One of my rain barrels has a quick connect on the spigot to which I connect the hose and soaker hose for the nearby bed. This barrel is on cinderblocks and the soaker hose operates as expected. I have another barrel on paver wall blocks stacked two-high and this one supplies a bed that is located much further downslope after a long garden hose run -- the water literally oozes and drips out of the soaker hose at a much slower rate but it does the job.
I don't have a drip irrigation system (well I do have some components I bought to try out, but they are still in their individual parts in the shopping bag -- I haven't had the chance to explore how I intend to use it...) but isn't it all a question of how high the barrels are positioned relative to where the water emitters go? I tend to just connect stuff and see what happens, but I'm sure more engineering minded folks will do the calculations first and could tell you how high.
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