Hmm, what is the general availability of water resources these would need in areas that would benefit most of them? If there is already droughts I don't think too much water usage could be afforded adding more vegetation.
If you are planting native trees there should be no problem - there have been droughts before and so the trees can handle them. You might need to water them for the first 5 years, but after that they should be okay.
Select trees that are not native (or native species but from a very different location) and you can run into problems.
Native trees aren't always the 'best' option in modified urban environments. Just becuase they grew on what used to be there doesn't mean they will like what is there now
