US Patent 7853363 Evapotranspiration unit connectable to an irrigation controller

An evapotranspiration (ET) unit includes a processor and a memory for storing a historical set of components of ET data. The processor can communicate with one or more environmental sensors each capable of generating a signal representing an actual value to be substituted for a corresponding one of the components of the historical set. Preprogrammed algorithms enable the processor to calculate changes to a set of watering schedules of a predetermined watering program of an irrigation controller based on any substituted actual value and the remaining historical set of components. The ET unit 10 communicates the changes to the irrigation controller. Optionally the ET unit 10 can receive and store a downloaded actual set of components of ET data and preprogrammed default algorithms calculate the changes to the watering schedules solely based on the downloaded actual set of components of ET data, thus alleviating any need for utilization of the historical set of components of ET data and any actual values generated by the environmental sensors.