Abstract: An apparatus for precoooling water supplied to an evaporative cooler of the type known as a "swamp cooler" includes inlet and outlet reservoirs secured by brackets to a bottom surface of the evaporative cooler water pan at substantially the same elevation. The inlet and outlet reservoirs are connected in fluid communication with a subterranean vessel enclosing a cooling coil. In use, water is supplied to the inlet reservoir and maintained at a predetermined level by a float valve assembly. A filtered drain line returns water from the evaporative cooler pan into the inlet reservoir from which it drains by gravity flow into the cooling coil within the subterranean vessel. The water circulates within the coil to precool and thereafter discharges and mixes within the vessel. A pump in the outlet reservoir pumps water from a return line communicating with the vessel back to the evaporative cooler.