Residential Irrigation System Rainfall Shutoff Devices

The amount of water that can be saved using rain shut-off devices varies, but in a year with average rainfall, savings are usually substantial.

Michael D. Dukes and Dorota Z. Haman

Agricultural and Biological Engineering Department

University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)

Rain sensors — also called rain shut-off (or moisture-sensing) devices — are designed to interrupt the cycle of an automatic irrigation system controller when a specific amount of rainfall has occurred.  They are small devices wired to the irrigation system controller and mounted in an open area where they are exposed to rainfall.

When a specific amount of rainfall has occurred, the rain sensor will interrupt the system’s common wire, which disables the solenoid valves until the sensor dries.

Advantages of a rain sensor . . .

  • Conserves water – prevents irrigation after recent rain events.
  • ·            Saves money – reduces utility bills by interrupting the irrigation system after adequate rainfall.
  • ·            Reduces wear on the irrigation system because the system runs only when necessary.
  • ·            Reduces disease damage by eliminating unnecessary irrigation events.
  • ·            Helps protect surface and groundwater by reducing the runoff and deep percolation that carries pollutants, such as fertilizers, into storm drains and groundwater.

Installation . . .

A rain sensor should be mounted where it will be exposed to unobstructed rainfall, but not in the path of sprinkler spray.  It is typically installed near the roofline on the side of a building.  However, it should not be mounted such that it comes into contact with water running directly off the roof.  If vandalism is not a threat, it can be mounted lower on a fence post or deck railing.  It is important that trees, overhangs, and awnings are not blocking direct rainfall onto the device.

The closer the sensor is to the controller, the shorter the wire and less chance for wire breaks.  Mounting the sensor in a very sunny, southern end of a building may cause the water to dry out sooner than desired.  Conversely, mounting on the northern end of a building with constant shade may keep it from drying soon enough.  A professional irrigation contractor should know how to correctly install a rain sensor. 

Potential Savings with a Rain Sensor . . .

The amount of water that can be saved using rain shut-off devices varies, but in a year with average rainfall, savings are usually substantial.  There are several factors involved in determining how much a sensor can reduce water usage . . .  

  • how often it rains
  • whether or not the controller is left on for automatic operation
  • the amount of water applied by the system per cycle.

If the water costs and the amount of water applied per watering cycle by the whole system are known, it is easy to calculate how much money is being saved each time the sensor interrupts the watering cycle because of rainfall.

Rain sensors should always be included in any new sprinkler system installation to ensure water is conserved.

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