I had that type of discussion quite a while back with my girlfriend while watching Star Wars EP3... the scene where it suddenly rains on Coruscant while they haul the mutilated Anakin from Sidious' ship into the med tower. Why should it even rain on Coruscant when there are no large scale oceans or lakes left where water could even evaporate into the sky.
Deviating a bit, but there apparently is quite a large total quantity of water on Coruscant. One large sea remains on the surface (or artificial reservoir), the Great Western sea. "Coruscant was once a world mostly covered in oceans. However, all natural bodies of water were drained and stored in vast caverns beneath the city as a result of years of overpopulation. The only body of water visible was the artificial
Western Sea, with many artificial islands floating on it, used by tourists on holidays."
http://starwars.wikia.com/wiki/Coruscant/Legends I don't know if it'd be enough for rain, how much surface area the sea covers for evaporation, water usage of the city to evaporate, rarity of the rain, etc. (I don't know how they'd drain an entire world of ocean underground either).
For terraforming, I distinctly remember reading a warning about how it could/will remove rare planetary features. Perhaps it was online, but thought it was in game. It's made sense to me. I don't remember terraforming 'destroying' districts, though I can keep an eye out for it next game when I go on a terraforming spree. It probably should keep the approximate same resources available - though maybe you could argue it became inaccessible or destroyed through terraforming (like food or energy).
