I'd like to increase the water collected in water catchers each time it collects e.g. if the default is 12 I'd like to double that to 24 each time.

How could I do that?

Thanks in advance.

try the water catcher boost plugin, it should do what you want

https://umod.org/plugins/water-catcher-boost

Nice one, I'll take a look when I get home :)