Actually, the more I think about it, it definitely is a siphoning effect. The water level always drops to around 1/3 when this happens, and I have 3 rain barrels all connected together. Too much of a coincidence for it to ALWAYS drop to 1/3, with me having 3 rain barrels. I have my overflow pipe coming up from the bottom of the middle barrel in the configuration, with a hose connected at the bottom and running into the yard. Ideally, as the water level rises, it hits the upper level of that pipe (inside the barrel) and starts draining out the overflow right down the center of the barrel and into the yard. HOWEVER, my center barrel also happens to be a closed system. Other than the overflow pipe, there is no outlet to exchange air for the water coming into the barrel (the outside barrels both are vented - one with downspout entry, the other with a nail-sized hole on the top). So, when that middle barrel completely fills up (like it does in a downpour), it then completely fills up the overflow pipe and siphoning effect occurs. It then siphons ALL the water out of the outside 2 barrels, until enough water is siphoned out so that the level drops to the top of the overflow pipe in the middle barrel. It then slowly levels out the water from that 1 full barrel to all 3 barrels. That's why it's always roughly 1/3 full after this phenomenon occurs. To fix this, I need to get some air exchange happening in that middle barrel. To do so, I have to eliminate the "closed system" aspect of my middle barrel. I can do that by simply putting a nail-sized hole in the top like I did in the other. I think I'll try that first. Oh, and the reason I didn't see this happen when I tested this before, is that I didn't let the water completely fill high enough and long enough for the siphoning effect to start. It would have been a period of time AFTER the barrels all appeared full. I obviously didn't wait long enough. I think that makes sense to me. Make sense to anybody else?

I live in Columbus and a local farm store sells empty plastic barrels about the same size as the ones in the catalogs for about 10 bucks. The only problem is they are used and Im not sure what was originally in them.

Good idea. Yes. It has a bottom spigot. We had planned to have them raised up on cinder blocks and have it all set up that way. It's a great idea to let the rain fill them up, check for leaks, empty them and re-level if needed. Common sense! I just needed someone else to think of it. :) Thanks so much!

I have an Ivy Rain Barrel under my downspout in the backyard. I bought it at a discount from the City of Houston's water barrel sale. You can check to see if your city has a water barrel sale. It's not the fanciest barrel, but it does the trick and doesn't breed insects.