Water Tanks, page-74

2 assumptions there zero
.
Firstly that the 135,000 - 160,000L tank is parked right next to the house. That doesn't happen with the big rural tanks because nobody in their right mind would want one up against the house replacing the garden and blocking windows. At 2.2m high and 9m diameter you might see that point.
20m away is close enough if you like your garden and don't like looking at a water tank.

Secondly don't assume that the end of the roof will be enough to fill it and keep it full. That will work for a small garden water tank but not something that often relies on the total annual rainfall being collected from the roof in order to get the family through the summer.

I'm lucky in having more than double the rainfall of many areas and 500sqm of roof to collect it but try 500mm of rainfall instead of 1,000m a year and 200sqm of roof. That's 100,000L of water and the 135,000L tank is only 3/4 full without using a drop in the house.

You just can't use those smaller garden tank set ups with full size rural tanks.