A method automatically adjusts the amount of water supplied by a water timer having a user interface and a controller programmed to provide control signals for controlling water flow control devices. The time of day on a base day is chosen, and a base ambient temperature and a base ambient humidity are sensed. Base ambient temperature and humidity signals are provided to the controller. The current ambient temperature and humidity are subsequently sensed and corresponding signals are provided to the controller on at least one day following the base day at a time corresponding to the chosen time of day. Control signals are modified, thereby modifying watering amounts, according to the base and current ambient temperature and humidity signals.