[chris (OP)]

Jenny wrote to say:

My 8y old asked this question last week, and I think she's probably right: since the human body is made up so largely of water, surely the ever-increasing global population is impacting on the quantity of fresh water available on the earth?

What does everyone think?

[Colin2B]

A very perceptive question.
In fact all living organisms are a high proportion of water. Humans about 60%, but some plants as much as 95%, so the more biomass the more water locked in. Obviously this is recycled when the organism dies, but I suppose the key question is has the total biomass on earth changed significantly over the past eons and particularly over the recent past.

[evan_au]

Humans about 60% (water)

So a 70kg human contains about 40 liters of water.
Humans drink 1-2 l per day urinate a similar amount, and generate another liter or so from their food, most of which gets breathed out in water vapor.
So you could say that a human has a water throughput of about 4 liters per day.

On the other hand, typical water consumption in Australia is around 300 liters per day per person, or 900 liters per day per household.

So the amount of water stored in human bodies (around 40l) or consumed by humans (around 4l/day) is tiny compared to what we use for showering, flushing toilets, washing the dishes and watering gardens. It's a pity that we treat water to extremely high purity, only to spray it on lawns where a bit or organic content would probably help the growth.

And then the "Yuck Factor" stops us treating and reusing the water (we think). Most towns dump treated waste water into streams, and then it is treated and drunk by the next town downstream.

A lot of water also goes into food production...