For now, I would use your imagination.
I don't want to use any solid model edits, or additions for the purpose of keeping it legal for private server use without exploit, so irrigation systems would be impossible for me to add.
I could, however, add a weather system to Orgrimmar, and possibly add some weather to pieces of Durotar to give it an oasis theme, much like how the Barrens is in 3.3.5.
Unfortunately it's hard to make things make sense in WoW, mostly because their climate differences are all over the place. Like how Dun Morogh is along Azzeroth's equator, but so happens to be a tundra... and how you can go from a snow filled zone to dry barren lands by simply moving into the next zone.
Some things in WoW you just have to accept...
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I searched all over the place for a spot to put this, but couldn't find one. Had no idea there was a such forum, so I put it in general as it pertains to both WoW and Molten-WoW.
If a mod moves it, I don't mind. As long as people can see it.