Actually you could even build a water recovery system, IE gutters into a tank which acts as the source for the sprinkler. A giant swamp cooler. There are "some" commercial buildings which have similar schemes. I thought about doing that one time, run say, a 3/4" pvc on the roof line drilled full of holes with a small pump. Would not take much.
Below, example, is or was the then new Penny's store at University Shopping center in Spokane. I used to maintain the AC on that roof a giant PITA. It had an expensive rubber membrane roof, covered with round river stones and was uncomfortable as 'ell to walk on. But it retained about 1" of water which acted as a giant cooling device.
On a side note, look how damn close the AC units are to the roof edge, There is no parapet there, and a couple of them are less than 8' from a Great Big fall. This was before OSHA and others decided to mandate fall protection on roofs.
