Known as the water cycle, sea water evaporates and forms rain clouds. These clouds rain on land, and the rain water flows downhill. Eventually, the water will reach the sea and the cycle will continue. But, some water is captured in reservoirs, or naturally top-up the underground water level, wells, springs, etc. Reservoir water is treated by settling to remove silt, and aerating, etc, and is then piped as drinking water to our household taps.