An irrigation system includes at least one environmental sensor, such as a solar radiation sensor that is installed on an irrigation site, and a soil moisture sensor that is also installed on the irrigation site. Programming allows an estimated ET value to be calculated based at least in part on the output signal of the environmental sensor. A pre-programmed watering schedule is automatically modified based on the estimated ET value to thereby conserve water while maintaining the health of plants on the irrigation site. The system automatically inhibits irrigation when an output signal of the soil moisture sensor indicates an amount of moisture in the soil is above a predetermined threshold.