Salinity control of sump water in an evaporative cooler (10) wherein a pump (19) continuously impels a water flow from a sump (11) to an evaporative surface where some of the water is evaporated by air flow from a fan (15, 16). Control is affected by sensing electrical resistance of the sump water with monel probes (36) energised with low voltage alternating current, and initiating an "ON" period of a timer (quad NOR gate) when resistance drops below a preset level. During the "ON" period a solenoid valve (25) is opened in a bleed line and some only of the pumped sump water is passed to drain, without interruption of cooler operation.