TREAD LIGHTLY
The science of water footprints
They say it takes 1000 litres of water to produce a litre of milk. But what do water footprints really mean? Nikki Macdonald investigates.
Remember Winston Peters’ $18 cabbages? The dire prediction about the impact of Labour’s pre-election water tax proposals must have involved a guesstimate of how much water it takes to grow a cabbage – albeit a vastly wrong one.
As competition for water intensifies around the world, the idea of water footprints is gaining currency as a way to compare the water efficiency of different foods. You’ve probably heard that it takes 1000 litres of water to produce a litre of milk, and rumblings and grumblings about the 16,000 litres needed to produce a kilogram of almonds.
The most widely quoted and consumer-friendly water footprinting method was pioneered by Dutch researcher Arjen Hoekstra.
It’s usually expressed as the sum of three parts: green water (rainwater taken up by a plant and evaporated out through its leaves); blue water (irrigation water drawn from rivers and aquifers); and grey water (a kind of pollution dilution factor – the volume of pure water required to dilute any resulting contamination, such as nitrogen from fertiliser runoff).
Broadly, the footprints show a hierarchy of water efficiency from sugar crops (roughly
200l/kg), to vegetables (300l/kg), to roots and tubers (400l/kg), to fruits (1000l/kg), to cereals
(1600l/kg) to oil crops (2400l/kg) to pulses (4000l/kg). And animal products are generally more water-thirsty than crops.
But how useful are the numbers to help Kiwis understand how efficiently our water is being used?
Plant and Food principal scientist Brent Clothier is calling from the United Arab Emirates, where he’s studying the water use of dates in the desert. It’s 9am and it’s already 36 degrees Celsius.
There, where it never rains, water is mined, not collected. Like a precious mineral, it’s been deposited over thousands of years, and when the aquifer is dry it won’t be replenished. Which means the community has to decide whether to desalinate water, which creates contaminating brine, or face the culturally fraught prospect of breaking the sacred Ramadan fast with dates grown from treated sewage.
And that’s the problem with Hoekstra’s water footprints, Clothier says. While it’s a good starting point for consumers, in terms of understanding what nature needs to grow particular crops, it doesn’t take into account water availability. The total footprint is the same whether all the water comes from rainfall, or whether it’s sucking dry a reservoir that will never be recharged. Which is probably how National arrived at its pre-election calculation that a water charge
‘‘We have a marvellous resource. Our real challenge is not to screw it over.’’
Brent Clothier
of 10 cents a litre would add $75 to a bottle of wine. The global total water footprint of wine is 869l/kg, but only 138l of that is the blue water, or irrigated water that winemakers would have to pay for. ‘‘Nature is all joined together and that’s necessarily complicated, and if you simplify it you could end up with erroneous conclusions and erroneous actions,’’ Clothier says. ‘‘And so saying that all the world needs 120l of water to produce a glass of wine doesn’t tell you anything about the difference between the Barossa Valley and Marlborough. One has rain, the other doesn’t . . .’’ And for animal products, the global average doesn’t factor in different farming methods. The global water footprint for beef is probably based on grain-fed animals, whereas most New Zealand cattle graze on grass. ‘‘So New Zealand’s huge advantage is that we have massive amounts of green water available,’’ Clothier says. ‘‘It runs out in summer so we have to go and pull out some blue water, and is there enough blue water to go around?
‘If you have a look at dairying in Canterbury, the answer is probably not, in the future. People are sucking on the groundwater, the groundwater feeds the Selwyn River, the Selwyn River has disappeared – surprise, surprise . . . The challenge is for us to work out how much blue water we need to produce our food and fibre, and how much grey water we produce as a consequence.’’
The tricky thing is the amount of blue, or irrigation, water required for each crop can vary hugely between regions.
AgResearch principal scientist Stewart Ledgard compared the water footprint of milk in Canterbury and