I thought we were ready for desert hiking when my wife and I moved to Arizona from Michigan. We liked the climate, and were amazed how our skin was dry and comfortable when it was 105 degrees (41 Celsius) outside. However, we noticed that people carried water bottles just to take the bus to work or to go shopping. We also noticed that soon we were drinking much more water than ever before. We needed to - especially if we were going hiking.
A desert isn't defined by heat. It is dryness that defines a desert. Heat is certainly an issue when you are hiking in the desert, but getting enough fluids is the larger problem.
Your first time hiking in the desert, you'll be surprised at how much water you need. You won't really feel like you are sweating that much, because in the dry desert air your perspiration can evaporate as fast as it comes out of your pores. This makes it very comfortable compared to the "sticky" heat of more humid areas, but it also hides your body's water loss.
It is easy to become dehydrated very quickly without realizing it is happening. Rule number one for desert hiking, then, is to carry much more water than you think you'll need. Drink your fill before starting as well. You may even want to wet your shirt before starting out, so the evaporation will cool you and reduce your sweating (and so reduce your water requirements).
On longer hikes, and especially on multi-day backpacking trips, be sure you know where you'll be refilling your water bottles. Have enough water carrying capacity for the longest dry stretch you'll be traversing. When in doubt, carry too much.
Desert Hiking Routines
A quote from the U.S. Army Survival Guide: "The body requires a certain amount of water for a certain level of activity at a certain temperature. For example, a person performing hard work in the sun at 43 degrees C requires 19 liters of water daily. Lack of the required amount of water causes a rapid decline in an individual’s ability to make decisions and to perform tasks efficiently."
In other words, if you have a limited water supply, you should limit your activities during the hottest time of the day. Get started hiking early, perhaps just before sunrise. That way you can get in some miles before the heat comes. Hiking in the evening may work as well. If there is a full moon, you might even try hiking from four in the morning until the heat starts.
Unlimited water? You still need to be careful. Your body cools itself by sweating, and it can only process so much water per hour. When it is hot enough and you are active, you can reach a point where you are perspiring faster than you can process the incoming fluids. In other words, a belly full of water won't help if it is being absorbed slower than it is used. Stop sweating during hot weather and high activity, and you'll quickly develop heat stroke, which requires immediate medical attention.
By the way, in that last case, if your water is truly unlimited (as when you are hiking along a desert river), you can wet your clothes to cool off, so you won't need to sweat as much. Watch for any little tricks like this that make your time in the desert not only more comfortable, but safer. There are beautiful deserts to go hiking in, and no reason to avoid them entirely. Just pick cooler times, and follow some of these basic guidelines.