I know this is not a new problem but I cannot find a definitive answer to the question despite searching the boards and the wiki.
My fortress has two wells, both of which had designated water source squares adjacent to them. One was over a hand-filled cistern, the other from a very large reservoir topped up from a nearby river. They were working just fine for several years. This past winter season (I'm in a biome that freezes) I started getting "Cancels Give Water" and "Cancels Fill Pond" messages with a reason of "No Water Source." Looking at the zones, both showed Water Source (0).
This is not a bucket problem because as soon as the spring thaw came through, the messages stopped and dwarfs were fetching water from ponds and brooks outside.
I started two projects simultaneously which I can think might have caused the problem; one, I emptied and dug out the cistern to provide additional capacity. While doing this, I set the zone next to it to inactive. Two, I started a project to pump from the larger reservoir to another at a higher z-level for trap purposes. I didn't get any further than just digging out the channel over it, though, no pumps were installed and no water pumped out. The capacity appeared to remain full throughout. Moreover, dwarves continued to drink themselves from that well all winter, they just wouldn't fetch from it to give water to wounded or to fill the other cistern once I got it back together.
I've done everything I can think of, deleting and re-creating the activity zones, but for the life of me I can't get it to show anything other than Water Source (0). It has not been long enough for the cistern to be refilled yet, so I can't say for sure that is broken, but I have no idea what is going on with the reservoir well. I have floored over the additional channel I dug over it, to no avail. I've also ensured that it is topped up by opening the flood gate and leaving it. The water beneath the well is 7/7 (it is three z-levels down; on the board it has been suggested that it must be only one down, but the wiki disagrees and besides, as I said this was working fine like that before and dwarves can still drink themselves). I have tried removing the zones entirely, since someone suggested that they are unnecessary for wells, but with or without does not making any difference: dwarves still drink, but won't fetch.
So; any ideas? Anything else to try? I really need to get this figured out before winter comes again or I get sieged and lose my access to external water sources! Oh, and I'm running 0.31.25.