Hiking in arid regions is basically an exercise in moving from one water source to another, carrying enough water to travel the distance safely. Time of day, intensity of effort, and temperature all influence the distance that your carried water supply will support. I am a firm believer in early starts in hot conditions, and resting during the heat of the day. I have consumed six liters in an afternoon of hiking and still been dehydrated. In cooler situations, that same amount of water could last more than two days.

There are other variables that enter into the picture as well - the water content of food you are consuming, wind, clothing, relative humidity, amount of conversation, and your degree of acclimatization. One of the key phrases in hiking in hot conditions is "Ration your sweat, not your water." Do not restrict your intake, but take it easy when it is hot. The siesta is a very rational procedures when it is hot.