Four square metres, kept badly and recorded carefully.
The butt is two hundred litres and fills from half a shed roof. It had rained more or less continuously for three weeks, so I assumed the thing was solving itself and stopped thinking about it.
Then I worked out what a dry fortnight actually costs. The beds want something like thirty litres a day between them in real heat. Two hundred litres is a week, and only if I am careful with it, which in July I am not.
So the butt is not a water supply. It is a buffer that smooths over a few dry days and runs out exactly when it would be useful. Knowing that is worth more than the butt was.
None of this is clever. It is just the first time I did the arithmetic instead of assuming that rain in May had anything to do with July.