Gravity can work & it is more reliable than pumps. BUT the challenge is that you’d have to raise the top of the water by 2.3 feet to get 1 psi, 4.6feet for 2 psi, & so on for any thing more complicated than a simple drip. For example, an irrigation system might need 20+/- psi minimum for its timer, sprayers, or even a standard soaker hose to work properly. So we just use rain barrels & gravity for the garden that’s at a lower grade than the house & mains water for the higher garden.

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?

When wood dries out, it shrinks and then leaks, so using wine barrels in the summer to catch rainwater probably is not going to work. I bought three plastic drums from a local company that was selling them for $5 each. Often things such as juice concentrate or other liquids are shipped in them. Ask around, someone in your area may be able to tell you who is selling them. Maybe if you indicate where you live, someone reading your post in your area may be able to help you out.

I know I'm responding to a 8-year-old post, but I want to offer my experience with a similar but much smaller situation. I used a solar pump to move the water from the reservoir up to the rain barrels, which are fitted with two spigots. Soaker hoses connected to the spigots irrigate the beds, and one has a solid hose attached to a Yard Butler hose bib extender and another short length of hose. This provided a way to fill a watering can for miscellaneous uses. The solar pump is on a timer but can be activated by a manual switch. Total head is about 45'.