which he drew similar conclusions. "It's useful in reducing greenhouse gas emissions and can be used to identify hot spots in a product or process," he said.
"For example, 40 per cent of a potato's carbon footprint comes from the fertiliser, but the (contribution from) pesticides is negligible."
He added: "Most growers understand the need for efficient use of these things anyway. But the industry is unsure what to do with carbon footprinting in total."
Lillywhite pointed out that Walker's Crisps formerly published a carbon footprint figure on its crisp packets, but the company subsequently "quietly dropped" it.
WATER FOOTPRINT INTERNATIONAL COMPARISON
Product and origin Volumetric Stress
Potatoes: UK 107 10
Potatoes: Israel 147 103
Strawberries: UK 176 12
Strawberries: Morocco 113 71
Data source: Dr Rob Lillywhite
Although strawberry growing in Morocco requires less water overall, as a
water-stressed country the impact is around six times higher. Israeli
potatoes have a slightly higher total water requirement but an
environmental impact put at around 10 times higher