What's to say, then, that PCs find enough water to refill their wineskins every few days? The most logical (really?) answer is that you have a map granular enough to cover the position of every small rill, stream, and pond. Of course, in reality you could never be expected to map with such delicate detail to every meter of crossed terrain. You could handwave and say it's fine; to me, that's a very unsatisfying option. You could just arbitrarily determine it (almost as bad, but what I tend to do) or you could make a table. Guess where there's a good table?
The Wilderness Survival Guide.
I was going to make a table myself for this post, but then I was suddenly struck with the inspiration to check there. I found one almost instantly. So, for the benefit of everyone who forgot or never new that this table existed...
I can only assume those abbreviations are Desert, Forest, Hills, Mountains, Plains and Swamps.