My guess, and it is only that, would be that all actual desert scenes were shot in or near to Palm Springs or Lancaster, desert communities not too distant from Hollywood and Los Angeles.
I am sure you already know it, but if you did not, you might go to www.us.imdb.com and you will find many films have on the left hand side of the page a location called "filming location" and many have been added that gives where it was filmed. Also if you inow of many locations or any other things about a particular film you can even add corrections to a film as I have done on some of the films I have been in or friends who had namers left out of films. This is a great site and you may find many things about films if you love to dig deeper into them.