h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One thousand litres! Yes: a thousand litres of water are required to grow a single kilo of spuds. And they're not the worst:

Maize 1400 litres per kilo,
Wheat 1,450 litres per kilo,
Chicken 4,600 litres per kilo,
Beef 42,500 litres per kilo!

Discuss (in the light of global water resources and requirements).

Pattern-chaser

"Who cares, wins"

I can't imagine that the kilo or so of spuds I grew in my garden last year were given a thousand litres of water under what growth conditions is that P-c?

I dunno, but here's where I got it from (I read something similar from a newspaper article yesterday as well): http://www.commondreams.org/headlines06/0228-05.htm

Pattern-chaser

"Who cares, wins"

How many litres, on average, fall from the sky onto potato plantations over the period of one year?

Anyone know?

"I dunno, but here's where I got it from (I read something similar from a newspaper article yesterday as well): http://www.commondreams.org/headlines06/0228-05.htm" eight litres to brush our teeth?! Where are they getting these statistics?

Well, in Ireland potatoes were the staple crop for a long time, and I haven't heard of any huge irrigation projects, so I'd imagine that rainfall would cover that.

Other than that, anyone care to guess why beef takes so much water than chicken? The only thing (offhand) I can think of is that the grass they (hopefully) eat contains more water than whatever chickens eat. I'd be a bit wary of that stat otherwise.

"Other than that, anyone care to guess why beef takes so much water than chicken?"

Cows are bigger than chickens and thus would need to drink more water.

The whole "how much water" argument is misleading anyway. In the West, for example, we usually have a lot of water because we have a lot of rain. The "here's how much water we use" thing doesn't really demonstrate anything. We couldn't bottle the water we "save" and then post it to the third world.

Those parts of the world that are "dry" need access to clean water and wells and that requires both investment and infrastructure - with both of these even dry areas can be supplied easily (look at Los Angeles). What any article on the potential plight in the third world of should concentrate on is how we can help provide this and how we can deal with potential (that's potential, not inevitable) conflicts arising from access to water.

Excluding the wetter western areas of the British Mainland, (the West Country, Wales, NW England, Scotland), annual rainfall is under 1m/year, and in some areas like the East Coast where potatoes are often grown, annual rainfall may be nearer to 700mm/year, with only about half ogf that being in the actual potato-growing season.
(See: http://www.hanby.co.uk/Rainfall%20Map.htm )

In practice, a home-grower might expect a crop of 2-3kg of potatoes per square metre. Agricultural yields of non-irrigated potatoes, (in mid and eastern England) seem to be roughly 42 tons/hectare (42,000kg/10,000m3), giving a yield of 4.2kg/m3
http://www.nbu.ac.uk/iccuk/indicators/21.htm

Even ignoring water running off and soaking away, which isn't consumed by the crop, it'd seem that the actual agricultural growing of rain-fed potatoes need only consume something like 350mm-500mm of rain per 4kg of potatoes - giving something around 100 litres/kg of water, and I'd expect that surplus water running off or soaking away would reduce that figure somewhat.

To get to a figure of 1000litres/kg, I guess either some large water allowance for pesticide/fertiliser production and/or transport has been factored in, and/or a large allowance made for potatoes grown with irrigation (which doesn't seem to be the way most UK potatoes are grown).

" for potatoes grown with irrigation (which doesn't seem to be the way most UK potatoes are grown)."

Surely, though, the amount of actual water required to grow potatoes would be constant the world over. It is possible that in very hot countries extra water would be required to deal with evaporation but I'm uncertain about the science or the exact details.

re water for cows, a cow is slaughtered for beef just before it is three years old. I don't know about chickens, but I suspect it's much younger. Therefore it takes longer to grow a kilo of beef.

SWL, that would make sense.

Leaving the tap running for one minute would arrive at 10 litres of water used which is the stat they provide.

Also it doesn't say as implied in the original post that it takes 1000 litres of water to grow a potato, it's to produce one.
So the figure may include supermarkets washing the potatos. Either that or market gardening uses a hell of lot more water than leaving the leaving the things in fields requires of water supplies.

I'm a bit confused by those figures. Farmers in drought hit bits of France are being asked to stop growing maize and switch to wheat in order to reduce water consumption.

If those figures are right, that wouldn't make sense...

sprout