Desert sun would be very strong for an RES since it is so dry. RES's come originally from the Mississippi mud flats (mouth of the Mississippi river), where the air is very humid, in addition to being hot. So even though the temps can reach a balmy 100 degrees in the shade down in Mississippi, it is a very humid wet heat, vs. AZ where the same 100 degree temp is extremely dry. So if you do take your turtle out doors, I would make sure that it would only be in a baby pool filled with water & basking dock for short periods of time (1/2 hour).
Heck, if I lived in AZ, I would consider fostering a Sulcata Tortoise if I were you (there are many who are in need of homes since people don't realize how big they are going to get when they buy them as babies)! They are really cool Desert tortoises & they can definitely weather the hot AZ sun.
(And of course, keep your pet RES as well....I just thought I would mention a Sulcata in case you never knew about them & had room in your family for one).
