Friday, August 25, 2006

Where Does the Rest of the Water Go?

The UN takes a look at water usage in the world.

The bad news is that a third of the world’s population, some two billion people, are already short of water. But things do not have to be this way.

Roughly 900m people, the assessment finds, live in river basins where there is barely enough to keep rivers flowing and lakes filled. Another 700m live in basins rapidly approaching this “closed” state, and a billion more live within reach of adequate water supplies but cannot afford to make use of them. The water table is falling fast in densely populated and poor regions of China, Mexico and India.

In theory, the world would still have more than enough water to feed everyone, under most scenarios. But it might require much more food to be traded from sodden parts of Europe, North America and Russia to parched bits of Africa and Asia. Needless to say, subsistence farmers in those continents are in no position to pay for imports of food—and will become even poorer, presumably, if their water runs short.

The main culprit is agriculture. It takes roughly 3000 litres of water to grow enough for one person for one day, or about a litre for each calorie.
I have read similar numbers before, and the thing I always wonder is, where does the rest of the water go? A calorie of carbohydrates is 1/4 of a gram. A liter of water weighs 1 kg. So less than 1/400 of the water ends up in the food we eat. Some ends up in the part of the plants that we don't eat. But where does the rest go? Too often it seems like people treat water like gasoline that gets used up when you use it. Unless the water is chemically changed into something else like carbohydrates, it just passes on.

I would guess that some of the water evaporates and falls as rain somewhere else (either on other land or over the ocean). Some sinks into the ground and ends up in the ground water. And the rest ends up in rivers. As far as I know, that is the only places it can go.

My point is that water that ends up in a river, or in an aquifer or falls to the ground as rain can be used by someone else. That of the 400 liters used, some amount (quite possibly most of it) is available for others to use and therefore counting it as "being used" by the farmer is deceptive.

If anyone knows where the water ends up, please leave a comment.

via The Economist

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