Thanks, I get it now. Nice and simple. My rainwater system is all gravity-fed, no pumps are used. Currently it consists of six 55-gal plastic drums (four plumbed together and two independent) with soaker hoses running through the garden beds. I have to manually turn on and off the water flow. I've forgot on occasion and drained all my water.
I was hoping the thing you and Art were describing would prevent that but I guess not.

-Blast