Depends on the server, one server I play on it it raining 75% of the time"Manual" as in setting up a bunch of pumps only to have to move them around later. It's not much different from pumping lava from the nether.
As for collecting rain, because it's a fairly good idea, in Minecraft it doesn't rain a lot. I'd see having to make quite a few barrels/raintanks/etc to go off of it.
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