When to irrigate can be determined two ways. These are by visual and physical inspection and by direct measurement of soil moisture.
Visual and physical inspection: The most efficient way to water your lawn is to irrigate it when it shows signs of stress from lack of water. Visual signs of water stress include the lawn turning a bluish gray color, footprints lingering after being made and grass blades folding in half.
Direct measurement of soil moisture: One way to measure soil moisture is with a soil moisture sensor. Sophisticated sensors will activate your irrigation system when water is needed. The more basic soil moisture sensors turn off your irrigation system when water is adequate.