Balzac, my water planning for the desert is pretty much the same as yours. I hike a lot in low humidity, hot, water-scarce parts of Arizona, New Mexico and West Texas. I typically figure on four liters per day unless it is exceptionally hot. When it gets really hot (>100 degrees F), I will either stay at home, go to sure water sources, or carry a 2 liter per day reserve. Water weight adds up quickly so my really hot weather trips are generally short, or postponed.

When I'm in the Sierra, I seldom carry more than a liter. Usualy, that will last me all day but on occasion I need to stop and filter.

I think than one can adapt physiologically to hot climates in ways that reduce ones need for water. I lived in the Pacific Northwest for a large part of my life before moving to the southwest. I drink less water now than I did when first I moved to Arizona. I also don't worry too much if I get a little dehydrated as long as I have water available to drink but I certainly don't push the issue very much.