According to discount pool covers, swimming pools loose around 40 thousand to 50 thousand litres of water per year through evaporation.  That equates to approximately the same amount of water 4 people would use showering per year (yep, I did the math based on 30 litres of water per person per shower per day .... perhaps).  That's a truck load of water.

So, if you own a pool, do our water reserves a  favor, put a cover on it!