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By Daniel Collins

DairyCattle

The single biggest consumer of water in New Zealand is the dairy industry. As of 2010, farmers are permitted to take about 4707 million m3 of water per year from New Zealand’s rivers and aquifers to irrigate pasture, most of which is for dairying. This is 44% of all consumptive uses (excluding the Manapouri hydropower scheme), and 68% of this water is allocated in Canterbury alone (21% in Otago).

Not all of this water is ultimately taken, though, and for various reasons. One is because irrigators typically ask for more water than they may need in average conditions as a form of insurance in dry times. What is actually taken is more like 50% of that allocated. In Canterbury during 2010-11, for example, 52% of the allocated groundwater was used and 50% of allocated surface water, each based on data from a fraction of takes.

While this water is used for irrigation, supplemental to whatever rainfall already occurs, not all of it will contribute to pasture growth. Some will be lost* to the ground as it is conveyed along canals to the pump house. Some will evaporate before reaching the roots. Some will infiltrate beyond the roots and feed a river or aquifer. But much will be transpired by the pasture or consumed during photosynthesis in the course of producing food for cows, which ultimately produce milk.

A much smaller amount of water is also used for other stages of the milk production process, such as milking shed wash-down, but irrigation is by far the biggest component.

So how much water does it take to produce a litre of milk?

This question actually comes up regularly. It was recently asked in the comments section of a NZ Herald op-ed:

‘I wonder how many litres of water it takes for a cow to produce one (1) litre of milk?
My first guess is about 10 litres of water but please correct me if you know better!’

And so I’d like to answer it here.

The answer is that it depends. It depends on the local climate, the local soils, the irrigation techniques used (if any), the type of pasture, the type of cow and its genetics, the type of milk that you buy in the shop, and what you mean by ‘water’.

In some regions, such as Waikato, almost all water used in the production of milk is rainfall. In Canterbury, with low rainfalls during the growing season, supplemental irrigation is necessary. In traditional water footprint calculations, rainfall is termed ‘green water’ and water taken from rivers or aquifers (mostly for irrigation) is ‘blue water’ (more on these definitions in a later post).

In some calculations, you can also represent water pollution in terms of water quantity, by determining how much water would be required to dilute the effluent to meet necessary guidelines. In water footprint calculations, this is termed ‘grey water’.

The best global estimate of the total water needed (green + blue + grey) to produce a litre of milk is about 1020 litres. Unfortunately, I know of no global estimates of the irrigation requirement, but total agricultural water use accounts for about 70% of total water withdrawals globally.

For New Zealand, AgResearch has studied how much water is required to produce milk in a subset of Waikato and Canterbury farms for the 2004-2005 period. They reported their results in terms of the litres of water required to produce one kilogram of fat-and-protein-corrected milk (FPCM). FPCM is a standardised measure of milk quantity. If we approximate 1 kg of FPCM as 1 L (the milk is mostly water, and water has a density of 1 kg/L), then we can get estimates of volume.

For Waikato, about 945 L of water (green + blue + grey) are required to produce 1 L of milk. For Canterbury, it’s 1084 L of water. These are remarkably close to the global best estimate. For blue water alone, Waikato uses about 1 L of water per litre of milk, while in dryer Canterbury, 250 L of water is required, almost all of it for irrigation.

But wait, there’s more!

We can estimate the irrigation requirement of New Zealand’s milk production another way. The volume of milk produced in NZ from January to December 2010 was about 15 million tonnes [PDF], or assuming the density of water again, then 15000 million L. Dividing the estimated actual water use of all pasture irrigation (which would be a bit more than just dairy irrigation) by this number gives an upper bound of about 160 litres of water per litre of milk for New Zealand as a whole. This is on par with the AgResearch numbers above.

Finally, what do these numbers actually mean? By themselves, they are a statement of the conversion efficiency of the social, physical, chemical and biological processes that transform water into milk. They can thus be used in evaluating whether there’s enough water available to produce a target quantity of milk. But the numbers are also useful when compared with one another, either with milk production at a different time, in a different location, or under different management practices. What the Waikato and Canterbury numbers tell us is that, in terms of just the water taken from rivers and aquifers, milk production in Waikato is more efficient than in Canterbury; the national average is somewhere in the middle. Of course, there’s more to the conversion of water into milk than just water quantity, but this is an important piece of the puzzle.


* Water isn’t exactly ‘lost’ in the water cycle, it just goes somewhere else, although it’s true that we don’t know exactly how much that may be. The miniscule exception is when water is altered chemically, e.g., photosynthesis or hydrolysis, and leaves the water cycle until respiration or combustion.