OK, my friend is looking for a manga series and I... have no clue:
"There's this boy that's either protecting or kidnapped an old man or a girl who can create water (the girl). It's important because they live in a desert world. The only scene I remember form the manga is that they stop an an oasis that is actually a giant plant that collects water, and it reproduces by sending spores in the air that looks like snow in the desert. The plant tries to eat some other people who have stopped at the oasis and the girl attampts to save them but ends up killing the plant with her water. And the the boy gets mad at her because she killed the plant, so she leaves soem water for the animals before she goes. I'm pretty sure I saw this manga around 2001 or 2002 and no later."
(note from me: this may not have been scanslated. she claims she saw me reading it back in high school (which would most likely make this raw) *no clue*)
I don't know... But some parts of your description sounded a bit like agharta... I didn't read much of Agharta though. So the events you're talking about are mysterious to me... Probably someone more knowledgeable than me can give you more information. (and confirm/deny my claim)