Yes, it's rainwater. I just didn't had the time and then forgot about it, thanks :D. 
Since it's rainwater, contained, for nobody knows how long, in a corrugated sheet metal tank, you shouldn't drink it. You can wash yourself with it or use it as drinking water for animals, but 'watering_place' is also not really correct, since it's missing an animal trough.
Any suggestions for a suitable amenity keyword? 