Why don't people just put a water sprinkler on their roof for cooling? Some air conditioners work with evaporative processes. So I'm not sure why people wouldn't just use a sprinkler that turns on for a minute every hour or half hour or whatever in order to wet the roof. The water will not only absorb the heat from the sun, but when it evaporates won't it also take away the heat from the house, thus cooling it down.
I'm not suggesting it's going to be as good as an air conditioner since it's outside, and only cooling the roof. But surely it will use very little power and water for a significant reduction in heat in the house, wouldn't it? Are there factors I'm missing?
 A: This will actually help to cool the house somewhat, in many conditions. It will help to keep the roof from getting so hot in the  summer. This helps to lower attic temperatures, which can help, especially in older houses with little attic insulation. I live in Texas where we usually have high temperature, low humidity, sunny, summers (good conditions for evaporative cooling). One of my friends used to keep a garden hose running on the roof of his older house, in hot weather. I have another friend who would spray water on his roof ever so often.
A: There are products sold that are intended to be connected to the outside water supply of the home and blow a fine mist towards an area where evaporative cooling is desired, such as an outdoor deck that gets very hot in summer. I have one. It works pretty well. 
The product mimics the cooling one experiences under a shade tree, which is referred to as "transpirational cooling". As trees release water into the air from their leaves by means of transpiration, the air is cooled as the water evaporates.
Whether or not one feels the product wastes water is a matter of opinion. Do air conditioners waste electricity?  I haven't attempted to measure the rate of water used, but if you don't have shade trees and you only use it occasionally, it  may not use all that much water, and it does provide a benefit, like an AC.
Of course if drought conditions exist one should restrict the use of such a product, like lawn watering restrictions during droughts. Per @my2cents we must weigh such things with consideration to our responsibilities to the environment.
Hope this helps.
A: This is a waste of water and will catastrophically exacerbate the effects of heat and drought when applied at large scale. Paint your roof white, insulate your house (roof, walls, floor, double glazing) and use solar power for your airco is the way to go. 
