Radio New Zealand - Te Reo Irirangi o Aotearoa

A senior scientist with Plant and Food Research says the emerging debate about "virtual water" will become as significant an issue as greenhouse gas emissions.

Virtual water is the amount of water required to grow, manufacture and market a kilogram of any food product.

Dr Brent Clothier, co-author of a paper on the issue published in this month's Royal Society of New Zealand magazine, says it takes 140 litres of water to produce a cup of coffee.

He says the virtual water in a plate of pasta amounts to 200 litres, mainly due to the irrigation of the wheat crop, according to Australian research.

Mr Clothier says measuring the virtual water content of export products is a necessary first step to demonstrating, then improving, production efficiency and performance.

While there's little accurate data on the virtual water content of food grown in New Zealand, he says, for apples it's about the same as in the Netherlands and for kiwifruit it's about 20% of that used in Australia.