Surin Solar Legend
Time to build an underground cistern. An Aussie I know actually built a whole, quite large, addition to his house, a large room waterproofed that he directed all his rainwater into. Not only did it help him through the year for water for the house and grounds, but he said it keeps that side of the house (the kitchen I think it is) cool all year round as well.
Like all aspects of my build I researched rainwater harvesting carefully before starting.
DOS does make tanks designed to go underground but as they have to designed to take the weight of the soil they are very expensive and only seem to be used on commercial builds where either space or aesthetics are the priority. I have 3 rai of land so space is not an issue and I don't care about aesthetics. The hole has to be excavated and the tanks needs to sit on a concrete pad. They also require a crane to be lowered into place which all adds to the cost.
As for building a concrete underground tank like a swimming pool with a lid, again much more expensive than over ground tanks and also more prone to cracks and leaks which are expensive to fix.
My tanks cost ฿6,500 each and due to their squat design are very stable. It would not be too difficult or expensive to add more tanks if needed.
Another reason I went for over ground tanks was the flexibility of the setup. I currently have 4 rainwater tanks reserved for drinking water, 2 for well water and the rest for rainwater for household use, but changing this setup is just a matter of turning a valve.