Just get a rainwater storage tank and install it according to instructions.

Pretend that the input from the pump is coming from a downspout.

I was clearly lacking in info. My first post didn't go through and I lost it, so I rushed through a second. I am looking to irrigate 20,000 square feet of lawn. Rough calculation are 1" of water a week. That is .6 gallons per square foot. That comes to about 1500 gallons a day. If I am irrigating that much each night, I don't think one rain barrel would be sufficient. I would need 27 barrels filled each night. If we received enough rain to keep up with that I wouldn't need to irrigate! :-) Thanks for the idea though. Perhaps I can do that with gardens around the house.