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 S cience for everyday life
How much water do we really need? 

As temperatures continue to rise over the long summer months, so too do our thirst levels and we need to drink lots of water to avoid getting dehydrated.

No one really knows exactly how much water a person should drink, or actually needs, since it depends on how active the person is and what the climate is like. But one thing is certain: like all living things from the tiniest insect to the tallest tree, the human body is composed mostly of water. And we all need water to stay alive. In fact, water is more important to us than food -- you can go without food a lot longer than you can go without water. If you went even a day without taking in any water you would fall seriously ill.

Though it's difficult to estimate exactly how much water a person needs, it's important to have a general figure of individual water requirements for survival, for planning purposes. Everyone from architects to town planners to water engineers needs this information. It's also a useful figure to have to help decide if a place or people is 'water stressed', meaning they do not have access to enough water.

Recommended basic water requirements for human domestic needs

Purpose

Litres per person per day

Drinking water

5

Sanitation

20

Bathing

15

Food preparation and cooking

10

The figures give here were first calculated by a scientist called Peter Gleick, and this total of 50 litres is called a 'recommended' requirement since it is what is needed to lead a normal life, not what is biologically or medically needed. One's biological needs should in any case be less than this total, as, if you desperately need water, you may not be too concerned about bathing.

A lot of the five litres needed for drinking is often consumed not as water but as drinks such as milk, tea, soft drinks, etc

These figures throw up a couple of interesting points.

By sanitation we mean the amount of water required for flushing and washing after going to the toilet. It is interesting that this requirement is so high, higher than what is needed for anything else. This is partly because toilets use up a lot of water, and also because sanitation is so important. This importance is more easily recognised when the required water is not available. After all, the second largest killer of people the world over is still poor sanitation; in other words, lack of access to those 20 litres of clean water.

A second point is that though the amount of water required for cooking is included, what's not is the water needed to grow the food to be cooked. All the grain, fruit and vegetables we eat need water to grow, and the amount of this water far exceeds our basic water requirements. The amount would be even greater for meat-eaters as you would have to include water for the crops the animals eat as well as the water the animals drink

Now that we know how much water is needed, let's contrast this with how much water is available. Not just in our cities, which never seem to have enough water, but globally.

Our teachers teach us that we live on a 'blue planet'. And that there is actually more water than land on our planet. So, shouldn't there be more than enough water to meet everyone's basic needs?

The problem is that when we speak of water as a human need we are talking about fresh, clean water that's easily available to meet our requirements for personal survival and for agriculture. There really isn't that much of this water to go around.

Despite our perceptions of the earth being a water planet, only a fraction of the water is accessible for human use. Of the total volume of water on the planet (an estimated 1,386,000 cubic kilometres), most of it -- 75.5% -- is undrinkable unusable salt water found in the world's oceans and seas. Only what's left, around 2.5%, is fresh water. Of this again, most (69%) is locked up in the form of ice in glaciers up in the mountains or in the north and south polar regions. Again, unusable. So, only a little over 30% of that 2.5% can be used by people. Sorry, not quite: a large chunk of this water is deep underground, in fact most of this 30% is groundwater. The tiny bit that's left over, usually calculated at between .3-.7% of the total water on earth is what's left for all the human beings in the world to share. Not much, huh?

-- Manoj Nadkarni

InfoChange News & Features, May 2006

 
 
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