The ability to water only when necessary is the ultimate goal of efficient irrigation scheduling. Today's technology allows for installation of sensors which can measure soil moisture, rainfall totals and even shut systems down in freezing weather.
These units are mounted in a location exposed to normal rainfall, but outside the watering spray of the sprinkler system. There are different designs, but most have settings that allow some sort of measuring to take place. Rain will cause the system to remain off during or after an event if sufficient rainfall is measured. The settings can be adjusted so a light shower will not effect the system operation or eliminate a scheduled watering when rainfall is not sufficient to make up for a normal application.
This type of sensor is very popular in commercial settings where ice on walkways or streets can cause liability. Freeze sensors have a place in the residential garden as well. Watering before or during heavy freezes can create problems with ice-laden shrubs and trees as well as creating the same liability concerns the commercial user experiences. The freeze sensor is mounted on an outside wall in a location most likely to experience freezing conditions. It will interrupt the signal to the control valves when temperatures fall below freezing. When conditions improve, the system will return to normal operation