I'd like to start capturing rainwater and using it to help supply the irrigation system I already have. It's a standard Rainbird sprinkler system that runs on full pressure water supplied by the city. From what I've read, I'd probably need a pump to get the rain water pressure to be high enough to use for the sprinklers.
I won't be able to store nearly enough rainwater to run the sprinklers entirely off of rain water. I think I'd be lucky to be able to store enough for one watering.
So what I'm stuck on is: is there anyway to plumb this so that the sprinklers would run on rainwater when it's available, but back-off to the regular domestic water when that runs out? Or supply it from both sources so that the domestic water continues to provide water after the rainwater runs out?
Is there some sort of Y-fitting that is able to give one supply priority? Or maybe it could be done with some check valves? Or something totally different?
Edit: The answers so far involve building a rainwater only supply system, and then using a float to add utility water to the tank when it's almost empty. I'd prefer not to do that since it would seem to put more stress on the pump and increase the risk of flood if the float valve fails.