I'm working on a fantasy setting that's literally a giant tree on a planetary scale. Magic is in effect, so concerns of how a tree that large could survive are irrelevant, but I need to figure out what water sources usable by it's inhabitants could feasibly exist on a tree of that size. I know bogs could exist, because bogs actually occur in the canopies of real life redwoods, but what about other sources? There's a highly diverse epiphyte community, including some plants that are normal-sized trees, as well as both normal-sized and giant equivalents of real life epiphytes, so there might be bodies of water inside the cups of giant bromeliads or something similar, but could lakes form on the surface of the tree itself? What about rivers and streams? I probably don't want anything so large as a sea, let alone an ocean, but, if possible, how big would lakes be able to get?
If it matters, the tree in question isn't attached to any true planet, when you reach the base of the tree, you find a field of spongey material that can't be seen through a thick, surrounding mist, and if you travel on that field more than a couple miles from any point of the trunk on this surface, you end up more or less back where you started. On the other hand, there is a fairly regular weather system, with rain and other precipitation falling on localized regions of the tree, and some areas being typically drier or wetter than others.
Though a root mass exists, it is, for want of a better way to describe it, "out of phase" with the plane of existence the rest of the tree is on. A few surface roots protrude in the area around the base of the tree, extending about half the distance that can be traveled out from the trunk. The other plane where most of the roots are could in theory be reached via magic, but no one's ever tried due to the sheer uselessness of the surface around the base of the tree, which inhabitants refer to as "the mist ring". There is no other effective mass of the "planet" other than the tree and the various lifeforms growing on it. From the base of the tree to the top of the canopy is roughly 10,000 miles, with a canopy spread of maybe 12,000 miles. It has definitive "upward" direction rather than growing from a central mass. The main trunk is relatively short and thick before masses of branches start splitting off - about 1,600 miles in diameter and branching off at a height of 2,000 miles. Some one race has a fairly extensive tunnel network inside the trunk and some branches, though not reaching the heartwood.
Gravity has a definite downward direction, but there's also a very slight pull toward the trunk. Something that falls off a branch will tend to "drift" towards the center of the tree as it falls. It's also worth note that gravity isn't nearly as strong as Earth's, somewhere between 0.2 and 0.5 Gs. The tree is fully stationary. I'm as of right now undecided as to how weather patterns move, but some clouds can definitely enter the canopy in a way that some precipitation might fall on lower branches but not upper ones.