In many places in the continental United States, a significant amount of natural rainfall occurs during the growing season. All of this rain is not available for plant use. A large percentage will be lost to evaporation, runoff, or deep percolation. What reaches the roots and is utilized by the plant is called effective rainfall. For example, this year in central Virginia (using a very conservative formula) we still end up with 12.26 inches of effective rainfall for the April to October growing season with an evapotranspiration (ET) plant water requirement of 34.25 inches for the same period. This means that natural rainfall provided almost 36% of the water required for the 2013 growing season. This example emphatically demonstrates the importance of a well functioning local rain sensor.
Much of the rain that occurs during the growing season is localized. We all know that it can rain 1.5 inches on one side of town and ¼ inch on the other. The only way to account for these large variations is to install a local rain sensor with your specific system. When functioning properly, this sensor will react to the rainfall that occurs at your specific location. It will interrupt any scheduled irrigation cycle and not allow it to operate until the rain sensor has dried out, roughly paralleling soil conditions.
If you have a sensor it must be installed at a location that is open to natural rainfall. It can’t be installed under the canopy of a tree or close to the controller just because this is convenient for the installer. It must be located where it will receive natural rainfall so that it can react to actual weather conditions. Most of these units, especially the wireless devices, rely on a small battery in the transmitting unit. These batteries usually need to be replaced annually. If this battery is dead or weak, the rain sensor will not operate properly or at all.
In summary, the local rain sensor is a critical component of your landscape sprinkler system. A fully functioning and well-maintained device will prevent the operation of your system when not needed and save you large amounts of water and dollars. As per the example in the first paragraph, this small device could have saved the customer over 1/3 of the cost of operation of their landscape sprinkler system in 2013. Many of the units available on the market today have the ability to be programmed to interrupt irrigation for extended time periods even after the hydroscopic disks have dried out and the rains sensor contacts have closed. Take advantage of this low hanging fruit today. Make sure that you have a local rain sensor properly installed and fully functioning.