Originally Posted by
cyccommute
While it is true that the energy to evaporate a given unit of water is the same independent of the source, it's not necessarily true that water sprayed on you is the same as water sweated out of you. Nor that water on the outside of you is the same as water inside you. Heat exhaustion and heat stroke are the result of dehydration or too little water inside your body for you to continue to function. To avoid them, you need water inside you. Pouring water on yourself may cool you but it does nothing to address the dehydration.
We are also talking about touring. While touring, as opposed to recreational riding, you probably won't know where water sources are. A liter of water wasted on the outside of you may be a liter of water you need at mile 40, 50 or beyond. If you have excess water, by all means, pour it on yourself. Nancysv is absolutely correct unless you can be sure you have excess water, it's better to have it on the inside than the outside. In other words, drink.
I'm not saying don't drink water. I am saying that given the option of sweating a cup of water or evaporating a cup of water off your skin, the second is the better option if it is really hot out. You will be evaporating that cup of water one way or another, doesn't matter if it's inside you or outside. If you are working and producing heat, and the air is hotter than your body, you will need to evaporate a certain amount of water or else your temperature will rise to unhealthy levels.
In desertlike conditions where it is extremely hot and extremely dry, the mass-rate that you need to evaporate water to keep cool while riding can easily exceed the maximum mass-rate that your body can absorb water, and the salt-loss rate that goes with that sweat can also exceed the amount of salts you take in. Splashing about half of your water on your skin will keep you cool without dehydrating you. You will still sweat as well, and you will still need to drink water. But you won't sweat as much and you won't need to drink as much.
Basically, riding in the desert you need to evaporate a certain amount of water each hour no matter what. Throwing a portion of that water on your skin saves you from dehydration.