I guess it would be impractical to install a large tank on your roof with piping down to catch rain water. Especially since you go months with no rain at all. But during the monsoon it would fill up and you would have your own water in case of shortages from the public supply. Course with heat comes evaporation so it would need a lid to contain the water but then how would you get rain. Umm, build a funnel on top so the water could run down the funnel into the tank but provide only a small opening for evaporation. Ha. Hot showers on demand.
