The statement that “it takes 20,000 litres of water to grow the cotton for a t-shirt” (and its many variations) has been published and republished by a multitude of organisations across the globe over many years.  The statement is commonly used to argue cotton production is unsustainable.  But how reliable is this claim?

Some reports state it takes 20,000 litres to grow one kilogram of cotton and sometimes it's reported as requiring this much water for a single t-shirt.  The figure has been so often quoted and re-quoted that it's difficult to determine:
•    the original source and date of this data (the World Wildlife Fund is often quoted)
•    under which production system(s) the cotton was grown
•    whether it refers to average global numbers or numbers from a particular cotton producing country or region
•    whether it refers to irrigation water extracted or actual water used by the crop
•    the weight of the t-shirt referred to

All of the above factors have a major bearing on how the 20,000 litre statement is calculated, and yet are very rarely if ever referred to.

A Cotton Research and Development Corporation (CRDC) investigation found that the  figure may come from a 1991 World Wildlife Fund Report titled “The impact of cotton on freshwater resources and ecosystems”.  This cites a range of 7,000 to 29,000 litres of water required per kg of cotton which references as its sources for this range:
•    Vaidya (1993) - looks to be a general handbook that is published regularly in India, and is thus likely a) not cotton specific and b) likely focused on India only - so the references definitely need further investigation and thus out of date;
•    Rehm (1996) - published in German so CRDC unable to check at this time how ;
•    Klohn (1998) – is the reference for the "other food crops" use of water per kg that are tabulated for comparison to cotton. 

While none of these sources could be reviewed directly and their methodology for calculating water use ascertained, they are all more than 20 years out of date and should not be relied upon.  The industry’s investigations did not uncover a more recent source for the 20,000 litre statement.

How Does This Relate to Australian Cotton?
It’s important to note that water use efficiency, in Australian cotton at least, has improved dramatically over the last 20 years, making the ‘20,000 litre’ statement even more redundant.  From 1992 to 2019, there has been a 48% decrease in the water required per bale of cotton or in other words, Australian cotton growers are now:
•    producing almost twice as much cotton from a megalitre of water, or 
•    using almost half as much water to produce a bale of cotton.

What we can also tell you is that for the Australian cotton industry, Cotton Australia and CRDC estimates:
•    It takes around 600 litres of water to grow the lint for a 150 gram cotton t-shirt.  By way of comparison, Sydney Water reports that the average water use per person per day in Sydney is about 324 litres (2017-18).
•    Based on industry water use and production data, Australian cotton growers can grow 1kg of fibre with just 2,400 litres of water.  This calculation is made using figures for Australian cotton production of 6 megalitres of water per hectare and a yield of 11 bales per hectare.

Whichever way we look at it, Australian cotton is nowhere near 20,000 litres per kg OR per t-shirt, and a 20 year old data reference should not be relied upon to make decisions about cotton’s sustainability.

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