This is a good question and is a matter that should not be taken lightly. The amount of water you're going to need will depend on a lot of factors. The main one will be the temperature. Obviously, there is a big difference between 40oC and 50oC. As a rule of thumb, for temperatures under 40oC, it is recommended to drink 4L (1 gallon) of water every eight hours, and that is for moderate exercise and for drinking only. Other factors include: biking during the daytime when the temperature peaks, the air humidity, the clothing you are going to wear, how strenuous the ride is, etc. So, I would say that if the temperature is around 40oC, if you don't bike during the 4-5 hottest hours of the day, and assuming a moderate climb and the proper clothing, 8 liters should be sufficient for one day. That would be 6-6.5L for drinking and the rest to brush your teeth and cook some noodles. But if you bike during the daytime, under 50oC temperatures while climbing steep hills, even with the proper clothing you can drink 8L in 2 hours.
Here are a few tips:
- Wear a synthetic long-sleeve shirt, pants, and hat.
- If you use sun lotion, make sure you use one made for sports, otherwise it could clog your skin pores, preventing you from sweating and resulting in overheating.
- Small water spray bottles are a good investment. There is nothing like spraying your face with water while you're biking when it's really hot. And it uses much less water than pouring from a water bottle.
- Hang a tennis sweatband on your handlebar to wipe away sweat from your face or body.
- Wrap your bottles in canvas (I learned that from the Bedouins in Syria) or heavy cotton socks (synthetic dries too quickly) and wet them regularly to keep your water cool. The physics of the evaporation will actually cool off the water inside the bottle.
- During midday, find a spot in the shade and wait for it to cool off. If there is no shade, create your own with a tarp or the fly of your tent.
- If you can, bike during the nighttime and/or start to bike before the sun rises. There is nothing like biking in the desert during the night.
- Always make sure you know where the next source of water is. Don't rely solely on your map; verify the existence of towns or waterholes by asking locals.
- If necessary, save the water you used to cook your noodles; you can use it to wash your dishes, to make tea, or to brush your teeth.
- One of the early signs of heat stroke is confusion; if you start to feel like you can't think clearly, stop biking IMMEDIATELY.
- If you feel like you're overheating, wet your sleeves and shirt with the spray; it will cool your body.