When my former company closed, they had several large white plastic 50 gal barrells (they were full of glue) that I grabbed (I grabbed a TON of stuff from there!). Made two into similar water barrells - but not connected. One is on the deck for the deck plants, the other collects rain from the greenhouse and waters the greenhouse plants. We don't get nearly as much rain here in IL as you do in WA, so we have to use the hose sometimes.
BUT we're not metered so my bill doesn't change.
I do it because my plants prefer rain water to city water. City water scorches the leaves.