Water is becoming an issue here in the U.S. for a number of reasons :
Rain water harvesting is a way to store water. Even if it is only used for non-potable purposes, it still helps save potable water. A 1500 square foot roof can catch 3700 gallons of water from 5 inches of rain (allowing 20 percent loss due to evaporation, first flow diversion, and non-smooth roof surfaces). There are 231 cubic inches per gallon. A 1500 square foot roof has 1500 X 144 = 216000 square inches of surface. Five inches of rain on a 216000 square inch surface is 1080000 cubic inches. Divide that by 231 and you have 4675 gallons of water. Take away 20 percent of that due to surface conditions, evaporation, and needed first flow diversion and you have 4675 – 935 = 3740 gallons. divide that by 5 = 748 gallons from 1 inch of rain. add another 20 percent loss and you have 600 (598.4) gallons. You can wash a lot of clothes with 600 gallons of water. Think about it. Its not against state law to set up a few rain barrels. Check out the rest of this blog and its accompanying websites for lots of info.
The runoff from the average house with 6 downspouts could easily fill 6 55 gallon rain barrels from just 1 inch of rain. A slow sand water filter, combined with a first flow diverter, will purify the water so it can be used without the danger of serious contamination.