How much water does a euc use?

Roger Godsmark of Forestry South Africa got very excited recently when he came across an article in the Business Day stating that “unlike bamboo’s 2 litres/day, eucalyptus trees use up to 200 litres/day”. When editor Chris Chapman asked me how much water a gum tree actually uses, my facetious reply was ‘how long is a piece of string’? Not a very clever answer, as he then asked me to explain myself in writing! Still, it got me thinking...

by Dave Dobson

More specifically, if I drove from Pietermaritzburg to Cape Town and told you I used 125 litres of fuel it wouldn’t  tell you how well I drove, but if I told you my fuel consumption was 8 litres per 100 kms it immediately gives an indication. The public debate around water use by gum trees is rather similar. Does the 200 litres/day that gum trees allegedly use refer to use by a single tree or by one hectare of trees? Can the water use by gum trees be compared with  water used to produce other crops? How long is this piece of string?

There are plenty  of academic articles written on water use by trees, but if you are like me, I start getting nervous when the authors start using terms like ‘standard deviations’ and ‘coefficients’ of variance.’ In this day of tweeting and sound bites we need something short and pithy to make an impression. (That is why 200 litres/day sticks.)

My starting point in figuring out just how much water a gum tree uses, was a comment years ago made by Dolf Schonau, that the limiting factor in plantation growth in this country was moisture. The question then arises whether  gum trees growing in the super drained soils at Eston producing a yield of 100 tons of fibre per hectare use the same amount of moisture as say gum trees growing at Boscombe and producing 220 tons of fibre per hectare. Obviously the answer is no, it is not possible. The amount of moisture available to the trees on each of these two sites is very different and this is reflected in their growth. So how does one come up with a comparison?

I then recalled a conversation with Peter Roberts, a past Director of the ICFR and a forest industry water guru. I had seen an article which listed some interesting figures providing global average water use requirements to produce some of the staples we eat, for example rice, wheat, potatoes and sugar. He kindly offered to extend the list for me but then being a true scientist started talking of ‘blue water’ and ‘green water’. My brain went into academic freeze! Still, Peter is a patient man and he quietly talked me through my panic! Blue water, by the way, refers to runoff and green water to evapo-transpiration. He then produced the following table for me, which I duly scribbled down. It is this table which produced the eureka moment and enabled me to come up with a comparable and sensible water use for the gum tree.

The numbers are based on ‘blue water’ i.e. runoff reduction, which is the yardstick used for water consumption for commercial forests.

So maybe this is the figure (36 litres/1 kg fibre) we should be using in our comparisons.  It makes sense and puts the water use debate into some perspective.

Now, as I mentioned at the beginning of this exercise, I started early, so it’s time for some breakfast. How about that bacon and eggs with toast and a little butter. Perhaps add a glass of fruit juice. Hmm… and the water required to produce such a meal? You can do the sums yourself!

Comparative water uses